Speed of the light  Obstacles with the light   Polarization of the light
Lighting and light intensity Force, work, power and energy  
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Created it, 05/10/15

Update it, 06/01/09

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PHYSICS     “1st PART” 

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 constitution…? 

The second, will allow us to specify the direction which we must allot under the terms : force - work - power - energy.


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.


In empty spaces (or almost vacuums) which separates étoilent 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 ; … And it was indeed one of them which had, the first, the occasion to measure this speed. Towards the end of the XVIIème 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.


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 3 000°C. 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.


Until now we considered three types of obstacle which can present itself on the way of a luminous ray.

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 î = ^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.

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.


The luminous effect more the current that everyone notes daily is the illumination of the buildings : residence, place of work, stores… 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 “inversely proportional” and “square”).

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 ….

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. 1. - FORCE

2. 1. 1. - DEFINITION

Force is called all that is able :

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

A man, an animal, water, the wind, the steam, a magnet 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.


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 :

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.


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 “sheep”, 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.


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) = F(N) . I(m)

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) = F(N) . I(m)

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.


Energy arises in multiple forms:

There exists still much of other forms of energy: nuclear energy, solar energy, wind power (energy of the winds)

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.



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