What Cauases Refraction?

This cause is a general law of nature unknown before Newton.

The reflection and refraction of light cannot be explained by known impulse alone.

When light passes from an expanded, lighter substance, like air, into a heavier, denser substance, like water — which would seem to offer more resistance — the light changes its path and bends toward a perpendicular drawn on the surface of the water.

M. Leclerc, in his Physics, stated the exact opposite out of inattention. In his book 5, chapter 8:

The greater the resistance of bodies, the more the light that enters them moves away from the perpendicular. Thus, the ray moves away from the perpendicular in passing from air into water.

Leclerc

Leclerc and his followers make this mistake.

This ray AE falling from the air into this crystal bends (figure 24).

It forms an angle with the perpendicular BE when it strikes the surface of this crystal.

That same ray, refracted inside the crystal, forms another angle with that same perpendicular, which governs its refraction.

It was necessary to measure this incidence and this bending of light.

This might seem very easy — yet the Arab mathematician Alhazen, Vitellio, even Kepler himself, failed at it.

Snellius Villebrod was the first — according to Huygens, an eyewitness — who discovered the constant proportion in which light bends in given media.

He used secants.

Later, Descartes used sines instead — which is exactly the same proportion, the same theorem, just under different names.

The larger the line AB you see, the larger line CD will be as well.

This line AB is what is called the sine of incidence.

This line CD is the sine of refraction.

These 2 sines — whatever their lengths — are always in proportion in a given medium.

But this proportion changes when the refraction happens in a different medium.

When light falls obliquely from air into crystal, it bends in such a way that the sine of refraction CD is to the sine of incidence AB as 2 is to 3.

This means simply that line AB in air is one-third longer than line CD in the crystal.

In water the proportion is 3 to 4.

Thus, it is clear that in all cases, for all possible angles of incidence, the refractive power of crystal is to that of water as 9 is to 8.

It is not only necessary to understand the cause of refraction, but also why all these refractions differ in their proportions.

Here all philosophers made hypotheses — and were wrong.

Newton alone found the true reason as a property belonging to all bodies.

Consider that the rays of light are in motion.

If they deviate when changing their course, it must be by some primitive law — and nothing should happen to light that would not happen to any bodies of equal smallness, all other things being equal.

Suppose a lead ball A (figure 25) is pushed obliquely from air into water:

What happens is the opposite of what happens to the light ray.

For the thin light ray passes into pores, whereas the ball, with its broad surface, strikes the surface of the water, which supports it.

This ball moves away from the perpendicular B at first.

But once it loses the oblique motion impressed on it, it then falls roughly along the perpendicular drawn from the point where it begins to descend.

It slows its fall in the water — as we know — because the water resists it.

But a light ray, on the contrary, speeds up in the water, because the water does not resist the rays that penetrate it.

There is, then, some force — of whatever kind — acting between bodies and light.

That this attraction, this tendency, exists, we cannot doubt: for we have seen light, attracted by glass, enter it without touching anything.

This force acts in a perpendicular line since it is the shortest path.

Since this force exists, it exists in all the parts of the body exerting it.

Thus, the parts of the surface of a body feel this power before it penetrates the interior of the substance, before it reaches the point toward which it is directed (figure 26).

Thus, as soon as the ray arrives near the surface of crystal or water, it already begins to turn a little toward the perpendicular.

It bends already slightly at C even before entering; the more it enters, the more it bends — because the closer it gets, the more it is attracted.

The ray must necessarily curve imperceptibly before penetrating straight into the crystal because there is no perfectly sharp angle in nature.

Continuous motion cannot change direction except by passing through all possible degrees of change.

It therefore cannot go from one straight line to another straight line all at once without tracing a small curve connecting these two lines.

Thus Leibniz’s principle of continuity and Newton’s attraction meet in this phenomenon.

This ray, therefore, does not:

• fall entirely perpendicularly
• follow its original oblique straight line when passing through water or glass

Instead, it follows a line that partakes of both paths and descends faster the stronger the attraction of the water or crystal.

Thus, far from water bending light rays by resisting them, as once believed, water actually bends them because it does not resist, but rather because it attracts them.

We thus say that rays bend toward the perpendicular, not when they pass from an easier medium to a more resistant one, but when they pass from a less attracting medium into a more attracting one.

“Attracting” means the point toward which a recognized force is directed.

• It is an indisputable property of matter, a property that is very evident between light and bodies.

Consider that since 1672, when Newton demonstrated this attraction, no philosopher has been able to imagine any plausible reason for this bending of light.

Some say: “Crystal refracts rays of light because it resists them.”

But if it resists them, why do the rays enter it more easily and with more speed?

Others imagine a matter in crystal that “opens easier paths on all sides”; but if these paths are so easy on all sides, why does light not enter without bending?

Some invent atmospheres; others vortices; all their systems collapse somewhere.

We must therefore, I believe, stick to Newton’s discoveries — to this visible attraction whose reason neither he nor any other philosopher has been able to find.

The followers of Descartes and Aristotle had revolted against attraction.

• Some refused to study it; others despised it and mocked it after barely examining it.

But I beg the reader to make these three reflections:

1. Attraction is a force by which one body approaches another, without any visible or known force pushing it.

2. This property of matter has been established by the best philosophers in England, Germany, Holland, and even in several universities in Italy, where somewhat rigid laws sometimes bar the way to truth.

The agreement of so many learned men — is this not a proof?

Certainly; at least it is a powerful reason to examine whether this force exists or not.

3. We should think that we understand no more the cause of impulse than of attraction.

We have no clearer idea of one force than the other: for no one can conceive why one body has the power to move another from its place.

We do not conceive, it is true, how one body attracts another, nor how the parts of matter gravitate mutually — as will be proven.

Nor did Newton ever claim to know the reason for this attraction.

He simply proved that it exists: he saw in matter a constant phenomenon, a universal property.

If a man found a new metal in the earth, would that metal exist any less because we did not know the first principles of which it was made?

Maupertuis discusses the metaphysics of attraction in the smallest and best French book ever written on philosophy.

People often say that attraction is an occult quality.

If by this they mean a real principle whose reason cannot be given, then the entire universe is in the same case.

We do not know how there is motion, nor how it is communicated, nor how bodies are elastic, nor how we think, nor how we live, nor how or why anything exists: everything is an occult quality.

If by this they mean merely an empty school expression, a word without an idea, then one must recognize that it was through the most sublime and most exact mathematical demonstrations that Newton revealed this principle — which some have tried to dismiss as a chimera.

The rays reflected from a mirror cannot come to us from its surface.

We have experimented that rays transmitted into glass at a certain angle return, instead of passing through into the air; that, if there is a vacuum behind this glass, the rays that had previously been transmitted return from this vacuum to us: certainly, there is no known impulse there.

We must necessarily admit another power; we must also acknowledge that in refraction there is something not understood until now.

What, then, is this power that bends this ray of light in a basin of water?

It is demonstrated (as we shall say in the next chapter) that what was once thought to be a single ray of light is actually a bundle of several rays, each refracting differently.

If, among the threads of light contained in this ray, one refracts, for example, at four measures from the perpendicular, another will bend at three measures.

It is demonstrated that the most refrangible rays — that is, those which, when bending as they leave glass and taking a new direction in the air, approach less to the perpendicular of the glass — are also those which reflect most easily and most quickly.

There is therefore strong evidence that the same law makes light reflect and refract.

Finally, if we find yet another new property of light that seems to owe its origin to the force of attraction, should we not conclude that so many effects belong to the same cause?

Here is this new property, discovered by Father Grimaldi, Jesuit, around 1660, and on which Newton pushed his investigation to the point of measuring the shadow of a hair at different distances.

This property is called the inflection of light.

Not only do rays bend when passing into a medium whose mass attracts them; but other rays, which pass through the air near the edges of an attracting body, visibly approach that body and deviate from their path.

Place (figure 27) in a dark place a thin blade of steel, or glass tapered to a point; expose it near a small hole through which light passes; let that light graze the point of the metal:

You will see the rays curve near it, so that the ray that approaches the closest to this point curves the most, and the ray that is farther curves less, proportionally.

Is it not highly likely that the same power which bends these rays when they are in the medium forces them to deviate when they are near the medium?

Thus, refraction, transparency, reflection — all are subject to new laws.

Here is an inflection of light that evidently depends on attraction.

A new universe presents itself to the eyes of those who wish to see.

We shall soon show that there is an evident attraction between the sun and the planets, a mutual tendency of all bodies toward one another.

But we also warn in advance here that this attraction, which makes the planets gravitate toward our sun, does not act in the same proportions as the attraction of small bodies that touch each other.

These are probably attractions of absolutely different kinds.

They are new and different properties of light and of bodies that Newton discovered.

It is not a question here of their cause, but simply of their effects, unknown until our own day.

Let no one think that light is bent toward and inside crystal in the same ratio, for example, as Mars is attracted by the sun.