How light is broken

The second property of the rays of light that must be carefully examined is that of deviating from their path when passing from the sun into the air, from the air into glass, from glass into water, etc. This change of direction in different media — this “breaking” of light — is called refraction. –>

It is by this property that an oar plunged into the water appears bent to the sailor who handles it; it is this that causes, in a bowl, when we pour in water, an object to become visible which we could not see before from the same position.

Finally, it is by means of this refraction that our eyes are able to see at all. The admirable secrets of refraction were unknown to antiquity, which nevertheless had them before its very eyes and used them every day — and yet not a single writing survives that suggests they had guessed the reason.

Similarly, even today, we are ignorant of the cause of the very movements of our body and the thoughts of our soul; but this ignorance is of another kind. We do not and never will have instruments fine enough to see the first springs of our own being; but human ingenuity has made itself new eyes, which have allowed us to observe, in the effects of light, almost all that it is permitted for men to know.

We must form here a clear idea of a very common experiment (figure 4). A gold coin is at the bottom of a basin; your eye is placed at the edge of the basin at such a distance that you cannot see the coin.

Water is poured in: you could not see it at first where it was; now you see it where it is not. What has happened?

The object A reflects a ray that strikes against the rim of the basin (figure 5), a ray that will never reach your eye; it also reflects the ray AB, which passes above your eye. Now you receive this ray A B; your eye has not changed place, so it must be the ray AB that has changed — it has clearly bent at the edge of the basin, passing from the water into the air; thus it strikes your eye at C.

But you always see objects in a straight line, so you see the object along the straight line C D, and therefore you see the object at point D, above the place where it actually is.

If this ray bends one way when it passes from water into air (figure 6), it must bend the opposite way when it enters from air into water.

I raise on this water a perpendicular; the ray A, which, leaving the luminous point, bends at point B and approaches this perpendicular in the water following the path B D; and this same ray D B, when passing from the water into the air, bends as it goes toward A and moves away from this same perpendicular: thus light refracts according to the media it crosses.

It is on this principle that nature has arranged the different humors that are in our eyes, so that the rays of light passing through these humors are bent in such a way that they reunite afterward at one point on our retina; and it is finally on this principle that we make eyeglasses, whose lenses undergo even greater refractions than those which occur in our eyes, and which, by bringing more rays together, can extend the power of our sight up to two hundred times — just as the invention of levers gave new strength to our arms, which are themselves natural levers.

Before explaining the reason Newton discovered for this property of light, you wish me to say how this refraction works in our eyes, and how the sense of sight — the most extensive of all our senses — owes its very existence to refraction. However well‑known this subject may be, beginners who might read this small work will be pleased not to have to look elsewhere for what they would like to know about vision.