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Snell’s law is a formula used to describe the relationship between the angles of incidence and refraction, when referring to waves passing through a boundary between two different isotropic media, such as water, glass, or air.

This formula is easy to establish in a wave lab with waves passing between deep and shallow water.

Since white light splits into different colours when passed through a prism, Snell’s law is often used as if light is a wave phenomenon.

However, light does not in fact act like waves. White light does not split into different colours when entering a prism, as Snell’s law would demand if white light was made up of many different waves. White light splits only at the exit.

If it was any other way, we would have noticed this long time ago. Plane glass sheets would have red light come out in a different place than blue light. Thick glass sheets, such as those used in large aquariums, would have multi-coloured fishes look like big smudges when viewed at an angle.

The fact that plane glass sheets do not smudge images, even when viewed at an angle, proves that light does not split on entry into glass. It splits only on exit, and only if the angle of exit is different from the angle of entry.

Red and blue light travelling through a plane glass sheet and a prism
Red and blue light travelling through a plane glass sheet and a prism

Snell’s law can therefore be used as supporting evidence for the position that light is not a wave phenomenon, but particles. The slalom analogy is better at explaining what’s going on than the wave analogy.

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