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Asked in GSEB Board July 2015 · Speed in vacuum and in a medium
Idea: in free space the speed of an electromagnetic wave is fixed by the properties of the vacuum alone, c = 1/(√μ₀ε₀) = 3×10⁸ m s⁻¹.
Neither μ₀ nor ε₀ depends on the frequency of the wave, so the speed cannot depend on it either.
Gamma rays, X-rays and microwaves differ by many powers of ten in frequency, but uλ = c keeps the product the same for all of them.
So in space all three travel at the same speed: V_g = Vₓ = Vₘ.
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