# What is the nature of refraction ?

## Main Question or Discussion Point

Take Electromagntic wave for example , when light pass from vacuum to one medium , how does the atoms of the medium bend the light towards a certain direction at the microscopic level ?

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As far as I know, when light changes from medium to another medium (i.e. a difference in the density of the substance) the light will change its speed as well. At the time it changes the speed it will also bend according to the Snell's law. So my advice if you really want to know the nature of Snell's law is to read an article about it.

Last edited:
Andy Resnick
Take Electromagntic wave for example , when light pass from vacuum to one medium , how does the atoms of the medium bend the light towards a certain direction at the microscopic level ?
By reducing the speed of propogation from c_0 to c_0/n. Momentum is conserved, so the direction of propagation changes.

If you think of light in terms of particles, then it's because of conservation of momentum.

If you don't want to use conservation of momentum, you can think of light in terms of a wave. You can try an experiment - Put thin oil and water into a container and separate them by a very thin plastic film. If you make a wave in one fluid you will see the wave 'change direction' slightly when it hits the other fluid, because of speed changes.

I have learned electrodynamics , whose explanation of refraction I think is quite phenomenological. Because it have not touched on the microscopic details when light enter the medium .

It is certain that the bending of light must be due to the interaction between photon and atoms , but I don't think the speed of photon will be slowed down after the interaction at the microscopic level . The slowdown of a light wave is a macroscopic phenomenon which invlove with lots of microscopic details . In this sense , conservation of momentum cannot ensure the photon will turn to a certain direction after interaction . It's possible for the phton turn to another direction and the momentum conservation law also holds .

So, what is the root cause of refraction ?

I have wondered this my self , and most people say the speed changes , but we all know that light always travels at c , so the speed is not changing , it is just the time lag from it interacting with the electrons and other stuff it is pretty complicated but anyways , when white light hits the glass why do the red photons take a different angle then the blue photons .