EXPLORE
OPTICSOSA ACTIVITY GUIDE

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Now lift your left hand, and wave at yourself. Which hand is your reflection waving with? You’ll see your reflec-

REFLECTION WITH REFLECT VIEW

tion is waving with its right hand. The virtual image is inverted, or swapped around - left becomes right!

Reflections are everywhere. When you look at yourself in the mirror,
or in a big window, you’re observing the wonder of reflection! So
how do reflections work? Do they have rules?

Reflection
Light travels in straight lines. It moves freely through the open air,
but when it comes across an obstacle, it can’t move around it. So
what does it do
A lot of the time, the light bounces off the object. This is calledreflection, and it’s how we see! Take a look at Figure 1. When
Figure 2: Properties of a Virtual Imagelight from the sun or your overhead light bounces off an object
and into your eye, your eye “sees” the object. Have you everbeen in a room so dark you can’t see your own hand in front of
your face? This is because without light, your eye can’t tell an
object is there!
When light from an object bounces off a reflective surface like amirror or a window, you see the object on the other side of theglass. This reflection is called a virtual image. Virtual means fakeor appearing to be real. This is because the object is not really
there - when you look in a mirror, the reflection makes it look likeyou’re standing in front of yourself, but you’re not really there!
Figure 1: How We See

Virtual Images
The virtual images created by a flat mirror or piece of glass follow some very strict rules, as shown in Figure 2. Take a look at yourself in the mirror. Notice how far from the mirror you’re standing. How far does
your reflection seem to be standing from the mirror, on the other side?
You should notice that your reflection is the same distance from the mirror as you are! This is because avirtual image is always the same distance from a flat mirror as the real object. Your virtual image is also thesame size as you are.

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ACTIVITY SHEETACTIVITY SHEET

Reflection with Reflect View

Reflection with Reflect View

We’re going to use the Reflect View to reflect pictures and words, and see for ourselves how virtual images work!
1. Write your name in the blank slot on the worksheet.
2. Place the Reflect View on the dotted line. Make sure the stand is facing upwards and the bevelled (stepped) edge istowards to.
3. Lean slightly to the left, until the star on the right matches up with the reflection of the one of the left.
4. Reach your arm around to the right hand side and trace the reflections you see on the Reflect View onto the paper.
What do you notice about the words/images on the right hand side? Measure the distance between eachmatching word or picture and the dotted line. What does this tell us about virtual images?
____________________________________________________________________________________________________________________________________________________________________________________________________
The words/images on the right hand side are the same size as those on the left, but they are inverted. They arealso the same distance from the dotted line as those on the left.
5. Fold the paper in half along the dotted line. Holding the folded paper up, shine the light of the Lazr finger
against the back of the paper, towards you. You should see both halves of the paper at once now.
What do you notice about the words/images on both halves? Do they match up? Don’t they?
____________________________________________________________________________________________________________________________________________________________________________________________________
The words/images match up. This confirms what we learnt above about virtual images formed by flat mirrors!
6. Unfold the paper and stand in front of the mirror, holding the paper in front of you. Observe the worksheet’s
reflection in the mirror.
What do you notice about the words on the right hand side/traced side?
____________________________________________________________________________________________________________________________________________________________________________________________________In the mirror, the words on the right hand side are the right way around, and those on the left are backwards. The

reflection of the right hand side is inverted, so it has turned it the right way around again!

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Figure 2: How objects have color

COLOR WITH M&M’S

Absorption

Color is everywhere. Just this morning, you had to choose what col-sunlight, like in Figure 2. We now know that it looks redor shirt or shoes you were going to wear. So what gives an object

because the red wavelength in the white light is reflecting off

its color? Why are some things red and others blue?

the ball and into our eye.

Let’s go back to the red ball. Say we take a look at it under

But wait - we said white light is made up of all the colors ofColor Starts With Light

light. So what happened to the rest of the light? Where did

To understand why objects have color, we need to understandthe blue wavelength of light go, for example?

where light gets its color.

The left over light that wasn’t reflected off the ball was

Light is a wave, and so has a wavelength. Take a look at Figure 1.

trapped inside the ball. This is why it couldn’t be reflected

The blue light has a longer wavelength than the red light. Wave-back for us to see. This is called absorption of light. A black

length is what gives light its color.

Figure 1: Wavelength gives light color

object, for example, absorbs all the light you shine on it,
and reflects nothing back - this is why it has no color! White

Reflection

objects reflect all the colors, and absorb none.

So what gives an object its color? The simple answer: light!
Without light, an object has no color. This is because without light, we can’t see! When you look at a red object, what
you’re actually seeing is red light bouncing off the object andinto your eye. The bouncing of light off an object is calledreflection.
Figure 1: Wavelength gives light color

White Light
When you see a red ball, however, there’s no red light shining on it, is there? Instead, there’s only the“clear” light coming from the sun or the light in your room. This “colorless” light is called white light.
White light is actually a mixture of the three primary colors of light: red, blue and green. These three colors can be mixed in different ways to form every color of light imaginable - the same way that we can mix
the three primary colors of paint to make other colors! This is why you can see all the colors of the rainbow under sunlight - all the colors are in the white light, ready to be reflected!

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ACTIVITY SHEET		ACTIVITY SHEET		

Color with M&Ms

Color with M&Ms

BROWN
BROWN

4. Switch off the Lazr finger and turn on the overhead light again.

BROWN

3. Switch on the white Lazr finger and hold it over the unsorted M&M’s. Sort them into each of their labelled color boxes.

YELLOW
YELLOWRED

2, Switch off the overhead light and darken the room as much as you can.

YELLOW

1. Mix up the M&Ms, and place them in the centre box of the worksheet.

RED RED

We’re now going to see just how much light affects our ability to see color by sorting M&Ms under colored light!

Were the M&M’s in the correct boxes? Why/Why not?
____________________________________________________________________________________________________________________________________________________________________________________________________
Under the white light, all the M&M’s should be sorted easily and correctly. This is because the white light con-

	
  
	
  

tains all the colors of light needed to see each of the M&M’s.

UNSORTED

Were the M&M’s in the correct boxes when sorted under the red light? Why/Why not?

UNSORTEDUNSORTED

5. Place the M&M’s in the unsorted box again and repeat the experiment using the red Lazr finger____________________________________________________________________________________________________________________________________________________________________________________________________
You probably found that some of the colors were in completely the wrong boxes! Others looked exactly likeeach other, but when you saw them under the white light again, they were completely different.
6. Repeat the experiment again using the green and then the blue Lazr finger, respectively. (This order is recommended, since the results just get crazier!)
Were the M&Ms in the correct boxes when sorted under the green or blue lights? What does this tell us
about how color works?
__________________________________________________________________________________________________

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GREEN

GREEN
GREEN

the main part of the M&M’s color wasn’t present, so it could not be reflected, and you could not see it.

ORANGE
ORANGEbut when the white light is switched on, it’s another color entirely! This is because the wavelength of light making up

ORANGE

could all look exactly the same. Sometimes an M&M could look definitively like one color under a certain color light,

BLUEBLUE

Under the colored lights, the M&M’s were probably more difficult to sort. Like with the red light, a group of colorsBLUE__________________________________________________________________________________________________

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TRANSMISSION WITH GUMMY BEARSWhite Light and Color

Take a look at a gummy bear. Now compare it to something solid,

The light coming from the sun or a flashlight is called white light. It looks clear, or invisible, but it actually

like a table or your shoe. You’ll notice it’s slightly different - if you

consists of a mixture of red, blue and green light. Together, these three primary colors of light can mix tolook closely, you can see inside it. Why do you think this is?

create any color of light imaginable!

Reflection and Absorption

When an objects looks red under white light, the red wavelength of light is being reflected off the object

Color depends on the wavelength of light. Red light has a different wavelength to blue light, for example.

and into your eye. The blue and green wavelengths are absorbed by the red object, and we don’t see them.

Light travels in straight lines. When it comes across an object, then, it
can’t move around it. So what does it do?

Transparency
The first thing it can do is bounce off the object. This is called reflec-

Clear substances like glass let all wavelengths, or colors,tion, and shown in Figure 1. This is what allows us to see the objectsof light through. A small amount of light is sometimes

around us: light from the sun or a light bulb bounces off the object

reflected, as well. Think about when you see your reflec-

and into our eye. This is why we can’t see in the dark - there’s notion in a window: a person standing outside would stilllight to bounce into our eyes!

be able to see you through the window, but you can see
yourself as well.

Light can also become “trapped” inside an object, as shown in Figure 2. This is called absorption. This
Some objects allow transmission of only certain wave-

light does not reach our eyes after it’s been absorbed, so we don’t see it.

lengths of light. These substances are colored, like the

Figure 1: Selective Transmission

Reflect View, or colored cellophane. Consider a blue
sheet of plastic, like in Figure 4. It allows blue light to be transmitted, but absorbs all the other wavelengths of light. Some of the blue light is also reflected, however; that’s why the sheet looks blue.

FigureFigure
1: Absorptionof Light
0: Reflection of Light

ght

Figure2: Reflection of Light
Figure 0: Absorption of Light

Figure0:0:Transmission
Reflection ofofLight
FigureLight

Figure 0: Absorption of Light	
  Transmission

Figure3: Transmission of Light
Figure 0: Transmission of Light
Figure 0: Absorption of Light

Figure 0: Transmission of Light	
  Sometimes light is neither reflected nor absorbed, however. How else would we see the trees outside thewindow, or what cool drink we have in our glass?
When light passes through an object, it’s called transmission. This is shown in Figure 3. Objects that allow
light to pass through them are called transparent. Objects that don’t allow any light to pass through arecalled opaque.

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ACTIVITY SHEET		
Transmission with Gummy Bears
We’re going to shine light through transparent gummy bears to see how light is reflected and transmitted.

CHALLENGE: SPECTRA OF COLORED LEDS
Spectrum of Light
Colored light is not always what it appears. A red LED does notconsist of only red light - instead, it consists of red, yellow and

1 Place each gummy bear on the white surface.

orange light as well. We can view the colors of light that a beam

2. Shine the red LASER pointed through the each gummy bear in turn. You may have to hold it quite close to
the gummy bear; not further than 2 cm away.

of light consists of - its spectrum - through a diffraction grating, asshown in Figure 1.

Does light pass through the red gummy bear? What color is it? Is any light reflected?
____________________________________________________________________________________________________________________________________________________________________________________________________The light from the red LASER pointer passes through the red gummy bear. Some of the light might also bevisibly reflected against the white surface.
Does light pass through the green gummy bear? What color is it? Is any light reflected?
____________________________________________________________________________________________________________________________________________________________________________________________________The red light does NOT pass through or reflect off the green gummy bear. This is because the green gummy
bear absorbs red light, and transmits/reflects only green light.
3. Now, shine the white Lazr finger at each of the gummy bears.

Figure 1: Spectrum of White Light through a Diffraction Grating

Selective Transmission

What do you notice about the white light when it passes through the red and green gummy bears?

Certain transparent objects allow only certain wavelengths,__________________________________________________________________________________________________or colors, of light to pass through them. This is known as__________________________________________________________________________________________________

selective transmission. A blue piece of cellophane, for ex-

The white light passes through both the red and green gummy bear, but appears as red and green, respective-

ample, transmits only blue light, and absorbs all the otherly. This is because the red gummy bear, for example, transmitted and reflected the red part of the white lightwavelengths. It also reflects blue light, which is why it ap-(remember, white light consists of ALL the colors of light) but absorbed the rest of the wavelengths.

pears blue to us. This is shown in Figure 2.

What do you notice about the all the results from the clear gummy bear, specifically? What do you noticeabout the black gummy bear, specifically?

Figure 2: Selective Transmission of Blue Light____________________________________________________________________________________________________________________________________________________________________________________________________
The clear gummy bear allowed all of the lights to pass through it unchanged. It is completely transparent. The
black gummy bear didn’t allow any light to pass through it. This is because it absorbed ALL the wavelengths oflight, and is opaque.

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ACTIVITY SHEET		
Spectra of Colored LEDs
We’re now going to use what we know about selective transmission to confirm that colored LED light consistsof a varied spectra.
Place the Reflect View about 5-10 cm from the wall/screen.

WHITE LIGHT & SHADOWS
Do you sometimes feel like you’re being followed? Don’t worry,
that’s just your shadow! So how does your constant companion
work? Does your shadow every change? And if so, why?

White Light
The light created by the sun is called white light. It’s the same kind

Shine the red LASER pointer at the Reflect View. What do you see?

of light you get from the room lights, a flashlight, or the white Lazr__________________________________________________________________________________________________

finger. It may look clear, or invisible, but that’s not the case at all!

__________________________________________________________________________________________________White light actually consists of a combination of the three primary
The red LASER passes through unhindered, since the Reflect View reflects/transmits red light.

colors of light: red, green and blue. Just like paint, you can combine these colors to make every color of light imaginable!

Shine the blue Lazr finger at the Reflect View. What do you see?
____________________________________________________________________________________________________________________________________________________________________________________________________

How Light Travels
A ray of light will always travel in a straight line. When you put a bunch of rays together, you get a beam oflight. Light sources, like flashlights, emit beams of light. When a beam of light leaves a light source like an

Can you think why this is? Use the diffraction grating to view the spectrum of the blue light and speculate.

LED or a Light BLOX, the rays starts to spread out, and move further away from each other.

____________________________________________________________________________________________________________________________________________________________________________________________________
No light passes through the Reflect View, since blue light does not have a red component.
Shine the green Lazr finger at the Reflect View. What do you see?
____________________________________________________________________________________________________________________________________________________________________________________________________
Again, use the diffraction grating to view the spectrum of green light and say why you think this is.
____________________________________________________________________________________________________________________________________________________________________________________________________A small amount of red light passes through the Reflect View, since green light has a red component.

When a ray of light hits an object, it can’t move around it - light can’t bend or curve around an obstacle. If
the object is transparent, the light passes through the object, or is transmitted. If the object is not transparent, but rather opaque, a shadow is formed.

Shadows
When light encounters an opaque object, it cannot pass through it. Therefore,
it must be either reflected, absorbed, or
both. In either case, the light never makes
it to the other side of the object.

Figure 1: How a Shadow is Formed

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Take a look at Figure 1. You can see a bunch of light rays leaving the light source on the left. They travel instraight lines, and when they meet the red ball, they are reflected or absorbed. The space behind the redball, however, now has no light rays, and is dark. This is a shadow.

ACTIVITY SHEET		White Light & ShadowsFirst, we’re going to create our own patch of white light to see what it’s made of!

The size of a shadow will change
depending on how far it is from the

1. Darken the room as much as you can.

light source. Take a look at Figure 2.

2. Switch on the red, green and blue Lazr fingers, or Light BLOX.

The red ball is far away from the light

3. Try to create a patch of white light on the blank screen or wall, using only these three lights. You can combine them in any way you want. Use the white Lazr finger as a guide for the kind of light you’re looking for.

source, and by the time the light rays
are blocked by the ball, they are very

What did you need to do to create white light? What is white light made of?

far apart and only a small area is left
without light. This produces a small
shadow.

__________________________________________________________________________________________________
Figure 2: A Small Shadow__________________________________________________________________________________________________
To create white light, you have to shine all three lights on the same patch of screen at the same time. When you
combine the red, blue and green light, you get white light!

Now take a look at Figure 3. Here, theball is close to the light source, and theNow, let’s create a shadow and see if we can figure out how it works.

light rays are still very close together.
This way, the ball blocks a lot of light

4. Place the long, thin object about 3 inches in front of the white screen. You’ll want the object to stand up straight.

rays, and a large shadow is formed.

5. Switch on the white Lazr finger, and shine it directly onto the object, towards the screen. Hold the light about
2 inches from the object.
Figure 3: A Large Shadow

Try to make the shadow bigger, then smaller, by moving the LIGHT. What do you have to do to make this happen?
____________________________________________________________________________________________________________________________________________________________________________________________________
To make the shadow bigger, you have to move the light closer to the object.

Shadows in Real Life: Sundials

Now try to change the size of the shadow by moving the OBJECT. What do you have to do?

A sundial is a device which tells the time of day using only a shadow from the light of the sun. It dates
back to the ancient Egyptians, and can still be used today!

__________________________________________________________________________________________________The sundial consists of an upright piece of metal (called a gnomon) attached at exactly 90° to another sheet of metal or stone. Carved into the stone or metal sheet are markings like one would find on
a clock - every hour, or even minute, is shown. When the sundial is facing in the right direction, theshadow cast by the gnomon from the sun will point at the time of day!

To make the shadow bigger, you have to move the object closer to the light. The same principle applies as__________________________________________________________________________________________________
above: the closer the object and the light, the bigger the shadow.
Make the shadow move to the left of the screen. What did you do? How can you make the shadow move to the right?
____________________________________________________________________________________________________________________________________________________________________________________________________
To make the shadow move to the left, you have to move the source of light to the right, and vice versa.

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CHALLENGE: TRANSMISSION & SHADOWS
Shadows

ACTIVITY SHEETTransmission & Shadows

Shadows are an absence of light. Light travels in a straight line,

We’re now going to use what we know about selective transmission to create shadows using selectively

and when it meets an obstacle, cannot move around it. The lighttransparent objects.

is then either reflected, absorbed, or transmitted. If the object istransparent, and the light is transmitted, the light will continue on

1. Place the Reflect View about 5-10 cm from the wall/screen.

its path on the other side of the obstacle. If the object is opaque,however, the light is either reflected, absorbed, or both. Either way,

Start out by shining the red Lazr finger at the Reflect View. What happens?

the light does not make it to the other side of the object, and a__________________________________________________________________________________________________

shadow is formed.

__________________________________________________________________________________________________You should see that the red light passes through the Reflect View, and shines on the screen behind it.

Selective Transmission
Some transparent objects allow only certain wavelengths, or colors, of light to pass through them. This is called selective transmission. A red piece of plastic, like the Reflect View, will allow only
red light to pass through it. All other wavelengths of light will beabsorbed, or blocked.

2. Stand the pencil vertically next to the Reflect View.
Shine the red light at the pencil on its own. What do you see? Is this what you expected?
____________________________________________________________________________________________________________________________________________________________________________________________________The pencil casts a shadow on the screen behind it. Regardless of the color of the light, the opaque pencil allowed none of the light to pass through it.
3. Move the pencil so that it stands between you and the Reflect View.
Shine the red Lazr finger at the pencil and the Reflect View together. What do you see on the screen?
____________________________________________________________________________________________________________________________________________________________________________________________________
You’ll notice that the red light passes through the Reflect View unhindered. The pencil blocks the light, however, and a shadow of the pencil is formed on the screen.
Now shine the blue light at the Reflect View and pencil together. What do you see?
____________________________________________________________________________________________________________________________________________________________________________________________________
This time, the entire Reflect View casts a shadow! None of the blue light makes it to the other side of the Reflect View, since the Reflect View absorbed all the blue light, and would only allow red light to pass.

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PEPPER’S GHOST
Think about the last time you stood in front of a window at night,
while the lights in your room were on. What did you see?

What Is Pepper’s Ghost?
By reflecting a real person against a pane of glass, one can create
the image of a ghost floating in “mid-air”. This is called Pepper’s
Ghost, and it’s been around for ages!

History of Pepper’s Ghost
Pepper’s Ghost was first mentioned over 400 years ago, but notdemonstrated until an inventor named Henry Dircks created the

Figure 1: Setup of Pepper’s Ghost

Dircksian Phantasmagoria in 1862. This was a device that theaters

How Pepper’s Ghost Works

could use to make ghosts appear and disappear on stage at a
moment’s notice. Sadly, it was a complicated trick that required
theatres to be completely rebuilt.
When a professor named John Pepper realized that the illusion could be more simply created, the Pep-

Figure 2: Pepper’s Ghost in Action

One of the ways that Pepper’s Ghost was created was with a special stage, shown in Figure 1. The stage
consisted of two parts: the visible stage (in pink), and a room to one side, hidden from the audience’s view(in blue). The hidden room was called the “blue room”, and this is where the person dressed as a ghost
would stand. The blue room was painted a dark color; usually black.

per’s Ghost that we know today was developed. Pepper first showed off the new trick during a perfor-

A sheet of glass was placed at a 45° angle between the stage and blue room. When the blue room wasmance of Charles Dickens’ A Haunted Man. The illusion was such a huge success that it was nicknamed

dark and the stage well-lit, the stage seemed empty - think about when you stand in front of a window

Pepper’s Ghost, instead of after its original inventor Dircks.

when it’s sunny outside. Can you see your reflection? No, only the objects outside!

Pepper’s Ghost has since been used in theatrical productions, magic shows and haunted houses - and isThen, when the stage lights were dimmed and a spotlight lit over the man in the blue room, his reflection

still in use today!

became visible in the glass. Since the audience couldn’t see the real man, it appeared as though a ghost
was floating on the stage!

Reflection and Virtual Images
When you look at yourself in the mirror, your reflection is called a virtual image. When something is virtual, it appears to be real, but isn’t actually - your reflection looks just like you, standing in front of you,but you’re not really there! Virtual images created by plane mirrors, or flat mirrors, follow very strict rules.
These virtual images are always as far from the mirror as the real object being reflected, and horizontally
inverted - left and right are swapped around. They’re also the same size as the original object!

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ACTIVITY SHEET		
Pepper’s Ghost
We’re now going to create an old-fashioned Pepper’s Ghost. For these activities, you’ll want to darken theroom you’re working in as much as possible - the darker the room, the clearer the images!
1. Place the printed picture on a flat surface. The surface should either be at eye level, or you should be able tocrouch down so that it is.
2. Hold the Reflect View over the picture at a slant. Rest the top end of the Reflect View on the table just below
the picture. Make sure the bevelled (stepped) edge of the Reflect View is facing towards you.

PEPPERGRAM™ VS HOLOGRAM
Holograms have been hugely popular in movies and video games
for the last 40 years. What do you think a hologram is? How is it
different to a normal projection, like in a movie theater?

Pepper’s Ghost
Pepper’s Ghost is an illusion that uses the law of reflection tocreate a scarily realistic ghostly image. It is often mistaken for a

3. Shine the Lazr finger onto the picture. Hold it about 10-20 cm from the picture, and be careful not to shine
the light through the Reflect View. You want to get as little glare as possible on the picture.

hologram, but it is, in fact, not a hologram at all!

4. Look directly into the Reflect View. You should be low enough that you don’t see the picture on the table.

The Law of Reflection

What do you see? Think about how far the virtual image of the picture appears behind the Reflect View, andhow it compares to the original picture.
____________________________________________________________________________________________________________________________________________________________________________________________________You should notice that the virtual image appears to be the same distance from the Reflect View as the original
picture. The virtual image is also inverted, or swapped around.
5. Replace the picture and Lazr finger with a video played on the smartphone. Something like a candle with ablack background works best.

Simply put, the law of reflection statesangle of incidence = angle of reflection
To find these angles, take a look at Figure 1. You
measure these angles from an imaginary line
called the normal, which is basically any line at 90°
to your reflective surface. The angle of incidence
is the angle between the ray moving from yoursource of light to the mirror (the incident ray) andthe normal. The angle of reflection, then, is the an-

Figure 1: The Law of Reflection

gle between the reflected ray and the normal.
What do you see now? Is it different to when you used the printed picture?
__________________________________________________________________________________________________How the PepperGram™ Works__________________________________________________________________________________________________

We’re going to create a Pepper’s Ghost using aThis proves what we know about virtual images - the candle floats the same distance behind the Reflect View as
the smartphone, and it’s swapped around!

PepperGram™; a device which makes a video
played on your smartphone appear to float in
mid-air. The PepperGram™ is made of stiff, reflec-

What could you do to make the candle appear further away than it does now? Use what you know about

tive plastic. When bent into shape and placed on

virtual images!

a smartphone, each of the four sides are at a 45°____________________________________________________________________________________________________________________________________________________________________________________________________

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Figure 2: The PepperGram™ Setup

angle to the screen, as shown in Figure 2.

To move a virtual image further away from the reflective surface, you need to move the object being reflected.

Figure 3 shows one of the four sides - they all work the same way, independently. While it may look like all

We could do that by moving the Reflect View further from the smartphone, or making the Reflect View bigger

four sides are working together to create a floating 3D image, the PepperGram™ would work just as welland using a bigger screen!

with only one side.

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The video played on the smartphone is

ACTIVITY SHEET		

reflected against the side of the PepperGram™ and directly outwards.

PepperGram™ vs Hologram

To the viewer, the image in the video

Let’s create a Pepper’s Ghost with the PepperGram™. Try to make the room as dark as possible. This will makefor clearer images.

appears to float inside the PepperGram™.
This is the Pepper’s Ghost, and it is a

1. Remove the protective film from each side of the PepperGram™.

virtual image. Your eye expects light to2. Fold along the three lines on the PepperGram™ - you might have to press quite hard to get the plastic tosnap into place. Your PepperGram™ should now look like an upside-down pyramid.

always travel in straight lines, so it is tricked

3. Wipe down the PepperGram™ with a clean cloth. You’ll want it free of fingerprints and dust for the best result.

into thinking that the video is actually

4. Search YouTube for any “4 Faces Hologram Videos”. For a jellyfish, try this one:straight ahead, inside the PepperGram™.

https://www.youtube.com/watch?v=WFvlnYxEJGU. Make sure your smartphone screen is set at its brightest.

This is why the virtual image appears the

5. Place the PepperGram™ in the middle of the screen. You can use one of the mounting squares to stick it
down so that it doesn’t move. Crouch down so that the PepperGram™ is at eye-level and play the video!

same distance from the reflective surface as
the real object actually is.

Figure 3: One side of the PepperGram™

Move the PepperGram™ slightly slower than eye-level. What do you see now? Using your knowledge of the

But it isn’t a Hologram!

law of reflection, why do you think it’s important that you look directly into the side of the PepperGram™?

The images created by the PepperGram™ may look just like holograms, but they aren’t! Holograms are__________________________________________________________________________________________________

actually three-dimensional “photographs” of objects. Shining lasers at the object from different angles,__________________________________________________________________________________________________

the size and shape of the object is recorded on holographic film, which produces a holographic print.

The reflected light rays are moving in only one direction - horizontally from the side of the PepperGram™. They
can’t move upwards to meet your eye above the PepperGram™, because then the angle of reflection would be

When you look into the holographic print, you see the 3-D object from a head-on perspective. The object

bigger than the angle of incidence, and the law of reflection would be broken!

appears to float behind the print, as if you were looking at it through a window. You can turn the print (oryour head), and the perspective of the picture will change - you’ll see the object from a different angle!
One can also shine a laser through the holographic print and project the image onto a screen, which canthen be viewed from different angles as well.

We’re now going to confirm that the PepperGram™ does not create holograms!
Pause the video. Take a good look at the jellyfish, then move around the PepperGram™ and compare. Whatdo you notice? Does the jellyfish “turn around”?
____________________________________________________________________________________________________________________________________________________________________________________________________The jelly-fish looks the same, no matter which side of the PepperGram™ you view it from. If the jellyfish were ahologram, you should see the back of the jellyfish when you go to the other side of the PepperGram™.
Now switch off the screen of the smartphone. Is the jellyfish still swimming? What does this tell us about the image?
____________________________________________________________________________________________________________________________________________________________________________________________________
Remember that a hologram is a photograph taken of an image. This means that you can’t “switch off” a hologram,
like you switched off your jellyfish. This proves that what we created was a Pepper’s Ghost, and not a hologram!

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MONOCHROMATIC LASER LIGHT
Working with LASERs, it’s immediately obvious that they don’t
behave like the light we encounter every day. What makes LASER
light different to the light emitted by a normal light bulb?

What is color?
Color is a property tied directly to the wavelength of light. Different colors of light have different wavelengths. When you look at a
red object, it looks red, because the object is reflecting red light.
When light consists of only one wavelength of light, we say it ismonochromatic.

White Light
So what color is the light emitted from the sun, or the overhead lights? Yellow? White? Does it even have a color?
Yes, it does! This light is called white light, and despite its name, it’s actually made up of a combination of the
primary colors of light. When red, green and blue light mix it creates the “clear” light we know as white light.
This is why we can see all the colors imaginable under sunlight, or the light froma flashlight, for example. Every wavelength of light is available to be reflected orabsorbed, so every color has the potential to be visible.

Diffraction Gratings
A diffraction grating is a transparent piece of plastic mounted in a frame, as
you can see in Figure 1. What you can’t see, is that the plastic is covered in

Figure 1: Diffraction Grating

thousands of tiny scratches. The scratches run from the top of the plastic tothe bottom, and are parallel to one another. They’re so thin that you can fit
hundreds of scratches on one mm of plastic!
When you shine a beam of light through a diffraction grating, it splits thelight up into its component wavelengths. All of the different colors of lightthat make up the beam of light can be seen side by side, as in Figure 2 - thisis called its spectrum.

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Figure 2: Spectrum of white light27

ACTIVITY SHEET		Monochromatic LASER Light
Let’s investigate one of the aspects of LASER light that makes it different from “normal” light.
1. Hold the diffraction grating up to one eye.
2. Hold the white Lazr finger at arm’s length, pointing towards your eye, and turn the Lazr finger on. Don’t lookdirectly at the light; rather slightly to the left or right of the source.

COLLIMATED LASER LIGHT
LASERs are often used during presentations, to highlight important
points. One could not do this with a normal light, though! Why can LASERs shine so much further than LED lights, without getting dimmer?

How Light Moves

What do you see? Record the order of the colors, starting from closest to the light source.

We can represent a beam of light as a collection of light rays, as__________________________________________________________________________________________________

shown in Figures 1 and 2. These light rays follow a very specific set of__________________________________________________________________________________________________

rules. The most important of these is that these rays always travel inWhite light is made up of a rainbow, all mixed together. The order of colours are blue, green, yellow, orange and red.

straight lines. They cannot curve, and unless they are refracted, theycannot bend.

3. Now hold the diffraction grating up to a flashlight, the overhead lights, or the sun.
Was the rainbow the same for all sources of white light? What do you expect to see for the colored Lazr fingers?
____________________________________________________________________________________________________________________________________________________________________________________________________
This spectrum is the same for all white light; always in the same order.
4. Repeat the process with the red, blue and green Lazr fingers.
What do the spectrums consist of? Was this what you expected? Why/why not? What is colored light really made of?
____________________________________________________________________________________________________________________________________________________________________________________________________You might’ve expected to see only red light for the red Lazr finger, blue for the blue Lazr finger, etc. Actually,colored lights are not one colour at all. They consist of a spectrum of colours as well, although the spectrum issmaller than that of white light. Red, for example, is mostly red with a little yellow and green.

Why is Light Bright?
The light rays making up a beam of light can be arranged ina lot of different ways. This affects how bright the light is.
Take a look at the ray diagram in Figure 1. The rays making
up this beam of light are very close together, so the beam will
be bright. The closer together the rays of light, the brighter,or more intense, the beam.
Conversely, Figure 2 shows a dim beam of light. The lightrays are far apart, so the beam is less intense, or less bright,

Often, when the rays of a beam of light leave its source, like

6. Hold the diffraction grating about 5-10 cm from the front of the LASER, and switch on the LASER pointer.

Figure 3. This means the beam starts out brightly, but loses____________________________________________________________________________________________________________________________________________________________________________________________________
Even though the LASER was split up into its components, they were all red. Hence, the LASER pointer consists

Figure 2: Dim Beam of Light

than the first one.

5. Point the red LASER pointer towards a blank wall or screen, approximately 15-20 cm away from the surface.
IMPORTANT: Never look directly into the LASER!

How is this different to the colored Lazr fingers? What does this tell us about what makes LASER light special?

Figure 1: Bright Beam of Lightan LED, the light rays start to spread out. This is shown in
intensity as the rays move further away from each other.
When the rays don’t spread out, but instead remain parallel toeach other, we say the beam of light is collimated.

Figure 3: Light Rays Spreading Out

of only one color light, i.e. only one wavelength. This is called monochromatic light.

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Calculating Area
In order to complete the activity sheet, you’ll need to know how to calculate the area of a circle and a rectangle.

ACTIVITY SHEET		Collimated LASER Light
This activity will showcase one of the properties of LASER light that makes it so unique.
1. Stick one of the blank sheets of paper against the wall, in line with the table top.
2. Stick the other sheet to the table, short end flush with the wall.
3. Mark off 1”, 3” and 6” lines on the paper on the table.
4. Place the Lazr finger at the 1” mark, and switch it on. Use a pencil to mark off the area of the circle of lightcreated. Repeat at 3” and 6”.
5. Remove the paper from the wall and calculate the area of each of the circles. Record your data in the table below.

Figure 4: Area of a Rectangle

Figure 5: Area of a Circle

6. Repeat steps 4 and 5 with the red LASER pointer and the Light BLOX, if you have one.
1”

Figure 4: Area of a RectangleFigure 4 shows the measurements required of a rectangle, which need to be3”

6”

Lazr Finger

substituted into the equation

𝐴=𝑏∙ℎLight BLOX

Figure 5 shows how to measure the radius of a circle, in order to calculate the area using

𝐴=𝜋∙𝑟2LASER

What do you notice about the size of the circle of light from the Lazr finger, and that of the rectangle fromthe Light BLOX? What could this mean, considering the nature of the light produced?
____________________________________________________________________________________________________________________________________________________________________________________________________
The circle of light becomes bigger the further away the source of light is from the wall. The rectangle behaves
the same way. This is because the light rays spread out as they leave the light source.
What do you notice about the size of the circle/ellipse of LASER light? What could this mean?
____________________________________________________________________________________________________________________________________________________________________________________________________The light from the LASER, however, stays the same size. This is because LASER light is collimated, i.e. the lightrays are parallel to one another. They do not spread out as they leave the light source. This is why the lightdoesn’t lose brightness, like the Lazr finger and Light BLOX.

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COHERENT LASER LIGHT

Interference and Speckle Patterns

LASER light has three main properties that distinguishes it from normal,

dergo constructive interference, and the resulting ray is amplified. If a crest and a trough meet, the rays

everyday light we come in contact with.

experience destructive interference, and the resulting ray is diminished.

When two rays of light meet, they interfere, and form one resulting ray. If two crests meet, the rays un-

Consider the phenomenon of diffuse reflection mentioned earlier. When an incoherent beam of light is

Light Waves

diffusely reflected, the rays of light cross and interfere, but the results are completely random. Hence, theA beam of light consists of a number of rays of light. Each ray ofbrightness of the beam of light remains fairly constant - at least to the naked eye.

light can be represented as a transverse wave. When all the rays oflight in a beam are in phase, we say the light is coherent.

When coherent light is diffusely reflected, however, the interference is more regular. This is because theincident light rays are in phase - their crests and troughs meet up in a more noticeable pattern when the rays

If two rays of light are in phase, it means their crests (and troughs)

are reflected. This results in a speckle pattern. The reflected light beam appears to be covered in dark spots

sync up. Figures 1 and 2 show the difference between coherent- these are areas of destructive interference - while the bright patches are areas of constructive interference.

and incoherent light. In Figure 1, you can see the peaks of the
waves are all aligned, forming a coherent beam of light. In Figure 2,
the peaks are randomly aligned - this is an incoherent beam of light.

Diffuse Reflection
The law of reflection states that angle of incidence = angle of reflec-

Figure 1: Coherent Lighttion. When a beam of light is reflected off a smooth surface, such as
in Figure 3, all the rays have the same angle of incidence, and hence
all move off at the same angle of reflection as well.
If a beam of light is shone on a coarse or uneven surface, however,
the rays are all reflected at different angles, as shown in Figure 4.
This is known as diffuse reflection.

Figure 3: Diffuse Reflection

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Figure 2: Incoherent Light

Figure 4: Regular Reflection

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ACTIVITY SHEET		Coherent LASER Light

MEASURING THE WAVELENGTH OF A LASER
Imagine water waves moving along the shore. What do you think

This activity will explore an aspect of LASER light that contributes to its amazing quality.
1. Switch on the Lazr finger and hold it close to the paper, at a small angle. The light should stretch out in frontof the Lazr finger in an ellipse.
Take a close look at the light. What do you observe? Is there anything unusual about the light?
____________________________________________________________________________________________________________________________________________________________________________________________________
2. Now, hold the LASER pointer the same way, and switch it on. The LASER light should form a long ellipse.

would happen is they suddenly met an obstacle, like a harbour
wall, which contained only a narrow entrance for them to movethrough? How would the waves behave once they passed throughthe hole in the wall?

Diffraction
Diffraction refers to how waves bend when they move through asmall opening, or around a narrow obstacle. Consider Figure 1.
This shows a wave moving through a small slit in a barrier. Youmight expect the waves to continue onwards in a uniform fashion,

What do you observe about the LASER light? Is there anything unusual happening? How is it different to thejust slightly shorter. Instead, the edges of the waves bend towards

light from the Lazr finger?

the barrier as they pass through.

____________________________________________________________________________________________________________________________________________________________________________________________________In the same way, waves diffract as they move past a narrow

You’ll notice that the LASER light isn’t consistent in brightness, like the light from the Lazr finger. The LASER

obstacle. This is shown in Figure 2 - the waves bend towards

light is covered in dark spots. This is the speckle pattern of the LASER light.

the obstacle, again, as they pass.

You might have to move between the Lazr finger and the LASER pointer a couple of times before you see
the difference in the light - keep going until you see it!
Now that you know what the speckle pattern looks like, move the LASER slowly while shining it on the pa-

Light is a wave, and hence it will diffract when faced with one

Figure 1: Diffraction through a narrow slitof the two abovementioned obstacles.

per. What do you observe about the speckle pattern? Why do you think this happens?

Diffraction Grating__________________________________________________________________________________________________

A diffraction grating is a piece of plastic containing thousands__________________________________________________________________________________________________of thin, vertical scratches, running from the top of the plastic

As you move the laser, the surface of the paper changes, even if you can’t see it. This changes the speckle pattern!

to the bottom. These scratches act as narrow obstacles, andthe light diffracts as it passes through the diffraction grating.

Figure 2: Diffraction around a narrow obstacle

3. See how many other coarse surfaces you can find to observe the speckle pattern on. The skin on the insideof your elbow is a good one.
Does the speckle pattern change? How so?
__________________________________________________________________________________________________The speckle pattern will be different for every surface you shine the LASER on! In the crook of your elbow, you

As the waves diffract and bend, however, they interfere with each other. When two crests meet, there isconstructive interference, and the wave is amplified, or made brighter. When two troughs meet, destructive interference occurs, and the wave is dampened, or made dimmer. This causes the interference pattern
you see emerging from the diffraction grating.

might observe more very bright spots than on the paper.

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Interference Pattern of LASER Light

ACTIVITY SHEET		

Different wavelengths of light diffract at different angles. If you

Measuring Wavelength of a LASER

were to pass white light through the diffraction grating, you
would observe its spectrum, or all the colors of light which it

We’re now going to use what we know about diffraction patterns to measure the wavelength of a red LASER.

consists of, side by side. As a test, hold the diffraction grating
up to one eye and take a look at a source of white light, like the

1. Stick the blank paper to a wall above a flat surface on which you can rest the red LASER pointer.

overhead lights. You should observe its spectrum!
LASER light is monochromatic, however - it consists of only one

2. Place the diffraction grating upright in front of the paper. Measure the distance between the grating
Figure 3: Interference Pattern of LASER Light

wavelength of light. Hence, all the light waves diffract at the sameangle, and we don’t observe a spectrum - rather, its interference pattern consists only of one color light,
shown in Figure 3. We can use this pattern to calculate the wavelength of the light!

Calculating the Wavelength of Lightand the wall, 𝐿, and record it in the table below.
3. Shine the laser pointer through the diffraction grating. If possible, place it on a small box or similar tohold it steady.
4. Mark off the central maximum and one of the first-order maxima on the sheet of paper with a pencil.
5. Remove the sheet and measure the distance between maxima. Record the distance 𝑋.
6. Repeat twice more with different distances 𝐿.

In order to calculate the wavelength of the LASER light, you need to be able to identify certain aspects ofDiffraction Gratinglines/mm

its interference pattern. Take a look at Figure 3 again. The brightest spot of light, right in the middle, iscalled the central maximum. The smaller dots on either side are referred to as secondary maxima. In white1

light, the secondary maxima are entire spectra.

2

Slit Width (𝑑)cm/lineDistance from grating to Distance from maximum
screen (𝐿) in cm
to maximum (𝑋)3

The formula for calculating the wavelength 𝜆 of light is as follows:

7. Substitute the values for measurements 1 to 3 into the equations below to calculate the wavelength of the LASER.

𝜆=(𝑋)(𝑑)𝐿
𝜆1=(𝑋)(𝑑)𝐿

where 𝑋  is the distance between the central maximum and one of the secondary maxima,𝑑  is the width of the slits of the diffraction grating, and𝐿  is the distance from the diffraction grating to the screen showing the interference pattern.

𝜆2=(𝑋)(𝑑)𝐿

𝜆3=(𝑋)(𝑑)𝐿

Average Wavelength

𝜆=𝜆1+𝜆2+𝜆3

Substitutions ofdata in formula

Final Value (cm)Final Value (nm)TIP: You need to convert your final value for from cm to nm in the table above. To do this, note that

1 cm=1×107 nm=10 000 000 nm
TIP: Red light usually has a wavelength of 650nm - 750nm. Check that your average wavelength falls in this range!

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CHALLENGE: TESTING THE LAW OF REFLECTION
Law of Reflection

ACTIVITY SHEET		Testing the Law of Reflection

When a ray of light is reflected on a surface, the incoming ray iscalled the incident ray, and the outgoing ray is called the reflected
ray. We can draw an imaginary line at 90° to the surface called thenormal. The angle between the incident ray and the normal is theangle of incidence. The angle between the reflected ray and thenormal is called the angle of reflection. The law of reflection statesthat the angle of incidence is always equal to the angle of reflection, as shown in Figure 1.

Figure 1: Law of Reflection
We’re going to test the law of reflection by drawing ray diagrams with the Reflect View.

1. Place the Reflect View on the dotted line. Make sure the stand of the Reflect View is facing upwards andthat the bevelled edge is towards you. The reflection of the star on the left should match up with theone on the right.
2. Reach over to the right hand side and trace the reflected rays as you see them on the Reflect View. Use
green for the reflected green ray, etc.
3. Use the protractor to measure the angle of incidence for each of the rays, as well as the angle of reflection. Record the angles in the table on the worksheet. NB: When measuring the angles, the dashed line
Angle of Incidence

acts are the normal, and the dark black line on top as the mirror.

Angle of Reflection

Green LightOPTIONAL: Place the small mirror upright on the line marked “mirror” on the activity sheet. Leave the slit

Red Light

caps on the Light BLOX. Place the green Light BLOX in line with the green incident ray, on the left handBlue Light

side. Do the real reflected rays and the traced reflected rays match up? You should find that they do!

Does the law of reflection hold? You can allow a couple of degrees difference between the angles.
__________________________________________________________________________________________________

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This activity guide and the Explore Optics Kit were developed by Laser Classroom for The Optical Societyof America's Centennial Celebration. Thank you for sharing your commitment to light, lasers and optics!
For more OSA Centennial Celebration information, visit WWW.OSA.ORG/EXPLOREOPTICS
For more kits and activities related to light, lasers and optics visit http://www.laserclassroom.com©201640