JEE Main · 2021 · 24 Feb 2021 · Shift 1 · EM Waves

The speed of an electromagnetic wave in a material medium is reduced below its vacuum value by the refractive index of the medium, which is $n=\sqrt{{\epsilon }_r{\mu }_r}$. Hence $v=\frac{c}{\sqrt{{\epsilon }_r{\mu }_r}}=\frac{3\times 10^8}{\sqrt{2\times 2}}=\frac{3\times 10^8}{2}=1.5\times 10^8$ m/s. Expressed in the units asked for this is $15\times 10^7$ m/s, so the required value is $15$. The frequency plays no part, since the speed of propagation is fixed by the permittivity and permeability alone.