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## Temporal and spatial changes to light when transiting different media

Hi
When light passes say from air to glass, there is a change in speed and wavelength but not frequency. So a red laser shone at a glass block will refract but is still the red light with its given frequency. Why is it wavelength and not frequency? I get the feeling that it has something to do with the frequency is a characteristic given to the wave by the source of the oscillation causing the wave motion and that wavelength more a property of the medium it is moving through. Any thoughts or directions for me to explore would be greatly appreciated.
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 Mentor I think your "feeling" is essentially right on. When you solve Maxwell's equations in the case of light passing from one medium to another with a different speed of light then within each medium you get the nice easy wave equation, and at the interface you have to include some boundary conditions. Once you specify the boundary conditions, you can figure out the wave in the rest of the medium pretty easily, so let's look at that in more detail. On the "source" side of the boundary you have an amplitude and a phase. That amplitude and phase has to be matched on the other side of the boundary. Since that phase varies over time and determines the frequency, the frequency must also match on the other side. The wavelength then follows simply from propagating with that frequency given the speed of light in the other medium.

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