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I would have said yes to both questions for an under grad course. E/M radiation will be affected by an atomsphere that can absorb and emit the wave - so propogration speed could vary in a non linear fashion - but that is kinda a very advanced view of a very subtle shift.
An aside - even though electrons travel through a wire at pretty much light speed (individually) en-masse electron drift is about 10% of lightspeed - due to collisions and a network haze of traffic though a metallyic crystaline lattice.
An individual quanta of E/M radiation at any particular frequency hitting an atomsphere will be absorbed and re-emitted by its atoms. The duration of absorption and re-emittance and how that might shift with the frequency or energy of the radiation hitting the atom is fairly advanced physics - not typically high school or under grad Uni physics I'd imagine? If the question is wanting you to know will high energy e/m radiation be absorbed and re-emitted by a gas faster or slower than lower energy e/m - or secondly have statistically more or fewer collisions - that is a pretty advanced question.
I'd treat any e/m radiation as travelling though a consistent media at the same pace, unless your radiation has a energy level so high that it starts to warp spacetime. So yes - shoot say a 10 ^ 80 Joule cosmic ray into the atomsphere and it will curve spacetime with its passage - but that isn't the pretext of the question.
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