Practice portal › Electromagnetic Waves › Speed of EM Waves and Media
Asked in GSEB Board July 2018 · Crossing an interface
Idea: at the boundary the wave does not stop and start again. The fields on the two sides must match at every instant, so the rate at which they oscillate cannot change.
That rate is the frequency, so u, and with it ω=2π u, is the same in the medium as in vacuum. It is set by the source, not by the material.
What the medium does change is the speed: v=c/n.
Since λ=v/u, the wavelength shrinks by the factor n, the wave number K=(2π)/λ grows by the same factor, and ω/K=v falls.
So of the four, only ω is independent of the medium.
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