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In his account of refraction, which can be found for example here, Descartes compares it with the motion of a tennis ball.

He says that going into a denser medium must reduce the total speed, while the tanget speed must be the same. This leads to the conclusion that the trajectory must bend away from the normal line.

This can be seen in his own figure.

This reasoning works for a tennis ball, but for light it is wrong: light bends towards the normal line.

Yet everybody says Descartes correctly deduced the refraction law. I don't understand.

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The point is the Descartes also argues that light should move faster in the denser medium, contrary to what happens with a tennis ball. In one of his notebooks, he wrote: "Since light can be produced only in matter, where there is more matter it is produced more easily, other things being equal; therefore it penetrates more easily through a denser than through a rare medium"

So he uses the tennis ball for intuition, and deduces the refraction it would have when its speed is reduced. But since the speed of light increases, the refraction is opposite.

(How this is to be reconciled with his view that the speed of light is actually infinite, I don't know.)

In any case, the greater originality in Descartes work is that it is quantitative. He not only says the light bends towards the normal, which was known since antiquity, but he clearly states that the sines of the angles are in proportion (and not the angles themselves, for example).

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