Help understanding Refractive Index

[johnnyapplese]

I get that the light changes speed as it enters a material with different densities, however I do not understand why a change in speed would cause a change in direction. For instance if I'm in a car and am going 55mph but slow down to 45mph I'm still going the same direction. It seems the only way for a light beam to change directions is if either the horizontal or vertical components of the light itself change in an uneven quantity.

 

[Dr Lots-o'watts]

Hi, here is something I just found where we clearly see refraction of a water wave.


I'll let someone else do the wording.

 

[Born2bwire]

It has to do with the fact that the phase of the wave needs to be continuous across the boundary. Because the wavelength is different in the transmitted media, the spatial phase dependence changes and the phase would gradually become more out of sync as the wave progresses along the boundary.

Another way to think about it is that the wave vector needs to be conserved. This is equivalent to the above because the spatial phase is related to kr.

A better visualization can be had from here:

Notice that the wavelength noticeably changes when it is transmitted. However, due to the refraction of the transmitted light, the phase is continuous across the boundary as the minima and maxima remain aligned.

 

[DrDu]

Even a car can change direction. How many car accidents are due to some wheel driving onto a sheet of ice? If the friction on one of the two wheels on an axis is different from that on the other end there will be a moment which tends to turn the car around.
In fact for light the situation is not that different. In the case of a light wave, there are always parts of the wave which enter the medium before other parts of the wave which excerts a moment on the wave.
However, it only works for extended objects like waves or cars.