What is Light?

by Rene Descartes

The whole conduct of our life depends on our senses.

• Of these, sight is the most universal and most noble.

The inventions which increase its power are the most useful.

• The recent invention of lenses increases it the most.
• This is proven by the new stars that they have revealed.

They have given us new knowledge of nature much greater and more perfect than what our forefathers had.

But, to the shame of our sciences, this invention was first found only by experience and fortune.

About 30 years ago, a man named

Jacques Metius, from the town of Alcmar in Holland, had never studied in his life.

• His father and brother made a profession of mathematics.

He:

• had a particular pleasure in making burning mirrors and glasses
• even composed winter with ice

30 years ago, he had several glasses of various shapes.

• He luckily looked through two
  • One glass was a little thicker in the middle than at the ends
  • The other, on the contrary, was much thicker at the extremities than in the middle

He applied them so happily to the two ends of a pipe and made the first telescope.

And so telescopes were made from this pattern without anyone knowing the shapes that these glasses must have.

Many good minds have cultivated this subject very much.

• They have found several things in optics which are worth more than what the ancients left us.

nevertheless, because that somewhat difficult inventions do not arrive at their last degree of perfection at the first attempt, there are still enough difficulties in this one to give me reason to write about them. And, especially since the execution of the things that I will say must depend on the industry of the craftsmen, who ordinarily have not studied, I will try to make myself intelligible to everyone, and to omit nothing.

nor to suppose that one must have learned from other sciences. That is why I will begin with the explanation of light and its rays; then, having made a brief description of the parts of the eye, I will say particularly in what kind vision takes place, and then, having noticed all the things that are capable of making it more perfect, I will teach how they can be there. added by the inventions I will describe.

Now, having no other opportunity here to speak of light than to explain how its rays enter the eye, and how they can be diverted by the various bodies which they encounter, there is no need for me to undertake tell what its nature is, and I believe that it will be enough for me to use two or three comparisons which help to conceive it in the way that seems to me the most convenient to explain all those of its properties that experience makes us known, and then to deduce all the others which cannot so easily be noticed. Imitating in this the astronomers, who, although their suppositions are almost all false or uncertain, nevertheless, because they refer to various observations which they have made, do not fail to draw from them several very true and very assured.

It has no doubt happened to you sometimes, while walking at night without a torch through rather difficult places, that you had to help yourself with a stick to guide you, and you were then able to notice that you felt, by the intermediary of this stick, the various objects which met around you, and which you could even distinguish if there were trees, or stones, or sand, or water, or grass, or mud, or something similar. It is true that this kind of feeling is a little confused and obscure in those who have not had a long use of it; but consider it in those who, being born blind, have used it all their lives, and you will find it so perfect and so exact that one might almost say that they see hands, or that their staff is the organ of some sixth sense given to them in the absence of sight. And, to draw a comparison from this, I would like you to think that light is nothing else, in the bodies that are called luminous, than a certain movement or a very prompt and very lively action which passes towards our eyes. through the air and other bodies.

See a vat, at harvest time, full of half-trodden grapes, and in the bottom of which a hole or two has been made, like A and B[1], through which the sweet wine it contains can flow.

Then think that, having no void in nature, as almost all philosophers admit, and nevertheless having several pores in all the bodies which we perceive around us, as experience can very clearly show, it is necessary that these pores be filled with some very subtle and very fluid matter, which extends without interruption from the stars to us. Now this subtle matter, being compared with the wine of this vat and the less fluid or coarser parts both of the air and of the other transparent bodies with the bunches of grapes which are among them, you will easily hear that, as the parts of this vin, which are for example towards C, tend to descend in a straight line through hole A at the same instant that it is open, and together through hole B; and that those which are towards D and towards E also tend at the same time to descend through these two holes without any of these actions being prevented by the others, nor also by the resistance of the bunches which are in this tank, notwithstanding that these clusters, being supported one by the other, do not tend at all to descend through these holes A and B, like wine, and even that they can however be moved in several other ways by those who tread on them. Thus all the parts of subtle matter touched by the side of the sun which looks at us tend in a straight line towards our eyes at the same instant that they are open, without hindering each other and even without being hindered by the gross parts. transparent bodies that are in between: either these bodies move in other ways, like air, which is almost always agitated by some wind, or they are motionless, as glass or crystal can be . And note here that it is necessary to distinguish between movement and action or inclination to move: for one can very well conceive that the parts of the wine which are for example towards C tend towards B, and together towards A, notwithstanding that they cannot actually move towards these two sides at the same time, and that they tend exactly in a straight line towards B and towards A, notwithstanding that they cannot move so exactly towards A in a straight line, because of the bunches of grapes which are between two: and thus, thinking that it is not so much the movement as the action of luminous bodies that must be taken for their light, you must judge that the rays of this light are nothing else than the lines along which this action tends. So that there is an infinity of such rays which come from all the points of the luminous bodies towards all the points of those which they illuminate, just as you can imagine an infinity of straight lines, along which the actions which come from all the points of the surface of wine CDE tend towards A; and an infinity of others, according to which the actions which come from these same points also tend towards B without one preventing the other.

Moreover, these rays must indeed always be thus imagined exactly straight, when they only pass through a single transparent body, which is everywhere equal to itself; but when they encounter some other bodies, they are liable to be deflected by them, or deadened in the same way as is the motion of a ball or a stone thrown through the air by those whom it encounters; for it is very easy to believe that the action or inclination to move, which I have said must be taken for light, must in this respect follow the same laws as movement. And, in order that I explain this third comparison at length, consider that the bodies which may thus be encountered by a bullet passing through the air are either soft, or hard, or liquid; and that, if they are soft, they stop and completely deaden its movement, as when it strikes against canvas, or sand, or mud; whereas, if they are hard, they send it back to another side without arresting it, and this in several different ways: for either their surface is all equal and united, or rough and unequal; and again being equal, it is either flat or curved: and being unequal, or its inequality consists only in that it is composed of several parts variously curved, each of which is in itself sufficiently united; or else it consists, besides that, in having several different angles or points, or parts harder than the other, or which move, and this with varieties which can be imagined in a thousand kinds. . And it should be noted that the ball, besides its simple and ordinary movement, which carries it from one place to another, can still have a second one which makes it turn around its center, and that the speed of this can have several various proportions with that of the other. Now, when several balls, coming from the same side, encounter a body whose surface is completely smooth and equal, they are reflected equally and in the same order, so that, if this surface is completely flat, they keep between them the same distance, after having met her, that they had before; and, if it is curved inwards or outwards, they approach or recede in the same order from each other, more or less, in proportion to this curvature. As you see here the balls A, B, C[2] which, after having met the surfaces of the bodies D, E, F, are reflected towards G, H, I.

And, if these balls encounter an uneven surface, such as L or M[3], they reflect to various sides, each depending on the situation of the place of this surface that it hits. And they change nothing but that in the way of their motion when its inequality consists only in that its parts are curved differently. But it can also consist of several other things, and cause by this means that, if these balls have previously had only a simple straight movement, they lose part of it and instead acquire a circular one, which may have various proportions. with what they retain of the law, depending on whether the surface of the body they encounter can be differently disposed: what those who play tennis experience enough when their ball meets false diamonds, or when they touch it in slanting with their racket, what they call, it seems to me, cut or curl.

Finally, consider that if a moving ball encounters obliquely the surface of a liquid body through which it can pass more or less easily than through the one from which it comes out, it turns away and changes its course on entering it: as for example if, being in the air at the point A[4], we push it towards B, it indeed goes in a straight line from A to B, if it is only its gravity or some other particular cause stop ; but, being at point B, where I suppose it meets the water surface CBE, it turns away and takes its course towards I, going again in a straight line from B to I, as is easy to be verified by experience. Now we must think in the same way that there are bodies which, being encountered by the rays of light, deaden them and deprive them of all their force, namely those which are called black, which have no other color than darkness; and that there are others which make them reflect, some in the same order as they receive them, namely those which, having their surfaces quite polished, can serve as mirrors, both flat and curved, and the others confusedly. to several sides; and that, again among these, some make these rays reflect without bringing any other change to their action, namely those which are called white; and the others bring with it a change similar to that which the movement of a ball receives when it is curled, namely those which are red, or yellow, or blue, or of some other such color: for I think I can determine in what consists the nature of each of these colors, and to show it by experience; but that goes beyond my subject. And it is enough for me here to warn you that the rays which fall on bodies which are colored and unpolished are usually reflected in all directions, even though they come only from one side.

Just as those which fall on the surface of the white body AB[5] only come from the torch C, they do not fail to be reflected so much on all sides that wherever we place the eye, as for example towards D, there are always several coming from each place of this surface AB which tend towards it. And even, if we suppose this body very slender, like a paper or a canvas, so that the day passes through it, even though the eye is on the other side than the torch, as towards E, it will not leave not to reflect towards him some rays from each of the parts of this body. Finally, consider that the rays are also diverted in the same way as was said of a ball when they obliquely encounter the surface of a transparent body through which they penetrate more or less easily than through that from which they come. and this way of turning away is called in them refraction.