Question

Question #12750

Answer 1

One simple answer is conservation of momentum.

Explanation:

This is not something I've really ever questioned - kudos for asking!

There are several ways to answer this question. We could look at the solution to the wave equation that comes out of Maxwell's Equations, but I suppose the simplest explanation is conservation of momentum. Although the particles of light, called photons, are massless, they do carry momentum. The magnitude of the momentum of a photon is given by:

`p=h nu = h c/lambda`

where `h` is Planck's constant, `nu` is the frequency, `c` is the speed of light, and `lambda` is the wavelength. So, in the absence of anything acting on the light, it will tend to continue in a straight line given by the direction of the momentum vector which is the direction of propagation.

A notable exception to this is refraction wherein light changes direction in a predicable way a the boundary between two dissimilar materials:

This change in direction is because of the interaction of the light with the electrons in the material which changes the speed of propagation, `c`. This change also changes the wavelength of the light in the material, but not the frequency. Therefore, from the equation above, you can see that the magnitude of the momentum is constant and `c` and `lambda` change in proportion even though the direction is changed.

You can actually derive the set of equations governing refraction using momentum conservation, in fact, René Descartes did so in 1637!

more information on Snell's law here

more information on the photon here