Discussion
If an EM wave travels in a medium with εr = 4, μr = 1, its speed (in ms-1) in terms of c (c = velocity of light in free space) is
Answer: C
The speed is \(\frac{c}{2}\).
The speed of an electromagnetic wave in a medium is \(v = \frac{1}{\sqrt{\mu\varepsilon}} = \frac{c}{\sqrt{\mu_r\varepsilon_r}}\).
\[ v = \frac{c}{\sqrt{1 \times 4}} = \frac{c}{2} \]
\(\frac{c}{4}\) forgets the square root, and 2c would mean the wave travels faster than in a vacuum, which a material medium does not allow. The refractive index of this medium is 2.
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