What Causes Refraction to Occur?

[austinv]

I'm very curious as to what actually causes light to bend when it encounters a medium with a different refractive index. Of course the speed of light changes, and this speed change is accompanied by a corresponding wavelength change so that the frequency remains the same and there is no discontinuity at the interface between media... but why does the light also change direction? And the fact that the direction change is specific and always occurs is interesting. Do we have any understanding as to why light will always bend toward the normal when entering a higher RI medium?

And why is it only when light impinges upon a different medium at an angle? For instance, light striking a surface orthogonally does not change direction.

Thank you very much!

 

[PhilDSP]

Hi Austinv and welcome to the forum!

A wave front approaching media from an angle will produce the situation where the edge of the wave contacting and penetrating the media first will be slowed while the wave edge outside the media will still travel at its original velocity. The difference in velocity between the 2 edges of the wavefront and the fact that momentum must be conserved along the wavefront will effectively pull the entire wave front towards the direction of the edge that contacted the media first.

 

[austinv]

Hi and thanks for your reply!

That actually makes total sense, but due to the extremely small scale of the wavelength of visible light, compared to the extent of the surface of a piece of glass for example, is it really true that the bending is occurring because the inside edge of the wave is in the medium and the outside edge is not?

The degree of curvature on the surface of a glass lens compared with a wavelength of visible light seems like there would be no difference between the light striking it straight on versus at an angle.

 

[PhilDSP]

The surface of the media doesn't need to be curved. Even with a flat surface the inside edge of the wave front will contact the media first. Another way to analyze the situation is to do ray tracing and invoke Fresnel's principle. You should end up with Snell's Law.