# How did Newton explain why a prism bends light rays causing the colors to separate?

We know today that light has wavelike properties. This explains why the rays are bent when they enter the prism. Light slows down when traveling through a medium, as the trough enters the prism before the crest, the bottom part of the wave slows down (before the top) and causes it to bend the ray, which results in the separation of colors, as each color has a different wavelength.

But as Newton believed that light is made of tiny corpuscles/particles, and didn't subscribe to the wave theory, how did he explain the science of refraction? Why does the prism bend the light ray? And more importantly how does white light get separated into different colors?

• FYI: Newton's big discovery was not that prisms disperse light—that already was known. But other natural philosophers thought that the prism somehow added colors to the light. Newton was the first to show that the colored rays could be recombined to make white light. He suggested that a prism separates white light into component colors that had been in the original "white" light ray all along. That discovery, by itself, was pretty big. IDK if he went on to speculate about how the prism dispersed the light. Commented Nov 15, 2022 at 21:11
• Newton explained the refraction of light towards the normal on entering a 'denser' medium to be due to the corpuscles experiencing an attractive force directed normally towards the surface of the medium. When the corpuscle emerges into the less dense medium the normal force will cause its path to bend away from the normal. Presumably the blue corpuscles experience stronger forces than the red, so the blue light bends more. Commented Nov 15, 2022 at 21:28
• @PhilipWood That is correct. Also, that is an answer. You should post it as such. Commented Nov 15, 2022 at 21:35
• Related Snell's law for classical particle?. The change in velocity is colour/wavelength dependant. Commented Nov 15, 2022 at 22:06
• Joseph h Thank you. I've taken your advice. Commented Nov 15, 2022 at 22:39

Newton explained the refraction of light towards the normal on entering a 'denser' medium to be due to the corpuscles experiencing an attractive force directed normally towards the surface of the medium. When the corpuscle emerges into the less dense medium the normal force will cause its path to bend away from the normal. Presumably the blue corpuscles experience stronger forces than the red, so the blue light bends more.

It is, at least superficially, easy to see how the corpuscular theory leads to Snell's law. There is no force on the corpuscles parallel to the surface, so their velocity component parallel to the surface is unchanged, that is, with the usual notation, $$v_1\sin \theta_1=v_2\sin \theta_2$$ That is $$\frac{\sin \theta_1}{\sin \theta_2}=\frac{v_2}{v_1}$$ So we have Snell's law if we assume that $$v_2/v_1$$ is a constant for the media. But this seems to me to be an unnatural assumption for corpuscles, and as M Enns has explained, if $$\frac{\sin \theta_1}{\sin \theta_2}>1$$ (for example if medium 1 is air and medium 2 is water), direct time-and-distance measurements for light (Fizeau, Foucault c.1850) showed that $$v_2.

• I'm not saying you're wrong (I don't know whether this is what Newton wrote or not) - but if "density" is mass per unit volume, such a model makes false predictions that Newton could easily have checked, e.g. at the interface between water and oil.
– g s
Commented Nov 15, 2022 at 22:49
• Newton's corpuscular theory assumed that light accelerated as it entered a denser medium and bent towards the normal - e.g. that it travelled faster in water than in air. At the time nobody had measured the speed of light in water. When this was finally measured in the mid 1800s and found to be slower than in air it was kind of the final nail in the coffin for Newton's corpuscular theory. Commented Nov 15, 2022 at 23:09
• Thanks Philip. But you haven't explained why light acts differently when it goes through a prism than when it goes through a medium with parallel surfaces. According to you it should behave the same way in both cases (i.e. it should bend away from the normal) but it doesn't. How did Newton explain that?
– Bach
Commented Nov 15, 2022 at 23:41
• @g s I put 'denser' in quotes for this reason. Perhaps I should have said 'optically denser'. Commented Nov 16, 2022 at 0:12
• @Bach When light enters and leaves a prism, it bends toward the normal on entry, and away on exit (assuming it is surrounded by air - or any less dense medium). Not toward the normal twice. Commented Nov 16, 2022 at 7:49