# Why Does Refraction Occur?

Why does refraction occur? I know that it's because light travels at different speeds in different mediums, but I don't see how that would cause a light wave to bend. I found "the wave must continuous across a boundary, which requires that the phase of the wave be constant on any given plane" at http://scienceworld.wolfram.com/physics/SnellsLaw.html" [Broken], but I don't really understand what that means.

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jtbell
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At the boundary, the "peaks" of the wave on one side have to match up with the "peaks" of the wave on the other side, and similarly the "valleys" have to match up; and they have to remain matched up as time passes.

The explanation I found easiest to understand is that the momentum can not change in the plane of the interface. You can derive Snell's law by requiring that the component of momentum be equal on either side.

Of course, the component normal to the interface will change.

I like the water analogy. In elementary physics we experimented with a wave tank, a large flat tray with an with about 1/2 inch of water. Half the tank had a flat spacer to create a shallow area with a step ledge to the deeper side. Then a wave generator (electric motor vibrating a wood stick on the water) was used to generate a plane wave. As the wave encountered the step at an angle (say 45 deg) the first part of the plane wave to pass the step slows down in the shallower water. The part of the wave still in the deep water sort of caught up to the slower part of the wave. The wave changes direction as a result of this speed change. A beautiful thing to observe on an overhead projector, see - http://en.wikipedia.org/wiki/Ripple_tank
My other favorite analogy is a marching band making a turn on the field. The marchers on the inside of the turn slow down and the ones on the outside speed up to keep the line straight, same idea.

I think I get it. Thanks for your help!