Waves Flashcards - Course Hero Waves can transfer energy and information from one place to another What are the two types of waves Mechanical and electromagnetic or em waves Which wave does not need matter to travel through Electromagnetic waves What are the two types of mechanical waves Transverse and longitudal Which type of wave is right angles to the direction of motion to the wave Transverse Which type of wave moves in the same direction as the motion of the wave Longitudal Movement that goes backwards and forwards a.k.a vibration, especially fast Define wave Oscillation travelling in a direction (eg light wave, infrared wave, water wave) Define amplitude Maximum displacement from the undos turned position What is the wave equation Wave speed, v = frequency, f x wavelength, (upside down y) Define wavelength Distance from one peak to the next peak (or one trough to the next trough) Define period and what unit you use The time for one whole oscillation.

Seconds Define frequency and it's unit Number if complete oscillations per second This is in "per second" which is called hertz (Hz) Define diffraction The spreading of waves as they pass by edges of obstacles Does diffraction occur with all wave types Yes What happens to waves when they pass the corner of an obstacle They curve around the edge When is the effect of diffraction small When fine gap size is large compared to the wavelength When is the effect of diffraction large When the gap of the obstacle is similar to the wavelength For sound is the frequency higher when the wavelength the have a shorter or longer length The frequency for sound is higher when the wavelengths are closer together Why is diffraction not really noticed in light Because light wavelengths are smaller than one-thousandth of a mm therefore not really noticed What are electromagnetic waves Transverse changes in the electric and magnetic fields in space Do em waves need a medium to travel through No they do not use a material medium When do all em waves travel at the same speed, and what speed is it They all travel at the same speed when going through a vacuum (300, 000, 000 m/s) Why are these em waves different They are produced in different ways and have special properties that depend on

 their frequency and therefore wavelength What is the continuous em spectrum Radio waves > microwaves > infrared > visible light > ultraviolet > X-rays > gamma (y) rays.

Run Miles In Very Unpleasant eXtreme Games What is the order of wavelength and frequency for the em spectrum Radio waves has the least frequency and it goes up for each wave Radio ways has the largest wavelength and goes decreases as you go up the spectrum What is the order of the visible part of the Em spectrum Red > orange > yellow > green > blue > indigo > violet Is the speed for all em waves constant? Yes If a wave has a higher frequency what does that mean for the wavelength The higher the frequency the shorter the wavelength Is the em spectrum continuous or non continuous Continuous What are radio waves used for Communication and information What are the uses for microwaves Heating up food and satellite communication and radar Dangers of microwaves Directly hear internal body tissue Uses of infrared Heating devices and in night vision cameras Dangers of infrared Readily absorbed by our skin which results to burning Uses of visible light By humans to see things, photography and types of fibre optics (endoscope) Uses of ultraviolet Responsible for skin cancers, can damage the eyes.

Sun is main source of UV radiation Uses if x-rays Examine internal structure of body in medical diagnosis, investigate the crystal structure of materials used in manufacturing Dangers of x-rays Can produce cell mutation and cancer, women who are pregnant should avoid during pregnancy and hospitals workers wear protective clothing Uses of gamma rays Used to treat certain types of cancer, sterilising medical equipment and food products Dangers of gamma rays Cause cell mutation and cancers. Workers must wear badges to monitor exposure or use remote handling outside What are analogue electrical signals Continuously variable voltages Can have either two possible values Advantages of digital transmission Can be regenerated, both amplifying and restoring their distinct '0' and 'I' shape electronically thus virtually eliminating the unwanted noise from the signal.

Optical fibres have wider bandwidth, meaning many digital signals can share same optical fibre > more information can be transmitted than by using an analogue signals What is the normal Construction line drawn at right angles to the mirror surface where rays of light strike it Where are all angles measured for light Between rays of light and the normal What is the law of reflection Angle of incidence = angle of reflection What does a virtual image mean Not a real image Bending of light that happens when light travels from one material Give examples of which substances light travels more slowly in Glass, perspex and water When does light bend towards the normal When the light rays travel from air into glass ( or similar substances) When does light bend away from the normal When the light ray travels out of glass back into air What is the refractive index The property of the material which determines the amount of refraction it does What is the law of the refractive index Sin I / Sin R How is the angle of incidence measured It is measured between the ray of light in air and the normal How is the angle of refraction measured It is measured between the ray it light in the material and the normal When does the ray not change direction when meeting the surface It doesn't change direction when the ray meets the surface at 9

0 degrees Does a tiny bit of reflection occur during refraction Yes The greater the side of angle i the higher...

The proportion of the ray reflected has increased When does total internal reflection occur When angle i is larger than the critical angle What is the equation for the critical angle N = 1/ sin c Physics ï¿½1st Partï¿½ - the Light The light PHYSICS ï¿½1st PARTï¿½ In the first chapter of this lesson of physics, we will look further into knowledge which we have on the light : how it is propagated, at which speed, which is its constitutionï¿½? The second, will allow us to specify the direction which we must allot under the terms : force - work - power - energy. 1. - THE LIGHT What is the light is a problem which occupied the spirit of the majority of the scientists of all times and which also interested all the people curious about the natural phenomena. More particularly as from year 1 600, and during nearly three hundred years, the physicists wondered whether the light consisted of very small corpuscles, in other words if it presented a corpuscular aspect, or of waves which were propagated in space like the waves on water and the sound in the air, i.e. if it presented an undulatory aspect. That seems a problem easy to solve.

Indeed, each one among us could distinguish without too much from difficulties the movement from a corpuscle active from a place with another, of an undulatory movement which is propagated while widening around its own source. Actually, there are reasons to be perplexed. It is that after having carried out various experiments, some made acceptable the corpuscular idea, others rather made it possible to retain the undulatory aspect. By considering these two aspects, we can compare a beam of light crossing space with a band of seals moving on the surface of the sea (figure 1-a). Each seal represents simultaneously a corpuscle of light and a light wave. Indeed, although moving with his/her companions from one point to another of the surface of the sea, and that by preserving a well defined direction, it follows, there or it is, the undulation of the waves. The three rectilinear files which make the band, while being always maintained in good order, bring back for us to the idea of the luminous ray which, in the figure 1-b, is represented by a straight line of yellow color. This comparison is only figurative and cannot give any explanation concerning the true nature of the light.

If we wish in knowing a little more, we should examine the behavior of the luminous rays in the most significant cases. We will describe in this lesson the principal luminous phenomena by limiting us to their observation. The luminous rays can be seen in summer, when, after a storm, the sun appears between the clouds. They can also appear when in an obscure part a beam of light penetrates through a small hole. These images give an idea of the trajectory of the luminous ray and highlight the following fundamental property : The light is propagated in straight line, at least as long as it does not meet an obstacle. 1. 1. - SPEED OF THE LIGHT In empty spaces (or almost vacuums) which separates ï¿½toilent them, there are floods of light which cross and go in all the directions, all at the same fantastic speed. For a long time, it was thought that the light moved away from its source at an infinite speed, which wants to say that one supposed that it was propagated instantaneously from one point to another of space, whatever the length of the course. But that is an error : the light always spends a certain time to go from a point to another.

The speed of the very large light being supposed, it proved to be necessary to measure this time, to consider a course very big length. They are very naturally the astronomers best placed to have such courses : the ground-moon ; ground-sun ; ï¿½ And it was indeed one of them which had, the first, the occasion to measure this speed. Towards the end of the XVIIï¿½me century, the Dutch astronomer ROEMER, who carried out observations on the movements of Jupiter and four of his satellites, noted that the latter, under certain conditions, had notable delays compared to their table of walk in space. This fact requires an explanation, because all the stars, large and small, are always very specific. ROEMER, after many observations, succeeds in determining with a great exactitude the revolution of a satellite of the Jupiter planet. Between in addition to, it knows that the ground and Jupiter turn around the sun. Using these data and by carrying out the many ones and complex calculations, it determines the points where Jupiter and its satellite must be a day given, at one hour and a minute precise. ROEMER then undertakes to check by the astronomical observation the exactitude of its calculations.

For this purpose, it awaits that the Earth and Jupiter are in conjunction, i.e. both aligned with the sun, the ground being in the medium as shown in the figure 2-a. It notes the exact moment then where the Jupiter satellite enters the zone of Shade of planet. It calculates then in how long three planets will be aligned, but this time with the sun in the medium (figure 2-b). The Earth and Jupiter are then in opposition. Moreover, its calculations enable him to specify the moment when the Jupiter satellite will disappear in the cone of Shadow. Its observations lead it to note, with surprise, that the penetration of the satellite behind its planet is carried out later that calculations envisaged it. This delay is 1.000 seconds. ROEMER seeks the cause of it, being certain exactitude of its calculations and observations. It notices whereas the Earth, when it is in opposition with Jupiter, is more distant from this planet than when it is in conjunction. It calculates whereas the difference of the distances between opposition and conjunction is approximately 298 million kilometers.

ROEMER explains the in the following way noted delay then: if the course of the light sent by the satellite towards the Earth were lengthened of 298 million kilometers and that the delay observed is 1.000 seconds, that means that this time is that necessary to the light to traverse 298 million kilometers. On the basis of these two numbers, it is easy to calculate the speed of the light. Indeed, by dividing a distance covered by time put to traverse it, one obtains speed. In our case, one finds 298.000 km/s. Ainsi, ROEMER can say that the light is propagated in the intersidereal vacuum at the fantastic speed of 298.000 km/S. Thereafter, after the experiment of ROEMER, other measurements were carried out under different conditions and with sophisticated measuring instruments. One thus could establish that the propagation velocity of the light in the vacuum is of 299.776 km /s, that is to say a speed slightly higher than that which ROEMER had found.

Practically, one rounds this number to 300.000 km /s, as that is mentioned figure 2. 1. 2. - OBSTACLES WITH THE LIGHT The light is born from the matter, i.e. substances which constitute the bodies. A simple electric bulb applies this assertion. The purpose of the electrical current traversing the tungsten wire of the bulb is exclusive to heat it until its temperature is of 3 000ï¿½C. A this moment, the wire emits light in the form of rays and in all the directions. It would be interesting to see how heat can be transformed into light. For that, it would be necessary for us to know more at bottom the structure of the matter, which will be made later on. For the moment, we will limit ourselves to follow the course of the luminous rays, by noting the causes which draw aside them from their natural direction rectilinear or which modify the color of it. Everyone can observe what occurs when one projects a ray of light coloured on a white body.

If the light is red, the white body appears red to us ; if it is yellow the body appears yellow, if it is green, the body to us appears green to us and so on. The white can be perceived only with the natural light, which is white, or with the almost white light of a bulb. These observations lead us to think that the color of the bodies is due, of the moment partly, with the type of light which they receive. Contrary to what occurs for the white body, one can observe that a black body, but of a black chechmate, appears always black under any light. The perception of the black indicates the absence of light. In a completely obscure room, one can note that the white, black or coloured bodies are all black and cannot be distinguished. In addition, if one successively observes a black body under various types of light, one notes that it never takes the color of the light which lights it. The white body, which is coloured differently according to the type of lighting, shows that the substances can return the rays which strike them. In addition, the black body erasing any trace of light, shows that the substances can absorb the rays which strike them.

The first phenomenon, in which the rays are returned in all the directions (figure 3), is called phenomenon of diffusion of the light. The second phenomenon, in which the rays are absorbed is called phenomenon of absorption of the light. Let us note that these two phenomena occur simultaneously and more or less each time a material obstacle is on the way of a ray. The absorption and the diffusion of the light are the principal causes of the coloring of the bodies. They make it possible moreover to show easily that the white light is actually a mixture of coloured lights. Let us take for example two objects, one red, the other green. Why this difference in color since both enlightened consequently white light ? The observer sees the red object, because this object diffuses rays of red light; in the same way the observer sees another green object because this last diffuses rays of green light. One can think that the matter constituting the two bodies transforms the white light into red light and green light. It is not the case! The reason is that the white light is made up of lights red, orange, yellow, blue, indigo and violet, with all the graduations.

Thus, the red object has this color under the white light because it absorbs all the coloured lights constituting the white light except the red light which it diffuses around him. In a similar way, the green object appears green to us because it absorbs all the colors of the white light except the green one which it diffuses around him. There are however matters which do not constitute an insuperable obstacle with the advance of the luminous rays. Let us consider for example the figure 4-a. Ray arriving on surface of glass, called ray incidental, changes direction as soon as it penetrates in the thickness of glass where it neither is diffused, nor absorptive. After having followed in all the thickness of glass a rectilinear course, it leaves there while taking again its direction initial but shifted compared to this one. The change of direction which undergoes the luminous ray while passing from the air in glass then glass in the air calls the refraction. The phenomenon of the refraction occurs each time a luminous ray passes from a transparent medium in another also transparent.

For example, it occurs when the light passes from the air in water, or conversely, as it is seen figure 4-b while following the course of the rays which go from the gull to the eye of fish. Let us notice on this figure that because of the refraction, the fish sees the gull at a place where it is not and so it can not perceive the danger which threatens it. We said about the figure 4-a that the outgoing ray of glass took again its direction initial but shifted compared to this one. We should specify that that takes place only when the two faces of the transparent body, the face by which the ray penetrates in the body and that by which it leaves there, are parallel. In the contrary case, the situation is quite different. Let us consider for example the prism represented figure 4-c. If the incidental ray is a pure light coloured, i.e. of only one color what is said still monochromic, on the outlet side of the prism the ray has a variable slope according to its color. Thus, the light violet with the greatest deviation. The other coloured lights undergo weaker deviations.

It is observed that the refraction is less and less marked as the colors pass from purple to the red. If, instead of a monochromic light one uses a white light like incidental ray, one observes that on the outlet side of the prism one obtains a range of coloured lights revealing in the white light all the colors going from purple to the red. These phenomena of decomposition and dispersion of the white light are at the base of the rainbow and the multicoloured flutter of the crystals. In the case of the rainbow, the function of the prism is achieved by the innumerable water droplets which, after a storm, are transported by the winds. 1. 3. - REFLECTION - DIFFRACTION - POLARIZATION OF THE LIGHT Until now we considered three types of obstacle which can present itself on the way of a luminous ray. The white body (or coloured) which diffuses the totality (or a part) of the incidental light ; The black body (or coloured) which absorbs the totality (or a part) of the incidental light ; The transparent body, which involves the formation of the phenomena of refraction, decomposition and dispersion. Three particular cases still remain us to be examined.

The first is that of reflective surface : the mirror (figure 5). The mirror, unlike the white body, is not restricted to return the incidental rays in all the directions. The rays which it returns, called reflected rays, are directed in well defined directions obeying all the same law called law of the reflection. This law is stated as follows: The incidental ray and the considered ray form with the perpendicular at the point of incidence two equal angles between them called angle of incidence and angle of reflection (figure 5). On this figure, the angle of incidence is indicated by the symbol (I) and the angle of reflection by the symbol (^r). The circumflex accents on letters I and ^r recall that one indicates angles. The angle of incidence and the angle of reflection being equal, one can in summary write ï¿½ = ^r. The same figure shows as the eye receiving the rays coming from the cone and reflected by the mirror is misled like was the eye of fish in the phenomenon of the refraction. Indeed, the eye seems to prolong the rays reflected behind the mirror and y to rebuild the image of the cone. But it acts there, obviously, of an illusory image still called image virtual, and turned towards the real object.

In the same way, when we look ourselves in a mirror, our face constitutes the real object and our image which we see is the virtual image. One can observe that the real object and its virtual image are symmetrical. This symmetry can be checked very simply. While taking as reference the base of the mirror, one notes that : - the object and the image are with the same height ; - the object on the one hand and the image on the other hand are with the same distance from the surface of the mirror ; - the object and the image are looked at. Now let us examine another phenomenon, that which illustrates figure 6. Let us note in the passing that its study shows us that the idea of the luminous ray is accepted with some reserves. In this new experiment, one makes fall a monochromic cone of light, a red light for example, on a glass plate striped vertically by fine incisions, parallels and very tight (figure 6). Glass, at the points where it is engraved, becomes almost opaque and thus does not let pass the light. On the other hand, between each pair of grooves there is a thin plate of glass through which the luminous rays can pass freely.

Under the conditions of the experiment, one can thus expect to see as many appearing on the screen fine bands of light and Shade that there are transparent plates and opaque grooves. However, that does not occur thus. On the screen indeed appears bands enlightened and obscure but much broader and more marked as much as one could expect it. In addition, the number of enlightened bands is lower than that of the transparent plates. One can thus deduce from it that the luminous rays from the grid do not arrive on the screen and that consequently they would be likely not to be propagated in straight line. This singular behavior of the light passing through a very thin slit is called the phenomenon of diffraction. Diffraction thus shows that the light is not always propagated like a ray. However, it is hard to imagine that certain rays are destroyed. Actually, all occurs as so some of them are reinforced and than others weaken. The result is that observed on the screen: very enlightened bands and others very black. The phenomenon is similar to that of the sound: two sound waves were propagated by giving rise to the phenomenon of interference. That resulted in a reinforcement or a weakening of the resulting wave which gave place to an intense or weak or even non-existent sound.

In this case, two or several rays are added giving place to a more intense light or, on the contrary, they are opposed by determining broad band black. That goes against the law of the rectilinear propagation of the luminous rays. The similarity with the sound waves leads us to say that with the light it can exist an interference of waves. The light thus consists of light waves, it is its undulatory aspect. Figure 7 represents another experiment whose results can bring us to a perplexity even larger. In this experiment, one observes that two crystals of a transparent matter, tourmaline, laid out in a given position, let pass the light. If one turns one of the crystals, the light does not pass any more. This phenomenon is explained if one has recourse to the undulatory aspect of the light. The light wave can be propagated in all the directions. However, in our experiment, one supposes that tourmaline transmits only one directed light wave in a well defined way compared to the crystal. More precisely, the outgoing ray of the first crystal can cross the second only if this one is in the same position as the first.

While turning one of the crystals, the light vibrates between them in a way such as it cannot cross the second. One can thus consider that it is stopped by this last which behaves then like an opaque body. The effect produced by the crystals of this type is called polarization of the light. On the polarization of the light the operation of certain filters for cameras is based. In this application, one eliminates the reflections being formed on surfaces brilliant from certain objects and particularly undesirable in the case of photography color. 1. 4. - LIGHTING AND LIGHT INTENSITY The luminous effect more the current that everyone notes daily is the illumination of the buildings : residence, place of work, storesï¿½ This luminous effect is commonly called lighting. Each one of us could note that the lighting of a body depended on the distance separating it from the source of light. It is indeed enough to approach and move away a flashlight from an unspecified object to note that when the lamp is close to the object, this one appears quite enlightened, when on the contrary the lamp is moved away, the object becomes less quite enlightened. The influence of the distance luminous-object source being established, let us make in kind measure it.

Let us consider the small installation represented figure 8-a and composed of a lamp and two screens of different surfaces. Let us light the small screen placed at 2,5 m of the lamp. Let us measure using a special apparatus (photocell) the lighting of the screen. Let us suppose that the number indicated by the cell is 20. Now let us light the large screen placed this time at 5 m of the lamp, figure 8 shows that the same quantity of light falls on the two screens. But, by measuring with the same cell the lighting of the large screen, i.e. its luminosity or the quantity of luminous rays that it returns, the number indicated by the apparatus is not any more 20 but 5, i.e. four times less. We have just noted that into doubling the distance source-screen, we divided the luminosity by 4. We can thus state an important law: Lighting is inversely proportional to the square of the distance. (The mathematical lessons explain what it is necessary to understand by ï¿½inversely proportionalï¿½ and ï¿½squareï¿½).

If thus, in our experiment the distance lamp-large screen had been 7,5 m, i.e. 3 times that of the lamp-small screen, lighting would have been 3 x 3 = 9 times less ; if instead of 3 times it had been of 4 times, lighting became 4 x 4 = 16 times less ï¿½. The figure 8-b represents an experiment which is used to compare the quantity of light emitted by two sources of light S1 and S2. The screen, which is represented there, is opaque on all surface, except the central circle (C) which is translucent, i.e. semi-transparent such as for example an oil spot on a paper sheet. If the light intensity of the S1 source is higher than that of the S2 source, a certain quantity of light coming from S1 filter cross-piece the circle (C) towards S2 and conversely. However, if the two light intensities are equal, the illumination of the zone (C) is equal on the two sides of the screen and consequently, no light filters neither on a side, nor other. It results disappearance from it from the spot. 2. - FORCE - WORK - POWER - ENERGY 2.

1. - FORCE Force is called all that is able : to deform a body ; to put a body moving if it is at rest or to stop it if it is moving ; to modify the movement of a body. Examples : On a bench, let us block a steel bar by one of its ends. Out of B in the direction of the arrow F. We support note that the bar becomes deformed and takes the B' position (figure 9-a). At a hook C, a helical spring R. Tirons the loose lead in the direction from the arrow F. We fix note that the spring lengthens (figure -9b). In these two experiments, there is deformation of a body. As long as the effort does not exceed a certain value, the bar and the spring regain their primitive shape when this effort ceases: it is said that it has elastic strain. Let us consider now a piece of wood B posed on a table T (figure 9-c). There remains motionless if nothing comes to move it. In addition, it cannot be put moving of itself. But if one draws it according to the arrow F, it moves : it is the actuation.

If one ceases drawing it, it stops almost immediately: it is the arrest due to the forces of friction. Lastly, if we release a paper ball at point A, it falls out of B, just below A. But if a draft occurs, it falls out of C : there was modification of the movement by the force of the wind ï¿½ A man, an animal, water, the wind, the steam, a magnet ï¿½ are able to exert forces. 2. 1. 2. - EQUAL FORCES Definition : It is said that two forces are equal when they produce, under the same conditions, the same elastic strain of a body. Thus, in experiment represented figure 9-a, if a person pressing on the bar in cause a drop in the end until B' then that another raises it at the same place, until B ", the distances B - B' and B - B" being equivalent, one says that the two people exerted equal forces. One can then write F = F'. It will be also said that these people exert equal forces if, simultaneously, one supporting and the other raising in the same point of the bar, this one does not undergo any deformation. It is said whereas the two forces balance.

2. 1. 3. - THE FORCES ARE MEASURABLE SIZES If one supports, without letting it rest on a support, a mass of one kilogram, one says that one exerts on this mass a vertical force directed upwards kilogram-weight (kgp). This force is balanced by the weight of this mass of 1 kilogram which is also a force of a kilogram-weight, but is directed vertically from top to bottom. The kilogram-weight (kgp), although still used, is an old measuring unit. The official unit is the Newton whose symbol is N. the relation between these units is given by the following equality : 1 kgp = 9,81 N. under multiple of Newton is the dyne which is worth 0,000 01 N or 10-5 N. 2. 1. 4. - ELEMENTS Of A FORCE A force can be broken up into a certain component count.

Thus, in the experiment of the figure 9-b : the point B where the force is exerted is its point of application ; the axis xy in which one draws the spring calls the line of action or direction of the force ; the following direction which the spring moves under the action of the force is quite naturally its direction ; the lengthening of the spring is a function of the size of the force, still called intensity, which one expresses in Newton or dyne. A force is thus characterized by four elements: its point of application, its direction, its direction and its intensity. When a force applied to a body moves this body, in other words, when a force moves its point of application, it is said that there is production of work. 2. 2. - WORK If one raises to 1 meter of the ground a body whose weight is 1 kgp (what amounts moving of one meter, in its own direction, the point of application of a force of a kgp) it is said that the work of the force is of a kilogrammeter (kgm).

In the same way, if one draws a carriage with a force from 1 kgp and that one makes him traverse one meter, one also says that the work of the force is of a kilogrammeter. More generally, when displacement takes place in the direction of the force, work W of the force expressed in kilogrammeters, is equal to the product of the force (kgp) by the length of displacement (in meters). From where the formula: W = F. I This relation shows well that there is not work that if there is displacement ; but displacement must be carried out in the direction of the force. The kilogrammeter is also an old unit. The legal unit is the Joule (J) and its value compared to the preceding one is given by the relation : 1 kgm = 9,81 J. under multiple is the erg which is worth 0,000 0001 J = 10-7 J. In the preceding relation, the units to be used are thus the Joule (J) for work W, Newton (N) for the force F, the meter (m) for the length of displacement. One writes then : W(J) = F(N) .

I(m) 2. 3. - POWER The work carried out by an electric motor gives insufficient information on its capacities. Thus, an engine which provides a certain work in one minute is not the same one as that which provides same work in one second. To be more precise, it will be said whereas the second engine is sixty times more powerful than the first because it can do the same work in sixty times less time. One will define the power P of an engine as being the work which it can carry out in one second, and more generally : the power is the work provided in one second. From this definition, one obtains the relation : P = W / t The unit of power is Watt (W) and its multiple is the kilowatt (kW) which is worth obviously 1 000 watts. The old unit, which one still finds on the maker badges of the engines, is the horsepower (ch) which is worth 736 W.

The preceding relation expressed with the units becomes : P(W) = W(J) / t(S) It is noticed that if carried out work is 1 joule into 1 second, one can then say that Watt is the joule a second (J / s) Watt, which is the mechanical unit of power is also the unit of electric output. Let us note finally the relation between the two old units of power (horse) and work (kgm) : 1cv = 75 kgm / s. 2. 4. - ENERGY 2. 4. 1. - DEFINITION It is said that a body has energy when it is able to produce a work. 2. 4. 2. - UNIT AND SYMBOL Energy is expressed like work in joule and has the same symbol W. 2. 4. 3. - POTENTIAL ENERGY AND KINETIC ENERGY That is to say a body (C) decree out of B, to 5 m of the ground and weighing 600 Newtons (what is equivalent to approximately 60 kgp) figure 10-a. Let us drop it without him to give speed at the beginning (one also says : without initial speed).

This body takes under the action of its weight, which is a constant force in size and direction, a uniformly accelerated movement which brings it on the ground at point D. Between the points B and D, the work carried out by the force F of 600 newtons is of : WJ = FN . lm WJ = 600 . 5 = 3 000 J During the movement, this work is stored in the body which arrives in D with a certain speed. That gives him what one calls of the kinetic energy. This one can for example be translated into D by the depression of a stake. It is said whereas the kinetic energy of the body C in D is 3 000 joules. Machines to insert the piles, called ï¿½sheepï¿½, and the power hammers are based on this principle. This kinetic energy of 3 000 joules would insert of 3 cm a stake which would resist the depression with a constant force of 100 000 newtons : W(J) = F(N) .

I(m) 3 000 J = 100 000 x 0,03 We will remember that the kinetic energy of a body is the energy acquired during its movement. Let us notice now that when the body C is stopped out of B, the force which acts on him and due to its weight, does not work since there is no displacement. However, this body is able to produce work, it thus has energy. We will say that this energy is in a potential state or that it is potential energy. We will retain that the potential energy of a body is the energy which this body has when it is at rest. 2. 4. 4. - PRINCIPLE OF THE CONSERVATION OF ENERGY Let us consider successively the figure 10-b with the body out of B, N, D. 1 - Out of B, its kinetic energy is null, since it is not moving.

Its potential energy is : W(J) = 600 x 5 = 3 000 joules We have then : Potential energy + kinetic energy = 0 + 3 000 = 3 000 joules 2 - The body is in N : Its kinetic energy is : W(J) = 600 x 3 = 1 800 joules Its potential energy is : W(J) = 600 x 2 = 1 200 joules We have at this point : Potential energy + kinetic energy : 1 800 + 1 200 = 3 000 joules. 3 - The body is in D. Its kinetic energy is, as we calculated already, 3 000 J. Its potential energy is null (the body cannot fall any more). We thus have now : Potential energy + kinetic energy = 0 + 3 000 = 3 000 joules. We see that at every moment the sum of the kinetic energy and the potential energy is constant. When it appears kinetic energy, it disappears an equal quantity from potential energy. This principle is general.

It is the principle of conservation of energy. 2. 4. 5. - ENERGY FORMS AND CONVERSION Energy arises in multiple forms: the water of a stopping (potential energy) while falling (kinetic energy) on the blades from turbines produces mechanical energy ; in their turn, the turbines actuate alternators and the mechanical energy is transformed into electric power ; An electric furnace transforms the electric power into calorific energy ; the chemical energy of a pile or an accumulator is transformed into electric power ; the starter equipping the engine with a car converts the electric power which it receives from the accumulator in mechanical energy ï¿½ There exists still much of other forms of energy: nuclear energy, solar energy, wind power (energy of the winds) ï¿½ We will note, to finish, that energy is reversible. I.e., for example, that the mechanical energy can be transformed into electric power and reciprocally; the electric power can be transformed into mechanical energy. LIGHT, SHADOWS AND REFLECTIONS - My Solution Guru LIGHT, SHADOWS AND REFLECTIONS LIGHT, SHADOWS AND REFLECTIONS Light, Shadows and Reflections 1. Source of light : An object which emits light, is called a source of light.

For Example : Sun, torch etc. 2. Luminous objects :These are the objects which emit their own light. 3. Non-luminous objects :These are the objects which do not emit light of their own. Such a body becomes visible when light falls on it. For example, the moon, the planets etc. 4. Pinhole camera :It is a device which forms a photograph-like image of a bright object on a screen. 5. Transparent objects :Those objects which allow all the light to pass through them are called transparent objects. 6. Opaque objects :If an object completely stops all the light falling on it, it is an opaque object. 7. Translucent objects :Some objects allow only a part of light falling on them to pass through, not all of it, such objects are called translucent objects. For example : a single thin sheet of paper. 8. Ray of light : A straight thin beam of light from a source to an object is called a ray of light. 9. Obstacle : An object which comes to the path of the light is called an obstacle. 10. Shadow :Opaque objects do not allow light to pass through them and cast dark patches behind them. These dark patches are called shadows. 11.

Formation of a shadow (i) All the opaque objects seem to form a dark shadow of their own. (ii) We need a source of light, an opaque object in the way, and a screen to see a shadow. (iii) Screen. This is a screen on which the shadow is formed. It may be a butter paper or simply ground. (iv) Shadow give us some information about shapes of objects. (v) The colour of the opaque object does not affect the colour of the shadow. (vi) All the space behind the opaque object, into some distance behind it seems to be filled with the shadow. 12. Image formed by a pinhole camera (i) Images formed by a pinhole camera are upside down. Here is a picture showing the ray paths of light, from an object far away to a pinhole, and then to screen. (ii) When sunlight passes through the leaves of a tree, the gaps between the leaves act as the pinholes. These natural pinholes cast nice round images of the sun. 13. Rectilinear propagation : Light travels in a straight line. It is called rectilinear propagation of light. 14.

Mirror : A smooth shining surface, which rebounds the light back in same or in different direction is called a mirror. 15. Reflection of light :When a ray of light falls on a smooth and polished surface, whole of light is return back in the same medium. It is called reflection. 16. Image formation by a plane mirror :We are able to see images through a mirror. Image formed by a mirror (flat) has following features : (i) Reflected image retains the colour of the object. (ii) Image is erected but laterally inverted. 17. Lateral inversion : Right side of the object appears as left side in the image formed by a plane mirror. For example, if we show our right hand, image in the mirror will show its left hand. 18. In a mirror, if you see another person, surely the other person can also see you in that mirror. Q.1. Rearrange the boxes given below to make a sentence that helps us understand opaque objects. OWS Q.2.

Classify the objects or materials given below as opaque, transparent or translucent and luminous or non-luminous : Air, water, a piece of rock, a sheet of aluminium, a mirror, a wooden board, a sheet of polythene, a CD, smoke, a sheet of plane glass, fog, a piece of red hot iron, an umbrella, a lighted fluorescent tube, a wall, a sheet of carbon paper, the flame of a gas burner, a sheet of cardboard, a lighted torch, a sheet of cellophane, a wire mesh, kerosene stove, sun, firefly, moon. Ans. (a) Transparent objects : Air, water, a sheet of plane glass. (b) Translucent objects :A sheet of polythene, smoke, fog, a sheet of cellophane, a wire mesh. (c) Opaque objects :A piece of rock, a sheet of aluminium, a mirror, a wooden board, a CD, an umbrella, a wall, a sheet of carbon paper, a sheet of cardboard.

(d) Luminous objects : A piece of red hot iron, a lighted fluorescent tube, the flame of a gas burner, a lighted torch, kerosene stove, sun, firefly. (e) Non-luminous objects : Include all (a) Transparent objects (b) Translucent objects (c) opaque objects and moon. Q.3. Can you think of creating a shape that would give a circular shadow if held in one way and a rectangular shadow if held in another way? Ans. Yes, there are many things which give a circular shadow if held in one way and a rectangular shadow if held in another way. For example : a cylinder, a circular disc, etc. Q.4. In a completely dark room, if you hold up a mirror in front of you, will you see a reflection of yourself in the mirror? Ans. No. In a completely dark room no image will be formed, because image by a mirror is formed due to reflection of light from the object and since there will be no light in a completely dark room, no image will be formed. Q.1. Name sources of light that are not hot. Ans. Tubelight, jugnu. Q.2.

List four natural sources of light. Ans. (i) Sun, (ii) Stars (iii) Fire (iv) Jugnu (Firefly) Q.3. Write down four man-made sources of light : Ans. (i) Candle (ii) Oil lamp (iii) Electric bulb (iv) Torch Q.4. Does the fire emit light? Ans. Yes, fire emits light. Q.5. Define luminous body. Ans. The objects which emit light of their own are called luminous bodies e.g., the sun, the stars etc. Q.6. What are translucent objects? Ans. Those objects which allow only a small part of the light rays to pass through them are called translucent objects. These are the objects through which one cannot see properly. Q.7. What are transparent objects? Ans. Those substances which allow light rays to pass through them completely are called transparent objects. One can see clearly through these objects. Q.8. Define opaque objects with example. Ans. Those materials which do not allow light to pass through them, are called opaque. Example : wood, stone etc. Q.9.

Classify the following into transparent, opaque and translucent objects : Wax, spectacles, a heap of salt, a stone, dense smoke, wood, skin, balloon rubber, membrane of a table, blood and milk. Ans. Transparent objects are : Spectacles, and membrane of table. Opaque object is : A stone, wood, a heap of salt, dense smoke. Translucent objects are : Blood, milk, wax, skin, balloon, rubber. Q.10. Close your eyes while looking toward bright sunshine. Now cover the eyes with your palm still keeping them closed. Do you notice any difference? On the basis of this experience, state whether your eyelids are transparent, translucent or opaque. Ans. On the basis of the experience we observe our eyelids are opaque. Q.11. Sometimes, you are able to see the Sun or the Moon behind the clouds. What can you say about the ability of such clouds to transmit light? Ans. We may say that cloud behaves as transparent object. Q.12. How will you convert a transparent glass sheet into a translucent sheet? Suggest any two ways to do it. Ans.

(i) By smearing a thin layer of oil we may convert a transparent glass sheet into a translucent sheet. (ii) By covering one side by butter paper. Q.13. Define reflection of light. Ans. When a beam of light strikes with a smooth surface, the whole light is return back into the same medium, it is called reflection of light. Q.14. Define Pinhole camera. Ans. It is a device which forms a photograph, like image of a bright object on a screen. Q.15. Define screen. Ans. Screen is a butter paper on which images are formed. Q.16. Give five examples of luminous objects. Ans. Sun, stars, electric bulb, burning candle, firefly etc. Q.17. On what factor does the proportion of light that enter an object depend? Ans. Optical nature of the object. Q.18. How are moon and planets visible to us through they are not luminous? Ans. Because they reflect light from the sun. Q.19. What is an obstacle? Ans. An object which comes to the path of light is called an obstacle. Q.20. Give examples of two substances through which light does not pass?

Ans. Wood and bricks are substances through which light does not pass. Q.21. Whether a shadow is two dimensional or three dimensional? Ans. Shadow is not just the two dimenstional outline that you see on the ground. All the space behind the opaque object up to some distance behind it, seems to be filled with the shadow. (See Fig. 11.2). Q.22. What we need in order to see a shadow? Ans. We need (i) a source of light, (ii) an opaque object in the way of light and (iii) a screen Q.23. Define Lateral Inversion. Ans. Phenomenon of changing left to right and right to left by the mirror, while forming images is called lateral inversion. Q.24. What happens when light falls on a mirror? Ans. Mirror is silvered on one side, so it does not allow the light to pass through it. It reflects almost whole of the light falling on it. Q.25. Define ‘Mirror’. Ans. A smooth shining surface, which rebounds the light back in same or in different direction is called a mirror. Q.26. Why is silver glass used as a mirror?

Ans. The silvered glass has a smooth surface and the smoothness helps in forming clear image. Silvering makes it shiny and the shiny surface helps in reducing the absorption. Q.27. Imagine a situation where air is opaque. Write down what would happen in your notebook. Ans. If air were opaque, then we could not see the things in the room or even very close to us. Q.28. While standing before a plane mirror, if you move your right hand, which hand does your image move? Ans. If we move our right hand, our image will move left hand. It is because in a plane mirror our “left appears right” and “right appears left”. This is called lateral inversion. Hence we can say that the plane mirror forms laterally inverted images. Q.29. Can you think of a situation where we can see the path of the light? Ans. In dark room we can see with torch light which goes straight. Similarly, dust particles become visible when light enters the room through fine hole. All these examples indicate that light travels in straight line. Q.30. Give two examples of substances through which light passes easily. Ans. Glass, water, etc. are substances through which light passes.

The substances through which light can pass are called transparent substances. Q.31. Under what special conditions, light does not travel in a straight line? Ans. If the size of the object in the path of light is very small (0.0001 cm/less) the light bends at the edge of the object. Thus in such case, light does not travel in straight line. Q.32. Does the direction of shadow change during the day? Does the length of shadow also change from season to season? Ans. Yes, the direction of shadow changes as the sun changes its position during the day. The length of the shadow also changes from season to season. Q.33. What change in the path of light takes place when light falls on a shiny surface? What is this called? Ans. The light comes back in the same plane, when light falls on a shiny surface.This is known as reflection of light. Q.34. How many reflected rays can there be for a given single incident ray falling on a plane mirror? Ans. For one incident ray, there is only one reflected ray. Q.35. If we are sitting in a room, we cannot see people sitting in the next room.

Why is this so? Ans. If we are sitting in a room, we cannot see people sitting in the next room because of opaque wall made of bricks between the two rooms. Q.36. Is the glass used in spectacles transparent or opaque? What would happen if it were not so? Ans. The glass used in spectacles in transparent. If it were not transparent, then we would not be able to see through spectacles. Q.37. Why do objects in a room become visible even if sunlight does not enter it? Ans. The objects in a room become visible even if sunlight does not enter it, due to the scattering of light. The air around the objects allows this scattered light to pass through it and we can see the objects. Q.38. What is reflection? Ans. When a ray of light falls on a smooth and shiny surface, the whole of light is sent back in the same medium. It is called reflection. Mirrors do not allow even a small amount of medium light to pass through them. Mirrors show regular and complete reflection. Q.39. How does light travel from one point to the other? Ans.

Light from a source of light spreads in all directions and reaches from one point to another point. Q.40. Why is the image formed in a pinhole camera inverted? Ans. In a pinhole camera image formed is inverted because the object is between radius of curvature and focus. Q.41. State any one difference between stansparent and translucent materials. Ans. In translucent materials only a small part of the light rays may be transmitted. Whereas in transparent materials light is transmitted almost completely. Q.42. What is scattering? Ans. When light falls on a rough and average shining surface, it is turned back in different directions. It is called scattering. The light rays are sent back in different directions due to uneven surface. Q.43. What does an object do the light that fall on it? Ans. It may allow the light to pass through as a glass sheet does. It may completely cur off the light from passing through by absorbing light, as the opaque bodies do. The object may scatter the light incident on it. Q.44. Why do we need a shiny surface for reflection? Ans. The extent of reflection depends upon the shine and smoothness of the surface.

More is the shine and smoothness of the surface, more will be the reflection. That is why, mirrors reflect most of the light falling on it. Hence, for reflection, shiny surfaces are required. Q.45. What are the characteristics of image formed by plane mirror? Ans. Characteristics of images formed by plane mirror are : (i) Plane mirror forms erect image. (ii) Image formed is of the same size as the object. (iii) The distance of image from the mirror is equal to the distance of object from the mirror. (iv) It is virtual, it can’t be obtained on a screen. (v) Image is laterally inverted. Q.46. What is shadow? How is it formed? How does the colour of an opaque object affects the colour of the shadow? Ans. A shadow is a dark outline or image cast by an opaque object that blocks light coming from a source of light. It is formed when light hits the opaque object which does not let the light pass through. Everywhere else around the opaque object, the light continues in a straight path until it bounces off the ground or wall behind the object. The wall or ground behind the Opaque object is the screen.

On this screen is a dark patch, or shadow, with the same outline as the object surrounded by light. The colour of the opaque object does not affect the colour of the shadow that we see. Q.47. Give the set up of an experiment to show the rectilinear propagation of light. Ans. We need : Three pieces of cardboard of the same shape and size, a thick nail, a candle, matches, a knitting needle or a similar long thin stick. Take the three pieces of cardboard. Place them one on top of another and make a hole in the middle of all three using a thick nail. Stand the three cards up on the table, each a short distance away from the next. You can keep them upright by placing them between books, notebooks, blocks of wood, etc. Take a candle which is of the same height as the holes in the cards. Light the candle and place it in front of the cards . Adjust the cards until you can see the flame of the candle from the other side. We see that the light of candle is visible only when the holes on cards lie in a straight line and if we disturb any of card then the light of candle disappears.

Thus on this observation we can say that “light propagates in a straight line.” Q.48. What are the differences between a shadow of a flower and its reflected image? Ans. Q.2. Fill in the blanks with appropriate words : (i) An object or material could be opaque, transparent, translucent or ………. . (ii) This is a …………… on which the shadow is formed. (iii) A ……………. Is a dark outline of an opaque object that blocks light coming from a source. (iv) Image formed by a pinhole camera is ……….. . (v) Image formed by a plane mirror is ………… . (vi) In a plane mirror our left hand looks like right hand. This phenomenon is called …………… . (vii) …………….. objects cast no shadow. (viii) Opaque objects cast …………. Shadows in morning and evening while …………. shadow at noon. (ix) Light travels in a ……….. . (x) Shadow is ……………. . Q.3. State whether the statements given below are True or False : (i) A torch bulb is a luminous object. (ii) Light travels in a straight line.

(iii) Image formed by a plane mirror is inverted. (iv) Light gets reflected when strikes a shiny surface. (v) Transport substances reflect all the light falling on them. (vi) All the shining bodies in sky have their own light. (vii) Jugnu (Firefly) is a living luminous body. (viii) When screen is far away from the objects the shadow will have no penumbra. (ix) A reflected image gives us more information about the object than a shadow. (x) In a mirror, if A is able to see B, B also see A. (xi) Objects around us seem to have different ways in which they interact with light. Q.4. Choose the correct option in the following questions : (i) Light travels in (a) Straight line (b) curved line (c) zig zag line (d) randomly (ii) When an opaque object comes in the path of light it forms (a) An image with colours (b) shadow (c) black and white image (d) depends on the colour of the light (iii) Which type of objects allow light to pass through them? (a) Translucent (b) Opaque (iv) Which is an example of a translucent object?

(a) A thin sheet of paper (b) A thin glass slab (c) A thin iron sheet (d) All of these (v) Bouncing back of light from shining surfaces is called (a) Reflection (b) Refraction (vi) What is lateral inversion? (a) Image becomes inverted (c) Right of the object appears left of the image (d) All of these happen (vii) Which letter of english alphabet will not show lateral inversion? (a) I, O, U (b) N, Z, X (c) I, X, E (d) A, E, I (viii) Which is a device to image the sun? (a) Plane mirror (b) Pinhole camera (c) A straight pipe (d) Glass slab (ix) Which of the following is a cold source of light? (a) Firefly (b) Tube light (c) The sun (d) Electric bulb (x) Out of these which one is not a man made luminous body.

(a) Electric bulb (b) Burning candle (c) Firefly (d) Oil lamp ANSWERS TO OBJECTIVE TYPE QUESTIONS Q.1.Match the items in Column A with Column B : Column A Q.2. Fill in the blanks : (i) luminous (ii) screen (iii) shadow (iv) upside down (v) erect (vi) lateral inversion (vii) transparent (viii) big, small (ix) straight line (x) Three dimensional (i) True (ii) True (iii) False (iv) True (v) False (vi) False (vii) True (viii) False (ix) True (x) True (xi) True. Q.4. Choose the correct option : (i) (a) A shadow is formed when an opaque object comes in the way of light. (ii) (b) A shadow is formed when an opaque object comes in the way of light. (iii) (c) Opaque objects do not allow the light to pass through them while translucent object allow only a part of light to pass through them. (iv) (a) Glass slab is transparent and iron sheet is opaque.

(v) (a) When a ray of light fall on a smooth and polished surface, whole of light is return back in the same medium, It is called reflection of light. (vi) (c) Right side of the object appears as left side in the image formed by a plane mirror. (vii) (a) Z and E will show lateral inversion. (viii) (b) Sun can be imaged by using a pinhole camera. (ix) (a) Firefly (Jugnu) emit short bursts of light due to photochemical reaction, which take place in its body temperature. (x) (c) Firefly (Jugnu) is a natural luminous body.

Question: What occurs when a ray of light meets an obstacle such as a fine wire?
Answer: Diffraction