Buku Teks Fizik Tingkatan 4/physics-form-4-chapter-6.pdf

Let’s StudyLet’s Study

6.1 Refraction of Light6.1 Refraction of Light
6.2 Total Internal Refl ection6.2 Total Internal Refl ection
6.3 Image Formation by Lenses6.3 Image Formation by Lenses
6.4 Thin Lens Formula6.4 Thin Lens Formula
6.5 Optical Instruments6.5 Optical Instruments
6.6 Image Formation by Spherical Mirrors6.6 Image Formation by Spherical Mirrors

What are the concepts and principles What are the concepts and principles
related to light and optics?related to light and optics?

How are the concepts of light and How are the concepts of light and
optics used to explain natural optics used to explain natural
phenomena and applied in daily life?phenomena and applied in daily life?

Why are fi bre optics and small Why are fi bre optics and small
lenses used in optical instrument lenses used in optical instrument
technology?technology?

CHA
PTE

R 6 Light and Light and
OpticsOptics

230230
Dew drops on grass can function as liquid magnifying
lenses that form images of objects around them. The eff ect
of light refraction is infl uenced by the size and shape of the
dew drops.

The thickness of the human eye lens is controlled by cilia
muscles so as to have diff erent focal lengths to form images in the
range of human vision. The concept of focal length adjustment
is applied by scientists and engineers to invent liquid camera
lens in smartphones. The focal length of the liquid lens can
be adjusted using electric fi eld in the electronic system of the
smartphone. As such, one liquid lens can replace a set of lenses
installed in the smartphone camera. The saving of space allows
the overall thickness of smartphones to be reduced.

Video on liquid camera lens

http://bt.sasbadi.com/p4231a

Learning Standards and Learning Standards and
List of FormulaeList of Formulae

PageInformation

231231
Refraction of Light6.1

Figure 6.2 Refraction of light

Observe Figure 6.1. What causes the phenomena?

i

r

Air

Incident ray

Refracted ray

Emergent ray

Normal

Glass block

INFOSMARTSMART

Optical density is not the same
as density which is defi ned
as mass per unit volume.
For example, the density of
oil is lower than that of water
causing it to fl oat on the
surface of water. On the other
hand, the optical density of
oil is higher than the optical
density of water.

Figure 6.1 Phenomena of refraction of light

These phenomena are known as refraction of light. They occur due to the change in
velocity of light when travelling through mediums of different optical densities as shown in
Figure 6.2.

Pencil appears bent in
the water.

Image of pencil appears bent
in the glass block.

232232 6.1.16.1.1
Incident ray

r
Air

Refracted ray

Normal

Glass block
i

Refractive Index
Refractive index, n determines the degree to which light bends
when travelling from vacuum to a medium. As such, we can define
refractive index as the ratio of speed of light in vacuum to the
speed of light in medium.

Refractive index, n = speed of light in vacuum
speed of light in medium = c

v

where c = 3.0 × 108 m s–1

Based on Figure 6.3, light rays bend towards the normal
when light travels from a medium of low optical density
(air) to a medium of high optical density (glass block).
This is because the velocity of light decreases when
light travels from an optically less dense medium to a
denser medium. Due to this, the angle of refraction, r
is smaller than the angle of incidence, i.

Figure 6.3 Refraction of light from
air to glass block (i > r)

Figure 6.4 Refraction of light from
glass block to air (i < r)

i

r

Air

Incident ray

Refracted ray

Normal

Glass block

Incident ray

r
Air

Refracted ray

Normal

Glass block
iBased on Figure 6.4, the light ray bends away from

the normal when light travels from a medium of
high optical density (glass block) to a medium of low
optical density (air). This is because the velocity of
light increases when light travels from an optically
denser medium to a less dense medium. Due to this,
the angle of refraction, r is bigger than the angle of
incidence, i.

When optical density
increases, value of refractive
index, n will increase. Value
of n is always more than
or equal to 1. The value n
depends on the wavelength,
λ of the light used. Change in
temperature can also change
the value n of a medium.

F ile

233233

Chapter 6 Light and Optics

6.1.26.1.26.1.16.1.1
Activity 6.1

Aim:  To compare refractive index of different mediums and relate the refractive index of a
medium to its optical density

Instructions:
1. Work in groups.
2. Gather information from reading resources or websites regarding refractive index for

several different mediums such as air, water, cooking oil, ice, glass, diamond and
other mediums.

3. Discuss:
(a) The relationship between refractive index of the selected mediums and their optical

densities.
(b) What are the physical factors that infl uence the value of refractive index of

a medium?
(c) Which medium has the highest value of refractive index?

4. Present your fi ndings.

Snell’s Law
Table 6.1 shows the refractive indices for several
mediums. Figure 6.5 shows light rays travelling
from water into three different mediums.

Table 6.1 Refractive indices for several mediums

Medium Refractive index

Vacuum and air 1.00

Olive oil 1.46

Perspex 1.50

Glass (crown) 1.52

Glass (flint) 1.66

Diamond 2.42

Figure 6.5 Light rays travelling from water into three different mediums

Water
50°

42.8°

50° 50°

Glass
(flint)

n1 = 1.33

n2 = 1.50

n1 = 1.33

n2 = 1.66

n1 = 1.33

n2 = 2.42
Water

Perspex

Normal Normal Normal

Water
Diamond

37.9°

24.9°

ISS ICS

234234 6.1.36.1.36.1.26.1.2
Figure 6.6 Law of refraction of light

Angle of refraction depends
on the angle of incidence
and the value of refractive
index of the glass block.

Figure 6.7 Bending of light ray

Incident ray

Medium 2
(n2)

Medium 1
(n1)

Refracted ray

Normal

Point of incidence
θ1

θ2

Medium
(n)

Air
(n = 1)

Normal

r

i

n = sin i

––––

sin r

When medium 1 is air (n1 = 1)
and medium 2, n2 = n

n = sin i
sin r

i is the angle of incidence in air,
r is the angle of refraction in
medium 2

• The incident ray, refracted ray and normal meet at one point and are in the same plane.

• Snell’s Law:  n1 sin θ1 = n2 sin θ2

n2
n1

= sin θ1
sin θ2

, where n1 = refractive index of medium 1
 n2 = refractive index of medium 2
 θ1 = angle of incidence in medium 1
 θ2 = angle of refraction in medium 2

Air

Air

Glass
block

Based on Figure 6.5, the value of n1 sin θ1 is the same as n2 sin θ2 for the three different
mediums. According to the law of refraction of light, when light travels from one medium
to another:

Observe Figure 6.7. Why does
light ray bend when entering a
glass block?

235235

Chapter 6 Light and Optics

6.1.46.1.46.1.36.1.3
Note: This experiment can
also be carried out using
perspex.

Glass block
Ray
box

O

P

Q

i

r

White paper

Figure 6.8

Experiment 6.1
Inference: Angle of refraction depends on angle of incidence
Hypothesis: The larger the angle of incidence, i, the larger the angle of refraction, r
Aim: To determine the refractive index of glass block

Variables:
(a) Manipulated variable: Angle of incidence, i
(b) Responding variable: Angle of refraction, r
(c) Constant variable: Refractive index of glass block

Apparatus: Ray box with single slit, protractor and ruler

Materials: Glass block, white paper and pencil

Procedure:

1. Set up the apparatus on a piece of white paper (as
shown in Figure 6.8).

2. Trace the outline of the glass block on the white paper
and draw a normal line at point O.

3. Draw fi ve lines at different angles of incidence,
i = 20º, 30º, 40º, 50º and 60º using a protractor to
represent fi ve incident rays.

4. Direct the light ray from the ray box at angle of
incidence, i = 20º. Draw the emergent ray PQ.

5. Remove the glass block and draw the refracted ray
OP. Measure the angle of refraction, r and record the
reading in Table 6.2.

6. Put the glass block back in place. Repeat steps 4 and 5
with angle of incidence, i = 30º, 40º, 50º and 60º.

7. Calculate the value of sin i and sin r. Record the values in
Table 6.2.

Results:
Table 6.2

Angle of
incidence, i / °

Angle of
refraction, r / ° sin i sin r

20
30
40
50
60

Analysis of data:

1. Plot a graph of r against i and a graph of sin i against
sin r on a different graph paper.

2. Calculate the gradient of the graph of sin i against sin r.
3. State the relationship between angle of incidence, i and

angle of refraction, r when light moves from air into the
glass block.

If a laser pointer is used in
this experiment, apparatus
set up needs to be
rearranged.

Demonstration video on
refraction of laser ray

http://bt.sasbadi.com/p4236

F ile

236236 6.1.46.1.4
Real Depth and Apparent Depth

Observe Figure 6.9. Why does the position
of the image of the fish appear closer to the
water surface?
 This situation is caused by refraction of
light. When light rays from the fish travel from
water to air, light is refracted away from normal.
This effect of refraction of light causes the image
of the fish to be closer to the water surface as
seen by an observer.

Activity 6.2

Aim: To draw a ray diagram to show real depth, H and apparent depth, h
Figure 6.10 shows a coin, placed under a glass block
with a thickness of 8.0 cm. Ray OPQ is the path of light
from the centre of the coin, to the eye of the observer.
You are required to draw another light ray, OP’Q’ from
point O to the eye of the observer. You can download
and print Figure 6.10 from the website given.

Instructions:

1. Work in pairs.

2. Draw lines QP and Q’P’ and extend both lines into
the glass block until they meet.

3. Mark the meeting point as point X. X is the
apparent position for the centre of the coin, that is
the image for point O.

4. Complete your ray diagram by drawing line OP’.

5. Measure:
(a) Real depth, H, which is the distance from O to the

surface of the glass block.
(b) Apparent depth, h, which is the distance from X to the

surface of the glass block.

6. Calculate the value of H—
h

. Compare the value of H—
h

with the refractive index of the glass

block, n. Relate real depth, H, apparent depth, h, and refractive index of the glass block, n.

Download Figure 6.10

http://bt.sasbadi.com/p4237

P
P'

Q'

O

Q

Observer

Glass block

Coin

8 cm

Figure 6.10

Conclusion:
What conclusion can be made from this experiment?

Prepare a complete report for this experiment.

Discussion:
1. What is the value of refractive index of the glass block?
2. State one precaution that needs to be taken to increase the accuracy of readings in

this experiment.

ICS

Figure 6.9 Effect of refraction of light

Air
WaWateterr

Actual position of fish

PPo isitition off
imimagagee ofof ffisishh

237237

Chapter 6 Light and Optics

6.1.46.1.4 6.1.56.1.5
Figure 6.11 shows that a diving instructor who is at the poolside sees the position of the
diver closer to the water surface. Figure 6.12 shows a diver who is at the base of the pool sees his
instructor further away from him. Can you explain these situations?

Figure 6.11 Position of image of diver from the
viewing angle of instructor

Figure 6.12 Position of image of instructor from the
viewing angle of diver

Experiment 6.2
Inference: Position of an image is infl uenced by position of object and refractive index of

different mediums

Hypothesis: The greater the real depth of an object, the greater the apparent depth

Aim: To determine the refractive index of water using non-parallax method

Variables:
(a) Manipulated variable: Real depth, H
(b) Responding variable: Apparent depth, h
(c) Constant variable: Refractive index of water, n
Apparatus: 1 000 ml beaker, ruler and retort stand with two clamps

Materials: Cork, two pins, cellophane tape and water

Procedure:
1. Set up the apparatus as shown in Figure 6.13.

Pin Po

Pin Pi

CorkImage of pin Po
y

Water
Beaker

Observer

Ruler

Retort stand

0
1

2
3

4
5

6
7

8
9

10
11

12
13

14
15

16
17

18
19

20
21

22
23

24
25

26
27

28
29

30
cm

Figure 6.13

Instructor
Normal Normal

Diver

Image of diver

Image of instructor

Diver

Instructor

Non-parallax method is
an important method to
determine position of
image accurately in optical
experiments.

F ile

238238 6.1.66.1.6
2. Stick a pin Po with cellophane tape to the base of the beaker.
3. Fill the beaker with water to a depth of 6.0 cm. This depth is real depth, H.
4. Observe the image of pin Po from above the water surface.
5. Adjust the position of pin Pi vertically until it appears in line with the image of pin Po.

At this level, position of pin Pi is at the same level as the image of pin Po.
6. Measure the distance y between pin Pi and the base of the beaker. Record the reading

in Table 6.3.
7. Repeat this experiment with real depth, H = 7.0 cm, 8.0 cm, 9.0 cm and 10.0 cm.

Results:

Table 6.3

Real depth, H / cm y / cm Apparent depth, h / cm

6.0

7.0

8.0

9.0

10.0

Analysis of data:

1. Determine the value of apparent depth, h which is equal to the distance between pin Pi
and the water surface using the formula, h = (H − y).

2. Plot a graph of H against h.

3. Determine the gradient of the graph.

4. Relate real depth, apparent depth and refractive index of water.

5. State the value of refractive index of water.

Conclusion:
What conclusion can be made from this experiment?

Prepare a complete report for this experiment.

Discussion:
1. If water is replaced by cooking oil in this experiment, what will happen to the gradient

of the graph?

2. State one precaution that needs to be taken to improve the accuracy of the results.

3. Discuss the advantage of non-parallax method compared to the method used in
Experiment 6.1 to determine the refractive index of a substance.

Based on the above experiment, the relationship between the refractive index of a
medium, n, real depth, H and apparent depth, h is:

n =
real depth

apparent depth = Hh

239239

Chapter 6 Light and Optics

6.1.66.1.6
Example 1

Figure 6.14 shows a light ray travelling from air into a
plastic block at an angle of 60°. The refractive index of
plastic is 1.49.
Calculate:
(a) angle of refraction, r.
(b) speed of light in plastic.

Solution:

(a) n = sin i
sin r

sin r = sin i
n

= sin 60°
1.49

r = sin–1 �sin 60°
1.49 �

= 35.54°

(b) n = c
v

v = c
n

= 3.0 × 108

1.49
  = 2.01 × 108 m s–1

Figure 6.14

60°

Plastic
Air

r

Example 2

When a coin inside a beaker containing a solution is observed, image of the coin is seen at a
height equal to 2

7  of the depth of the solution. What is the refractive index of the solution?

Solution:

Based on Figure 6.15,

Apparent depth, h = H − 2
7  H

= 5
7  H

Refractive index of solution, n = H
h

=
H

5
7 H

= 7
5

= 1.4
Figure 6.15

h = H –    H
H

Solution

Coin

2–7

2– H7

Image of coin

Solving Problems Involving Refraction of Light

240240 6.1.76.1.7
Formative Practice 6.16.1

1. Figure 6.16 shows the path of light which travels from medium 1 to medium 2.

Medium 2

Medium 175°

55°

Figure 6.16

(a) Write an equation to relate the light that travels through the mediums.
(b) Determine the refractive index for medium 2 if the speed of light in medium 1

is 3.0 × 108 m s–1.
(c) What is the speed of light in medium 2?

2. Figure 6.17 shows a tank fi lled with oil to a height of 3 m. Th e oil has a refractive
index of 1.38. What is the apparent depth of the tank as seen by the observer from
above the tank?

3 m
Oil

Tank

Observer

Figure 6.17

241241

Chapter 6 Light and Optics
Activity 6.3

Total Internal Reflection6.2

Photograph 6.1 Phenomenon of
total internal refl ection

Photograph 6.1 shows a turtle below the
surface of water. A reflected image of the turtle
can be seen on the boundary of water and air.
Why does this happen?
 This phenomenon is known as total
internal reflection of light. Total internal
reflection only occurs when light travels from a
medium of high optical density to a medium of
low optical density.

Aim: To observe the phenomenon of total internal refl ection and determine the critical
angle of glass

Apparatus: Semi-circular glass block, ray box, power supply and protractor

Material: White paper

Instructions:
1. Draw a straight line AB and a perpendicular line PQ on a piece of white paper.
2. Set up the apparatus as shown in Figure 6.18. Point O is the centre of the semi-circular

glass block.

Ray box

White paper

Incident
ray

Refracted
ray

Reflected ray

i

r
O

P

Q

A B

Semi-
circular
glass
block

Figure 6.18

3. Switch on the ray box so that a ray of light is directed along QO. Observe the refracted
ray in air.

4. Shift the ray box so that a ray of light is directed to point O with a small angle of
incidence, i. Observe the refracted ray in air and the refl ected ray in the semi-circular
glass block.

5. Repeat step 4 with an increasingly bigger angle of incidence, i until it is almost 90º.
6. Adjust the position of the ray box so that the refracted ray travels along OB, that is

angle of refraction, r = 90º. Mark the path of the incident ray.

242242 6.2.16.2.1
7. Remove the glass block. Draw the incident ray and measure the angle of incidence. This
angle of incidence is known as the critical angle of glass, c.

8. Place the glass block back to its original position. Observe the refracted ray and
refl ected ray when:
(a) angle of incidence is smaller than the critical angle, and
(b) angle of incidence is larger than the critical angle.

Discussion:

1. What is the critical angle of glass?

2. Describe the propagation of light ray through the glass block when the angle of
incidence is smaller than the critical angle.

3. What happens to the light ray travelling through the glass block when
(a) the angle of incidence is smaller than the critical angle?
(b) the angle of incidence is larger than the critical angle?

1  When the incident angle
is less than the critical
angle, the ray is refracted
away from the normal.
A weak reflected ray can
also be detected.

2  When the incident angle is
the same as the critical angle,
the refracted ray travels along
the glass-air boundary. The
reflected ray appears brighter.

3  When the incident
angle is greater than
the critical angle, there
is no refracted ray.
Total internal reflection
happens.

Figure 6.19 shows light rays travelling from glass (high optical density) to air (low optical
density) for three different angles of incidence.

Figure 6.19 Light ray travelling from glass to air

The phenomenon of total internal reflection happens when light travels from a medium
of high optical density to a medium of low optical density, with the angle of incidence larger
than the critical angle. Critical angle, c is the angle of incidence in the medium of high optical
density when the angle of refraction in the medium of lower optical density is equals to 90º.
Does the critical angle of a medium depend on its refractive index?

(a) Incident angle, i < c (b) Incident angle, i = c (c) Incident angle, i > c

r = 90°Air
Glass

c

r

Stronger
reflected ray

Air
Glass

Weak reflected
ray

Refracted
ray

Normal Normal Normal

Refracted
ray

Air
Glass

Totally
reflected ray

Incident angleIncident angleIncident angle

i i

No ray

243243

Chapter 6 Light and Optics

6.2.16.2.1
Activity 6.4

Aim: To discuss the relationship between critical angle and refractive index

Instructions:
1. Work in pairs.
2. Study Figure 6.20 that shows a light ray travelling from glass to air with an angle of

incidence, i equals to the critical angle, c.
Normal

r = 90°
O

A

B
c

Air
n2

Glass
n1

Laser
pointer

Figure 6.20 Relationship between critical angle and refractive index

3. Discuss and complete the following statements.

(a) Snell’s Law for light ray travelling from glass to air is:

n1 sin θ1 = n2 sin θ2, where n1 = refractive index of
 n2 = refractive index of
 θ1 = angle of incidence in
 θ2 = angle of refraction in

(b) Value θ1 = c, θ2 =  and n2 = .

With this, n1 sin  = n2 sin .

n1 =
1

In general, the relationship between the critical angle, c and the refractive index, n for a
medium in air is n = 1

sin c
. If the refractive index of diamond is 2.42, then the critical angle for

diamond can be calculated as follows:

sin c = 1
2.42

= 0.4132
 c = sin−1 (0.4132)
  = 24.4°

Critical angle for diamond, c is 24.4°.

Critical angle for a medium depends on the optical density of the medium. The higher the
refractive index of a medium, the smaller the critical angle of the medium.

Algorithms

244244 6.2.26.2.2
Natural Phenomena and Applications of Total Internal Reflection in Daily LifeNatural Phenomena and Applications of Total Internal Reflection in Daily Life

Photograph 6.2 Night view of the River of Life and Blue Pool, Jamek Mosque, Kuala Lumpur

The beauty of decorative lamps shown in Photograph 6.2 is due to total internal reflection. Can
you state other phenomena involving total internal reflection?

Activity 6.5

Aim:  To gather information and discuss natural phenomena which involve total internal
refl ection

Instructions:

1. Work in groups.

2. Gather information from various reading resources and websites regarding:
(a) natural phenomena which involve total internal refl ection.
(b) applications of total internal refl ection in daily life.

3. Present your fi ndings in the form of a mind map.

ISS ICS

245245

Chapter 6 Light and Optics

6.2.36.2.3
Activity 6.6

Aim: To observe the phenomenon of total internal refl ection in a water stream
Apparatus: 1.5 litre plastic bottle, plastic basin, laser pointer, wooden block and

retort stand
Materials: Water and cellophane tape

Instructions:

1. Punch a hole at the side of a plastic bottle. Then, seal the hole with cellophane tape.

2. Set up the apparatus as shown in Figure 6.21(a).

Retort
stand Plastic

bottle
Water

Cellophane
tape

Wooden block Plastic basin

Laser
pointer

Retort
stand Plastic

bottle Water

Wooden block Plastic basin

Laser
pointer

(a) (b)

Figure 6.21

3. Carry out this activity in a dark room for clearer
observation.

4. Remove the cellophane tape so that water fl ows out from
the hole into the basin.

5. Direct the laser beam towards the hole as shown in
Figure 6.21(b). Observe the colour of the water stream.

6. Record your observations.

Discussion:
What will happen if the water stream is replaced by an
oil stream?

Based on Activity 6.6, a laser beam which enters the water stream experiences repeated total
internal reflection until it comes out from the end of the water stream. This situation shows that
the water stream acts as a light pipe to guide the laser beam from one end to another end.
 If the water stream is replaced by an oil stream, light beams will experience total internal
reflection even more times. This is because the refractive index of oil is higher than the refractive
index of water. The critical angle of oil is smaller than the critical angle of water.

Note: This activity can
also be carried out using
an optical fibre kit.

Demonstration video on
total internal refl ection

http://bt.sasbadi.com/p4246

246246 6.2.36.2.3
Mirage
On a hot and bright day, a car driver sees the blurry image of a puddle of water on the surface
of the road ahead. When he gets nearer to the puddle of water, he discovers that the puddle of
water does not actually exist. This natural phenomenon is known as mirage which is caused by
refraction and total internal reflection of light. Figure 6.23 shows the process of formation
of mirage.

Figure 6.23 Formation of mirage

Figure 6.22 Formation of rainbow

1

2

3

4

When white light from the Sun enters water
droplets, the light experiences refraction and
disperses into different colours.

All these different colours experience total internal
reflection on the surface of the water droplets.

Light rays reflected experience refraction and
dispersion again when moving from water to air.

Colours of the rainbow are seen by the observer.

Reflected light rays are then gradually refracted towards the normal
and reach the eyes of the observer. The observer will see the image of a
cloud as a puddle of water on the road surface.

1

3

Layers of air above the road have different
optical densities. The layer of air just above the
road surface is hotter than the upper layers.
The layer of hot air has smaller optical density
than cold air.

Road

Observer
Cloud

Image of cloud

Cold air
(denser)

Hot air
(less dense)

1

2
3

Light which travels from the upper layer to the lower
layer are gradually refracted away from the normal.
When the angle of incidence is greater than the critical
angle of air, total internal reflection occurs.

2

OObseeeervr er

Raysys ffrom
the e Sunn

PuPurrpplelee

RedRed

1

2

3

4

Natural Phenomena that Involve Total Internal Reflection

Formation of rainbow
Formation of rainbow is a phenomenon caused by refraction, dispersion and total internal
reflection when light passes through water droplets in the air. Figure 6.22 shows the formation
of rainbow.

247247

Chapter 6 Light and Optics

6.2.36.2.3
Cat’s eye reflector

Used for the purpose
of safety for road users
at night.

Light rays from car
headlights enter the
reflector and experience
total internal reflection on
the back surface of
the reflector.

Applications of Total Internal Reflection in Daily Life

Figure 6.24 Applications of total internal reflection in daily life

BObject

Image
of object

C

D ObserverE

Barrier

AA

FF

Inner core

Outer cladding

eeeeeeeeerrrrrrrrr r ccccc

Inner core has
a high refractive
index

Coating has a low
refractive index

Light signal

Optical fibre

Prism periscope

Used to see objects behind a barrier.
 Made up of two right angle prisms fitted at both ends of a long tube.
 Light rays from an object travel along the normal to the side AB

of the upper prism (pass through the opening of the periscope).
The light rays reach side AC without refraction. The angle of
incidence is 45º and is larger than the critical angle of the prism,
which is 42º. Therefore, total internal reflection happens at side
AC and the light rays are reflected downwards.

The reflected light rays travel downwards along the normal to
side DE of the lower prism.

Once again, the reflected light rays experience total internal
reflection at side DF. Finally, the reflected light rays emerge from
side EF without refraction and enter the eyes of observer. The
image formed is upright and of the same size as the object.

Widely used in the fields of telecommunication
and medicine.

Made from pure glass or plastic fibres.
 Inner core which has a high refractive index is

surrounded by a layer of cladding with a low
refractive index.

Light signals enter one end of the optic fibre and
experience total internal reflection continuously in the
inner core until they reach the other end. As such,
signals can be sent rapidly without disturbances from
electric signals.

Light rays
enter

Light rays
exit

Total internal
refl ection
happens Doctors use endoscope to

examine the internal organs.
Engineers use fi bre optics to
examine the interior of complex
machines. Communication
experts use fi bre optics for
sending data speedily.

CAREER
iNFO

248248 6.2.36.2.3
Solving Problems Involving Total Internal Reflection

Example 1

Example 2

Figure 6.25 shows a light ray travelling from air to a prism with
refractive index of 1.49.
(a) Calculate the critical angle of the prism.
(b) Complete the path of the light ray until it exits the prism.

Solution:

(a) sin c = 1
n

= 1
1.49

c = sin−1� 1
1.49�

= 42.2°

Critical angle of prism, c is 42.2°.

(b)

45°
45°45°

45°
45°

A B

C

Figure 6.26

In Figure 6.26, angle of incidence (i = 45°)
is larger than critical angle (c = 42.2°) at the
sides AB and AC. Total internal reflection
happens and light ray emerges from the
side of BC along its normal.

Figure 6.27 shows the path of light ray travelling through
optical fibre from end A to end B.
(a) Explain the change in direction of the light ray

at points Q, R, S, T and U.
(b) If the refractive index of the optical fibre

is 1.51, determine the value of angle x.
(c) Why must the material of optical fibres be of

high optical purity?

Solution:
(a)  At point Q, refracted light bends towards the normal. At points R, S and T, total internal

reflection happens. At point U, refracted light bends away from normal.
(b) n1 sin θ1 = n2 sin θ2
  1 × sin 45° = 1.51 sin x

sin x = 1 × sin 45°
1.51

= 0.468
  x = sin−1 (0.468)
   = 27.9°
(c)  High purity material allows the critical angle along the fibre optic to be consistent. All

signals which enter the fibre will experience total internal reflection.

45°

45°

Optical fibre

A

Air Air
B

U

TS

R

x
Q

Figure 6.25

Figure 6.27

249249

Chapter 6 Light and Optics

6.2.46.2.4
Formative Practice 6.26.2

1. Figure 6.28 shows the path of light which travels from air to a prism.

45°

45°

A

BC

Figure 6.28

(a) Determine the angle of refraction of light ray in the prism. Th e refractive index of
the prism is 1.50.

(b) Will the light ray experience total internal refl ection at the side AB of the prism?
 Explain your answer.

2.  Figure 6.29 shows a fi ne optical fi bre made up of inner core which has a high refractive
index surrounded by cladding material of a low refractive index.

Critical angle of
the optical fibre

Claddingncladding = 1.40

Inner core

ncore = 1.80

Figure 6.29

(a) Determine the critical angle of the optical fi bre.
(b) What is the advantage of fi ne optical fi bre?

250250
Lens is a piece of transparent material
such as glass, perspex or plastic which
has two surfaces with at least one
surface curved. There are two main
types of lenses, convex lens and concave
lens as shown in Figure 6.30.

Image Formation by Lenses6.3

Activity 6.7

(a) Convex lens  (b) Concave lens

Figure 6.30 Types of lenses

Lenses

http://bt.sasbadi.com/p4251

Aim: To show convex lens as converging lens and concave lens as diverging lens

Apparatus: Convex lens, concave lens, ray box, power supply, three slit plate, pencil
and ruler

Material: White paper

Instructions:
1. Set up the apparatus as shown in Figure 6.31.

Ray box Three slit plate  Cylindrical convex lens

White paper
Path of parallel rays

Figure 6.31

2. Trace the shape of the convex lens on white paper.

3. Direct three parallel rays of light from the ray box towards the convex lens. Observe the
beams of light after passing through the convex lens and record your observations.

4. Make two marks on each path of light before and after passing through the lens.
Remove the convex lens and draw a ray diagram for the paths of light.

5. Measure the distance between the centre of the lens, P and the point of focus, F of the
three paths of light. Record your reading.

6. Repeat steps 2 to 4 by replacing the convex lens with a concave lens.

251251

Chapter 6 Light and Optics

6.3.16.3.1
7. Determine the point, F where the three beams of light appear to diverge.

8. Measure the distance between the centre of the lens, P and point F. Record your reading.

Discussion:

What happens to the three beams of light after passing through:
(a) convex lens   (b) concave lens

The above activity shows that parallel light rays which pass through convex lens will
converge at a focal point, F. Therefore, convex lenses are known as converging lenses. On
the other hand, parallel light rays passing through concave lens appear to be diverging from a
focal point, F. Therefore, concave lenses are known as diverging lenses. Table 6.4 shows the
differences between convex lens and concave lens.

Table 6.4 Differences between convex lens and concave lens

Convex lens Concave lens

Axis of lens

Principal axis
FO

f

F

Axis of lens

Principal axis
O

f

Focal point of convex lens is real because refracted
light rays converge at this point. As such, focal
length, f is said to be positive.

Focal point of concave lens is virtual because
refracted light rays appear to diverge from this
point. As such, focal length, f is said to be negative.

Table 6.5 Explanation of optical terms used

Optical term Explanation

Optical centre, O Point at the centre of the lens. Light rays passing through the optical centre are
not refracted.

Principal axis   Straight line through the optical centre of a lens and the centre of curvature of both
surfaces of the lens.

Axis of lens Straight line through the optical centre and perpendicular to the principal axis.

Focal point, F Point located at the principal axis of a lens.
• For convex lens, light rays parallel to the principal axis will converge at this point

after passing through the lens.
• For concave lens, light rays parallel to the principal axis appear to diverge from

this point after passing through the lens.

Object distance, u Distance between object and optical centre of a lens

Image distance, v Distance between image and optical centre of a lens

Focal length, f Distance between focal point, F and optical centre, O of a lens

252252 6.3.16.3.1
Focal Length for Convex Lens
Convex lenses of different thickness have different focal lengths.

Activity 6.8

Aim: To observe real image and estimate focal length of a convex lens using distant object

Apparatus: Convex lens, metre rule, lens holder and white screen

Instructions:

1. Set up the apparatus as shown in Figure 6.32.

2. Place the convex lens on the lens holder and position it towards an open window.

Window
of laboratory

Convex lens

White screen

Lens holder

Figure 6.32

3. Place the white screen behind the lens and adjust its position closer or further from the
lens until a sharp image is formed on the screen.

4. Measure the focal length of the lens, that is the distance between optical centre of the
convex lens and the screen. Record your reading.

Discussion:

Why is the activity above not suitable to be used to estimate the focal length of a
concave lens?

253253

Chapter 6 Light and Optics

6.3.26.3.2
6.3.36.3.3

Position and Characteristics of Images Formed by Lenses
Ray diagrams can be used to determine the position and characteristics of images formed
by convex lens and concave lens. There are three rays used in the construction of a ray diagram
(See Table 6.6).

Table 6.6 Rays used in the drawing of a ray diagram

Table 6.6 Rays used in the drawing of ray diagramsConvex Lens Concave Lens

1. Ray which passes through the optical centre
travels in a straight line through the optical
centre without being refracted.

O

1. Ray which passes through the optical centre
travels in a straight line through the optical
centre without being refracted.

O

2. Ray which is parallel to the principal axis is
refracted and passes through the focal point, F.

OF F

2. Ray which is parallel to the principal axis
is refracted and appears to come from focal
point, F.

O
F F

3. Ray which passes through the focal point, F is
refracted parallel to the principal axis.

OF F

3. Ray which travels towards focal point, F is
refracted parallel to the principal axis.

O
F F

254254
Activity 6.9

Aim: To determine the position and characteristics of images formed by convex lens and
concave lens for different object distances

Apparatus: Convex lens ( f = 10 cm), concave lens ( f = –10 cm), ray box with transparent
paper marked with an arrow as object, power
supply, lens holder, white screen and metre rule

Material: Graph paper

A  Convex lens

Instructions:

1. Set up the apparatus as shown in Figure 6.33.

White
screen

Ray
box

Convex lens

Transparent paper with
arrow as object

Lens holder

Metre rule

v

u

Figure 6.33

2. Place the convex lens so that its object distance, u = 30.0 cm, that is u > 2f. Adjust the
position of the screen until a sharp image of the arrow is formed on the screen.

3. Observe the image formed on the screen and record its characteristics in a table. You
can download and print the table from the website given.

4. Choose a suitable scale and draw the ray diagram for convex lens of f = 10.0 cm and
u = 30.0 cm on a piece of graph paper. You can scan the QR code given for steps to
draw complete ray diagrams.

5. Determine the position of image and record it in the table downloaded.
6. Repeat steps 2 to 5 for object distance u = 20.0 cm, 15.0 cm and 8.0 cm.

B  Concave lens

Instructions:
1. Hold the concave lens at object distance, u = 20.0 cm, 15.0 cm and 10.0 cm in front of

your eyes and look at the writing in your Physics textbook through the concave lens.
Record the characteristics of the images you see in the table downloaded.

2. Draw the ray diagrams for concave lens of f = –10.0 cm with u = 20.0 cm, 15.0 cm and
10.0 cm on a piece of graph paper.

Discussion:
1. State the positions of convex lens which form real image and virtual image.

2. What are the common characteristics for real images formed by a convex lens?

3. What are the characteristics of images formed by a concave lens?

Video on steps to draw
ray diagram

http://bt.sasbadi.com/p4255b

Download table

http://bt.sasbadi.com/p4255a

255255

Chapter 6 Light and Optics

6.3.36.3.3
Table 6.7 and Table 6.8 show ray diagrams as well as characteristics of images for convex
lens and concave lens respectively.

Table 6.7 Formation of image by convex lens

Object position Ray diagram Image position Characteristics
of image

Object at infinity

I

FF

• Image distance:
v = f

• Image at F

• Real
• Inverted
• Diminished

Object O beyond 2F
(u � 2f )

F

O

I
2F

2FF

• Image distance:
 f � v � 2f
• Image between
 F and 2F

• Real
• Inverted
• Diminished

Object O at 2F
(u = 2f )

F

O

I

2F
2FF

• Image distance:
v = 2f

• Image at 2F

• Real
• Inverted
• Same size as

object

Object O between F
and 2F
( f � u � 2f )

F

O

I

2F
2FF

• Image distance:
v � 2f

• Image beyond 2F

• Real
• Inverted
• Magnified

Object O at F
(u = f )

F2F
2FF

O

•  Image at infinity • Virtual
• Upright
• Magnified

Object O between F
and optical centre
(u � f )

F

O

I

F

• Image distance:
v � u

• Virtual
• Upright
• Magnified

cssss

256256 6.3.36.3.3
Activity

Linear Magnification

Observe Photograph 6.3. When an object is seen
through a magnifying glass at a distance less than
its focal length, the image formed is magnified.
The size of the image formed by a convex lens
depends on the position of the object.

6.10

Photograph 6.3 Image magnified
through a magnifying glass

Table 6.8 Formation of image by concave lens

Object position Ray diagram Image position Characteristics
of image

Object O beyond 2F
(u � 2f )

F2F

O
I

• Between optical
centre and focal
point

• Image distance:
v � f

• Virtual
• Upright
• Diminished

Object O between F
and optical centre
(u � f ) F2F

O

I

• Between optical
centre and focal
point

• Image distance:
v � f

• Virtual
• Upright
• Diminished

Aim: To generate ideas on magnifi cation of image with the help of ray diagrams

Instructions:
1.  Work in pairs.
2.  Visit the websites given on the simulation of magnifi cation using convex lens.
3.  Choose a suitable object height. Begin with this object

at a position far from the convex lens.
4.  Move the object slowly closer to the lens. Observe how

the position and size of the image change.
5.  Sketch ray diagrams for the following situations:

(a) Image size smaller than object size.
(b)  Image size larger than object size.

Discussion:

1. Which position of the convex lens forms
(a) a magnifi ed image?
(b) a diminished image?

2. What is the relationship between image height, object
height, image distance and object distance?

Simulation of magnifi cation
of image

http://bt.sasbadi.com/p4257a

http://bt.sasbadi.com/p4257b

ICS

257257

Chapter 6 Light and Optics

6.3.36.3.3 6.3.46.3.4
Formative Practice 6.36.3

Figure 6.35 shows an image seen through a convex lens with focal
length of 10 cm.

1. What are the characteristics of the image?

2. Draw a ray diagram to show how the image in the fi gure
is formed.

3. Suggest a suitable position to place the object to produce an
inverted image.

12

13

19

23

21

7
15

18

Activity 6.10 shows that image size formed by a lens depends on the position of the object
from the centre of the lens. Comparison between image size and object size is made based on the
ratio of image height to object height. Th is ratio is known as linear magnifi cation, m.

Linear magnifi cation, m =
image height, hi

object height, ho

Object

hi

Image2F 2FF F

u v

ho

Figure 6.34 Formation of image by convex lens

Based on Figure 6.34, ratio of image height to object height
is also the same as the ratio of image distance to object distance.

Linear magnifi cation, m = image distance, v
object distance, u

As such, linear magnifi cation can be formulated as,

m =
hi
ho

= v
u , where hi = image height

ho = object height
 v = image distance
 u = object distance

Figure 6.35

F ile
Linear magnifi cation, m does
not have a unit.

m � 1 Image diminished
m = 1 Image same size

as object
m � 1 Image magnifi ed

258258 6.3.46.3.4
Thin Lens Formula6.4

You have studied the use of ray diagrams to determine the position and characteristics of images
formed by convex lens and concave lens. Other than ray diagrams, thin lens formula can be used
to solve problems regarding lenses.

Th in lens formula gives the relationship between object distance, u, image
distance, v and focal length, f for a lens as:

1
f  = 1

u  + 1
v

(a)  Position of camera lens close to object  (b) Position of camera lens far from object
 produces a large image produces a small image

Photograph 6.4 Position of camera lens from object

Photograph 6.4 shows the images for diff erent object distances with a camera lens of the
same focal length.

Inference: Image distance is infl uenced by object distance

Hypothesis: When the object distance increases, the image distance decreases

Aim: (i) To study the relationship between object distance, u and image distance, v for a
convex lens

(ii) To determine the focal length of a thin lens using lens formula

Variables:
(a) Manipulated variable: Object distance, u
(b) Responding variable: Image distance, v
(c) Constant variable: Focal length, f
Apparatus:  Convex lens ( f = 10.0 cm), lens holder, 6 V bulb, a wooden block, power

supply, cardboard with a small triangular shaped hole, white screen and
metre rule

Experiment 6.3

259259

Chapter 6 Light and Optics

6.4.16.4.1
Procedure:
1. Set up the apparatus as shown in Figure 6.36.

ObjectBulb

Convex
lens

Cardboard with
triangular
shaped hole

Metre rule

u

v

White
screen

Wooden
block

Figure 6.36

2. Switch on the bulb and start the experiment with object
distance, u = 15.0 cm. Adjust the position of the screen
until a sharp image is formed.

3. Measure the image distance, v and record the reading in
Table 6.9.

4. Repeat steps 2 and 3 with object distance, u = 20.0 cm,
25.0 cm, 30.0 cm and 35.0 cm. Record the readings in
Table 6.9.

Results:

Table 6.9

u / cm v / cm 1
u  / cm−1 1

v  / cm−1

15.0
20.0
25.0
30.0
35.0

Analysis of data:

1. Plot a graph of 1
v

against 1
u

.

2. Determine the gradient of the graph, m.

3. Determine the intercepts for both axes.

4. Using lens formula and the graph plotted, determine the focal length of the lens in
this experiment.

Conclusion:
What conclusion can be made from this experiment?

Prepare a complete report for this experiment.

Discussion:
State a precaution to be taken to increase the accuracy of the results of this experiment.

Note: Th is experiment
should be carried out in a
dark room.

Video demonstration of
non-parallax method

http://bt.sasbadi.com/p4260

260260 6.4.16.4.1
Solving Problems Involving Thin Lens Formula

Table 6.10 Sign convention used in thin lens formula

Positive (+) Negative (−)

Focal length, f Converging lens or convex lens Diverging lens or concave lens

Image distance, v • Real image
• On the opposite side of the object

• Virtual image
• On the same side as the object

Example 1

A thin convex lens has a focal length of 12 cm. Determine the characteristics and the position
of the images formed and linear magnifi cation when the object distance is:
(a) 18 cm
(b) 4 cm

Solution:

(a)  u = +18 cm
 f = +12 cm

Th in lens formula, 1
f  = 1

u  + 1
v

1
v  = 1

f  − 1
u

= 1
12 − 1

18
   v = +36 cm

Linear magnifi cation, m = v
u

= 36
18

= 2

Image is real, inverted and magnifi ed.
Image is located 36 cm from the lens and
on the opposite side of the object. Image
is magnifi ed 2 times.

(b)  u = +4 cm
 f = +12 cm

Th in lens formula, 1
f  = 1

u  + 1
v

1
v  = 1

f  − 1
u

= 1
12 − 1

4
   v = –6 cm

Linear magnifi cation, m = v
u

= 6
4

= 1.5

Image is virtual, upright and magnifi ed.
Image is located 6 cm from the lens and
on the same side as the object. Image is
magnifi ed 1.5 times.

261261

Chapter 6 Light and Optics

6.4.26.4.2
Formative Practice 6.46.4

1. A concave lens with a focal length of 25 cm forms a virtual image of an ant at a distance
of 20 cm from the optical centre of the lens.
(a) What is the original position of the ant?

(b) Draw a ray diagram to show the formation of a virtual image of the ant.

2. A small bulb is at a distance of 1.6 m from the screen and a thin convex lens with a focal
length of 30 cm is placed between the bulb and the screen. Determine two positions of the
convex lens that can produce a sharp image on the screen.

Example 2

Example 3

An object with a height of 9 cm is placed 60 cm from a concave lens with a focal length of
30  cm. Determine the position and size of the image formed. State the characteristics of the
image formed.

Solution:

u = +60 cm
f = −30 cm

Th in lens formula, 1
f  = 1

u  + 1
v

1
v  = 1

f  − 1
u

= 1
–30 − 1

60
v = –20 cm

Linear magnifi cation, m = hi
ho

= v
u

hi
9  = 20

60

hi = 3 cm
Image is virtual, upright and diminished.
Image is 20 cm from the lens and is on
the same side as the object. Image height
is 3 cm.

Figure 6.37 shows a straight wire placed along the principal
axis of a thin convex lens with a focal length of 12 cm. X
and Y are 24 cm and 18 cm respectively from the lens. A
cricket takes 6 seconds to move from X to Y. What is the
speed of the cricket’s image?

Solution:

u1 = +18 cm u2 = +24 cm f = +12 cm

Th in lens formula, 1
f  = 1

u  + 1
v

1
v1

= 1
f  − 1

u1

= 1
12 − 1

18
   v1 = +36 cm

1
v2

= 1
f

– 1
u2

= 1
12

– 1
24

v2 = +24 cm

Speed of the cricket’s image = 36 – 24
6

= 2 cm s–1

Convex
lens

Cricket

Straight wire
YX

Figure 6.37

262262 6.4.26.4.2
Activity 6.12

Aim: To justify the usage of lenses in optical instruments

Apparatus: Magnifying glass, compound microscope and telescope

Instructions:

1. Place the magnifying glass, compound
microscope and telescope on separate tables.

2. Divide the class into three groups. Each
group is given 20 minutes to observe objects
through the optical instruments and study the
structure of the optical instruments.

3. Record the fi ndings.

Discussion:

1. State the characteristics of the images formed by
the lenses used in magnifying glass, compound
microscope and telescope.

2. Justify the usage of lenses in magnifying glass,
compound microscope and telescope.

Photograph 6.5

(a) Magnifying glass

(b) Microscope

(c) Telescope

ISS ICS

Activity 6.11

6.5.16.5.1

Optical Instruments6.5

Use of Lenses in Optical Instruments
Lenses in optical instruments have many benefi ts in our daily lives.

Aim: To study the use of lenses in optical instruments

Instructions:

1. Work in groups.

2. Gather information from reading resources or websites
on the use of lenses in optical instruments, such as
magnifying glass, compound microscope and telescope
in the following aspects:
(a) Use of lenses in the optical instrument
(b) Function of the lenses used

3. Present your fi ndings.

Use of lenses in
optical instruments

http://bt.sasbadi.com/p4263

263263

Chapter 6 Light and Optics
Figure 6.38 shows the uses of lenses in optical instruments such as magnifying glass,
compound microscope and telescope.

HISTORY
INTEGRATION OF

In the mid-17th century, Antonie
van Leeuwenhoek (1632–1723)
successfully invented the
single-lensed microscope that
can make linear magnifi cations
of 300 times. He succeeded
in observing and drawing
microorganisms.

HISTORY
INTEGRATION OF

In 1609, Galileo Galilei (1564–
1642) invented the telescope
to observe the four Moons
which orbited Jupiter. This
success triggered a revolution
in astronomical studies.

Figure 6.38 Figure 6.38 Uses of lenses in optical instrumentsUses of lenses in optical instruments

I am a microbiologist. I use
a microscope to examine
various microorganisms.

I am a gemologist. I use a
magnifying glass to identify
and evaluate gemstones.

I am an ophthalmologist.
I use a magnifying glass to
examine eyes.

I am an astronomer. I use
a telescope to study
celestial objects.

I am a geologist. I use a
microscope to study and
identify specimens of rocks
and minerals.

Video on the discovery of
image of black hole using
Event Horizon Telescope

http://bt.sasbadi.com/p4264

264264 6.5.16.5.1
Activity 6.13

Designing and Building Compound Microscopes and Telescopes Using Convex Lenses

Aim:  To design and build a compound microscope and a telescope using convex lenses

Instructions:

1. Work in groups.

2. Gather information on compound microscope and
telescope from reading resources or websites based on
the following:
(a) Type of lens used and its function
(b) Criteria for choosing objective lens and eyepiece lens

of a compound microscope which can produce the
largest image

(c) Criteria for choosing objective lens and eyepiece lens
of a telescope which can produce the clearest and
brightest image

(d) Draw ray diagrams to show the formation of images
in compound microscope and telescope

3. Discuss the information and complete the K-W-L Chart as
a guide to design and build your compound microscope
and telescope. You can download and print the form
from the website given.

4. Sketch the design of your compound microscope and
telescope.

5. Build your compound microscope and telescope based
on your sketch.

6. Comment on the effectiveness of the design and improve
on the design produced.

7. Present your group’s design, and the compound
microscope and telescope built.

Download K-W-L Chart

http://bt.sasbadi.com/p4265c

Video on the working
principle of telescope

http://bt.sasbadi.com/p4265b

Video on the working
principle of microscope

http://bt.sasbadi.com/p4265a

ISS ICS

265265

Chapter 6 Light and Optics

6.5.26.5.2
Compound Microscope

Figure 6.39 Formation of image by a compound microscope

Objective lens
(L1)

Eyepiece lens
(L2)

Final image

Observer

Fo

Fo

I2

I1

Fe

O

Fe

Made up of two convex lenses with short focal lengths.
Objective lens has focal length, fo and eyepiece lens has
focal length, fe. Focal length fo is less than focal length fe.

Distance between objective lens and eyepiece lens > fo + fe.

Object distance is between fo and 2fo. Objective lens forms
the fi rst image, I1 which is real, inverted and magnifi ed. I1
lies between Fe and the optical centre of the eyepiece lens
and becomes the object for the eyepiece lens.

Eyepiece lens functions as a magnifying glass. Eyepiece lens
forms the fi nal image, I2 which is virtual, magnifi ed and
inverted compared to object O (Figure 6.39).

Normally, eyepiece lens is adjusted so that the fi nal image,
I2 is at the near point of the observer’s eye to achieve the
clearest vision.

Magnifi cation of compound
microscope

http://bt.sasbadi.com/p4266

Normal adjustment of a
compound microscope can be
done by adjusting the eyepiece
lens so that the fi nal image is
formed at the near point of the
eye, which is 25 cm away.

F ile

266266 6.5.26.5.2
Telescope

Figure 6.40 Formation of image by a telescope

Objective lens
(L1)

Eyepiece lens
(L2)

Observer

I1

fo fe

Fo , Fe

Virtual image
at infinity

Made up of two convex lenses. Objective lens has a long
focal length, fo and eyepiece lens has a short focal length,
fe. Distance between objective lens and eyepiece lens is
fo + fe.

Parallel rays from a distant object can be focused at the
focal plane of the objective lens to form the fi rst image,
I1 which is real, inverted and diminished. Th e fi rst image,
I1 acts as the object for the eyepiece lens. Eyepiece lens
forms the fi nal image, I2 which is virtual, magnifi ed and
inverted compared to the object (Figure 6.40).

Normally, image I2 is located at infi nity. Th is is known as
normal adjustment.

Normal adjustment of telescope
can be done by adjusting the
distance between the objective
lens and the eyepiece lens,
L = fo + fe. This enables the fi nal
image to be formed at infi nity for
a comfortable vision.

Magnifi cation of telescope at
normal adjustment, M is

M =
fo
fe

.

F ile

267267

Chapter 6 Light and Optics

6.5.26.5.2
Applications of Small Lenses in Optical Instrument Technology
Th e advancement of technology in optical instruments has successfully produced small lenses.
Th ese lenses are widely used in smartphone cameras and closed circuit television (CCTV).

Activity 6.14

Aim: To discuss the application of small lenses in optical instruments

Instructions:

1. Work in groups.

2. Gather information on the application of small lenses in smartphone cameras and CCTV
based on the following aspects:
(a) Use of small lenses in optical instruments
(b) Function of the small lenses

Photograph 6.6 Lenses in a
smartphone camera

Photograph 6.7 Small size CCTV

3. Present your fi ndings in graphic form.

Small lenses in smartphones

http://bt.sasbadi.com/p4268

Normally, smartphones and CCTV have cameras of high resolution to capture pictures and
record videos clearly. As such, lens is the most important component in smartphone cameras and
CCTV. Th in smartphones and small size CCTV have small size convex lens. Th is lens can form an
image that is real, diminished and inverted at the sensor. Minimum distance between the sensor
and the centre of the lens has to be the same as the focal length of the camera lens as shown in
Figure 6.41. As the focal length of the camera lens cannot be of zero value, the overall thickness of
a smartphone and CCTV is limited to the focal length of the camera lens.

Object distance Focal length

Lens
Field of
vision Size of

sensor

Figure 6.41 Formation of image by small lens in smartphone camera and CCTV

ISS ICS

268268 6.5.36.5.3
Formative Practice 6.56.5

A pupil is supplied with an eyepiece lens which has focal length, fe = 7 cm and four pieces of
objective lenses A, B, C and D as shown in Table 6.11.

Table 6.11

Lens Focal length of objective lens,
fo / cm

Magnifi cation of
telescope, M

Diameter of objective
lens / cm

A 14 5.0

B 14 10.0

C 70 5.0

D 70 10.0

1. Complete Table 6.11.

2. State two lenses which can produce the largest image.

3. State two lenses which can produce the brightest image.

4. Based on your answers in 2 and 3, state the most suitable lens to be used as the objective
lens of a telescope. Explain your answer.

Activity 6.15

Aim: To discuss the limitation to the thickness of a smartphone due to the thickness of
camera lens

Instructions:

1. Work in groups.

2. Discuss the limitation to the thickness of a smartphone due to the thickness of camera
lens.

3. You can obtain information through reading resources or websites.

4. Present your fi ndings.

Logical Reasoning ISS ICS

269269

Chapter 6 Light and Optics

6.5.36.5.3
Image Formation by Spherical Mirrors6.6

Spherical mirrors are part of a sphere that has been cut as shown in Figure 6.42. If the inner surface
of the part that has been cut refl ects light, the mirror is a concave mirror. If the outer surface of
the part that has been cut refl ects light, the mirror is a convex mirror.

Object

Surface that
reflects light

Object

Surface that
reflects light

(a) Concave mirror (b) Convex mirror

Figure 6.42 Spherical mirror

Look at Photograph 6.8. Concave surface and convex
surface of a steel spoon act as concave mirror and convex mirror
respectively. Can you state the characteristics of the images
formed by concave surface and convex surface of the spoon?

i i i i

i i i

C

i

i i i i

Observer

Observer

Wide field of visionNarrow field of vision Very narrow field of vision

Observer

Plane mirror Convex
mirror

Concave mirror
C

Figure 6.43 Fields of vision in front of plane mirror, concave mirror and convex mirror

Figure 6.43 shows the fi elds of vision of an observer in front of a plane mirror, concave
mirror and convex mirror of the same size.

Photograph 6.8 Images formed by
surface of spoon

Concave
surface

Convex
surface

Formation of image by
spherical mirrors

http://bt.sasbadi.com/p4270

Spherical
mirror

Spherical
mirror

270270 6.6.16.6.1
Activity 6.16

Aim: To gather information on optical terms related to spherical mirrors

Instructions:
1. Work in groups.
2. Gather information from various reading resources and websites regarding the following

terms:

• principal axis • focal point, F
• object distance, u  • image distance, v
• focal length, f • centre of curvature, C
• radius of curvature of mirror, r

3. Present your fi ndings.

Figure 6.44 shows the optical terms as used in spherical mirror ray diagrams. Table 6.12
explains these optical terms.

Convex mirror

P F C

f
r

Concave mirror

PFC

f
r

Principal
axis

Principal
axis

Figure 6.44 Ray diagrams of spherical mirrors

Table 6.12 Explanation of optical terms related to spherical mirrors

Optical Term Explanation

Principal axis Straight line passing through the centre of curvature, C and pole of the spherical
mirror, P

Centre of curvature, C Centre of sphere which produces a concave or convex mirror

Radius of curvature of
mirror, r

Distance between the pole of spherical mirror, P and the centre of curvature,
C

Focal point, F A point on the principal axis of the spherical mirror,
• for concave mirror, light rays which are parallel to the principal axis will

converge at this point
• for convex mirror, light rays which are parallel to the principal axis appear

to diverge from this point

Object distance, u Distance between object and the pole of spherical mirror, P

Image distance, v Distance between image and the pole of spherical mirror, P

Focal length, f Distance between focal point, F and the pole of spherical mirror, P

ICS

271271

Chapter 6 Light and Optics

6.6.16.6.1
Guide to Drawing Spherical Mirror Ray Diagrams
Look at the guide in Table 6.13 to draw ray diagrams of spherical mirrors.

Table 6.13 Guide on drawing ray diagrams of spherical mirrors

Concave mirror Convex mirror

1. Light ray passing through C is refl ected back
to its original path.

FC P
Principal axis

1. Light ray travelling towards C is refl ected back
to its original path

F C
Principal axis

P

2. Light ray parallel to principal axis is refl ected
to the focal point, F.

FC P
Principal axis

2. Light ray parallel to principal axis is refl ected
as if it originates from the focal point, F.

F C
Principal axis

P

3. Light ray passing through F is refl ected parallel
to the principal axis.

FC P
Principal axis

3. Light ray travelling towards F is refl ected
parallel to the principal axis.

F C
Principal axis

P

INFOSMARTSMART

Radius of curvature of
mirror, r is two times the
focal length of the spherical
mirror, f, that is r = 2f.

272272 6.6.16.6.1
Activity 6.17

Aim:   To draw ray diagrams to show the image position and
determine the characteristics of images formed by a
concave mirror and a convex mirror

Instructions:

1. Work in groups.

2. Visit the websites given and carry out the simulation in
the websites.

3. Based on the simulation, complete Table 6.14 and Table
6.15. You can download and print the tables from the
websites given.

4. Draw ray diagrams to show the image position and state
the characteristics of images formed by concave mirror
and convex mirror.

Table 6.14 Position and characteristics of images by a concave mirror

Object position Ray diagram Image position Characteristics of image

Object at infi nity

Object beyond C
(u � 2f )

Object at C
(u = 2f )

Object between F
and C
( f � u � 2f )

Object at F
(u = f )

Object between F
and P
(u � f )

Table 6.15 Position and characteristics of images by a convex mirror

Object position Ray diagram Image position Characteristics of image

Object beyond F
(u > f )

Object between F
and P
(u < f )

Simulation of images for
concave mirror and
convex mirror

http://bt.sasbadi.com/p4273a

Download Table 6.14 and
Table 6.15

http://bt.sasbadi.com/p4273b

ICS

273273

Chapter 6 Light and Optics

6.6.16.6.1
Table 6.16 Position and characteristics of images by a concave mirror

Object position Ray diagram Image position Characteristics
of image

Object at infi nity

F

I

C P

• Image distance:
 v = f
• In front of mirror

• Real
• Inverted
• Diminished

Object beyond C
(u � 2f )

I
C

O
F P

• Image distance:
f � v � 2f

• In front of mirror

• Real
• Inverted
• Diminished

Object at C
(u = 2f )

F
I

C

O
P

• Image distance:
 v = 2f
• In front of mirror

• Real
• Inverted
• Same size as object

Object between F
and C
( f � u � 2f )

F
I

C
O

P

• Image distance:
 v � 2f
• In front of mirror

• Real
• Inverted
• Magnifi ed

Object at F
(u = f )

F P
C

O
• Image at infi nity
• Behind mirror

• Virtual
• Upright
• Magnifi ed

Object between F
and P
(u � f )

F P
C

O
I • Image distance:

v � u
• Behind mirror

• Virtual
• Upright
• Magnifi ed

Table 6.17 Position and characteristics of images by a convex mirror

Object position Ray diagram Image position Characteristics
of image

Object beyond F
(u � f ) I

C

O

F' P F

• Image distance:
v � f

• Behind mirror

• Virtual
• Upright
• Diminished

Object between F
and P
(u � f )

I

C

O

F' FP

• Image distance:
v � f

• Behind mirror

• Virtual
• Upright
• Diminished

274274 6.6.16.6.1
Photograph 6.9
Blind spot mirror

Applications of Concave Mirror and Convex
Mirror in Daily Life
Photograph 6.9 shows a blind spot mirror. Th is
mirror is a convex mirror. What is the use of
this mirror?

Activity 6.18

Aim: To gather information to justify the use of concave mirror and convex mirror in daily life

Instructions:
1. Work in groups.

2. You can gather information from reading resources in the library or the websites
regarding:
(a) the use of concave mirror and convex mirror in daily life.
(b) the importance of the mirrors.

3. Present your fi ndings in a mind map.

Figure 6.45 Applications of concave mirrors in daily life

Cahaya dan Optik

Dental mirror
A dental mirror forms an
upright and magnifi ed
image to examine the
teeth.

Applications of Concave Mirrors in Daily Life

Refl ector in car headlight
A parabolic concave mirror is used as a
refl ector in car headlights to maintain light
intensity even at a distance.

Cosmetic mirror
A concave mirror is used
as a cosmetic mirror to
produce a magnifi ed
image for applying
make up.

ISS ICS

275275

Chapter 6 Light and Optics

6.6.26.6.2
Formative Practice 6.66.6

Figure 6.46 Applications of convex mirrors in daily life

1. Figure 6.47 shows a pupil looking in the direction of a
plane mirror and a convex mirror of the same size.
(a) Complete the path of light for both types

of mirrors.
(b) Which type of mirror can produce a wider fi eld

of vision?

2. Adelia holds a shiny steel spoon with its back (convex surface) facing her eyes at a distance
of approximately 30 cm. She sees an upright image of herself.  However, when the spoon
is changed so that the front (concave surface) of the spoon is facing her eyes, an inverted
image is observed.
(a) Explain this situation.
(b) Why is an upright image not seen on the front surface of the spoon at that distance?

Applications of Convex Mirrors in Daily Life

Vehicle rear mirror
Vehicle rear mirrors provide a wide fi eld of
vision to enable the driver to see vehicles
coming from behind.

Security mirror in
buildings
Convex mirrors are used
in buildings or shopping
centres for surveillance
purposes.

Blind spot mirror
A convex mirror is
placed at sharp corners
to widen the fi eld of
vision of the driver.

C
Normal

Plane mirror

Pupil Pupil

Convex
mirror

Figure 6.47

276276 6.6.26.6.2
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42
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277277

Chapter 6 Light and Optics
SELF REFLECTIONSELF-REFLECTION

Performance EvaluationPerformance Evaluation

1. New things I learnt in this chapter on light and optics are ✎ .

2. Th e most interesting thing I learnt in the chapter on light and optics is ✎ .

3. Th ings I still do not fully understand or comprehend are ✎ .

4. My performance in this chapter ,

Poor  1 2 3 4 5   Excellent

5. I need to ✎  to improve my performance in
this chapter.

1. Diamond is a precious stone that always appears shiny. Critical angle of diamond in air
is 24°.
(a) (i) What is the meaning of critical angle?

(ii) Determine the value of refractive index of diamond.
(b) Figure 1 shows three diamonds of diff erent shapes. A ray enters each diamond as

shown in Figure 1. Complete the path of light rays until the ray emerges again into air
in Figure 1.

P  Q  R

70°
45°

35°

Figure 1

(c) State the physics concepts involved in this phenomenon.

Download and print
Self-refl ection Chapter 6

http://bt.sasbadi.com/p4278

278278
2. Figure 2 shows a glass block with a refractive index of 1.50 placed between the eyes of the
observer, E and the object P. If the thickness of the glass is 30.0 cm, what is the distance
between object P and its image?

E

30 cm

Object P Observer

n = 1.50

Figure 2

3. Figure 3 shows a light ray travelling from air to water
and then entering a glass block. Th e refractive index of
water is 1.33.
(a) Determine angle x.
(b) If speed of light in air is 3.0 × 108 m s−1, what is the

speed of light in water?
(c) Between water and glass, which medium has a higher

optical density? Explain your answer based on
Figure 3.

4. Mariam carried out an experiment with a semi-circular glass block and a ray box. Figure
4 shows the path of light ray entering the glass block at point R and travelling towards the
centre of the semi-circular glass block, point S.

Ray box

45°

S

R Semi-circular
glass block

Figure 4

(a) Why does the light ray not change direction at point R?
(b) If the refractive index of the glass block is 1.52, determine the critical angle in this

medium.
(c) Draw the path of light ray aft er point S and mark the value of the angle of the light

ray with the normal at point S.

Figure 3

Air

Water

Glass

55°

x

279279

Chapter 6 Light and Optics
5. When light from a star travels into the Earth’s
atmosphere, its direction of travel will change.
Th is situation is shown in Figure 5. Th e change of
direction is represented by the angle
Δθ = i − r.
(a) Speed of light in air is 299 910 km s−1 and

speed of light in vacuum is 3.00 × 108 m s−1.
(i) Calculate the refractive index of air.
(ii) Explain the value of refractive index

obtained.
(b) Value of Δθ on a hot night is diff erent from

that on a cold night. State a logical reason for
the diff erence.

(c) Rajiv returns from school in a school van on
a hot and bright day. Rajiv can see a puddle of
water on the surface of the road ahead. When
the van reaches the location of the puddle of
water, Rajiv discovers that the puddle of water
does not actually exist. Explain this phenomenon.

6. Figure 6 shows an object and its virtual image formed by a convex lens.

I

Principal axis
Virtual image Object

Figure 6

(a) One of the characteristics of image I in Figure 6 is that it is virtual. State the other
characteristics of image I.

(b) Complete the ray diagram in Figure 6 and determine the position of the lens and
focal point of the lens. Mark the position of the focal point of the lens with, F.

(c) If the object is slowly moved away from the lens, state two changes that might happen
to the image without drawing a ray diagram.

7. A sailor in the navy is looking at the situation on the surface of the sea through a submarine
periscope. He found that the Sun is setting. Th e captain of the submarine told the sailor that
the Sun had in fact already set.
(a) Is the statement of the captain of the submarine true? Explain your answer.
(b) Explain the formation of image in a prism periscope for object with obstruction in

front with the help of suitable ray diagrams.

r

i

Figure 5

280280
8. A lighted candle is placed in front of a concave mirror with a focal length of 2.4 cm. A
white screen is moved behind the candle to catch a sharp image as shown in Figure 7.

Principal axis

Screen

Image of
candle

Concave mirror

Figure 7
(a) On Figure 7,

(i) mark the focal point of the mirror with F and centre of curvature
with C.

(ii) draw a complete ray diagram to determine the position of the object.
(b) How can the magnifi cation of the image be increased?

9. (a)  Explain the way to construct a compound microscope using two lenses. In your
answer, state the type of lens that you chose, estimate the focal length of the lenses and
characteristics of the image formed by each lens.

(b) Why is an astronomical telescope not suitable to be used to see distant objects on the
surface of the Earth?

(c) How can you modify a compound microscope to become an astronomical
telescope?

10. Table 1 shows the speed of light in vacuum and two materials for making optical fi bre.

Table 1

Medium Speed of light / m s–1

Vacuum 3.00 × 108

Material I 2.01 × 108

Material II 1.96 × 108

(a) Identify suitable mediums to be used as core and cladding of optical fi bre. Explain
your answer.

(b) Determine the critical angle of the optical fi bre.
(c) Why must the surface of optical fi bre be very smooth?

281281

Chapter 6 Light and Optics
Enrichment Corner
Enrichment Corner

11. Amin carries out an experiment to investigate
the relationship between real depth, H and
apparent depth, h for an object in a liquid. Th e
apparatus set up is shown in Figure 8. Pin A
is placed at the base of a tall beaker. Liquid is
poured into the beaker until pin A is at a depth
of 5.0 cm. Th e real depth, H for pin A is the
distance of the pin from the surface of the liquid.

Another pin, pin B is adjusted until the
image of pin B in the plane mirror is in line
with the image of pin A when observed from
above as shown in Figure 9.

Apparent depth, h for pin A is the same
as the distance between the image of pin B
and the surface of the liquid. Distance x, can
be determined by measuring the distance
between pin B and the plane mirror. Th e
distance from the surface of the liquid to the
plane mirror, z is also measured.

Th is procedure is repeated for real depth
of liquid, H = 10.0 cm, 15.0 cm, 20.0 cm,
25.0 cm and 30.0 cm. All readings are recorded
in Table 2.

Table 2

H / cm x / cm z / cm h / cm

5.0 30.8 27.0

10.0 29.5 22.0

15.0 28.3 17.0

20.0 27.0 12.0

25.0 25.8 7.0

30.0 24.6 2.0

(a) Based on the results of this experiment, determine the relationship between h and H
and then deduce the value of the refractive index of the liquid.

(b) Draw suitable ray diagrams regarding the formation of images that can be seen
by Amin.

(c) Discuss the importance of plane mirror and non-parallax method in
this experiment.

Observer Cork

Image
of
pin A
Image
of
pin B
Pin A

Pin B

Real
depth

x

H

x

z

Retort
stand

Plane
mirror

Tall
beaker

Image of pin A

Water

Image of pin B
in plane mirror

Plane mirror

Figure 8

Figure 9

282282
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Buku Teks Prinsip Perakaunan T5 MPEI.pdf


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