Question

In an ionized medium, the phase velocity $v_p$, group velocity $v_g$ and the speed of light are related as

• A. $v_p>v_g>c$
• B. $v_p=v_g=c$
• C. $v_p<v_g<c$
• D. $v_p>c$, $v_g<c$

Answer 1

The correct option is D $v_p>c$, $v_g<c$

Phase velocity of e.m. wave in free space $c=\frac{1}{\sqrt{{\mu }_0{ϵ}_0}}$
Phase velocity of e.m. wave in ionised medium
$v_p=\frac{1}{\sqrt{{\mu }_0{\epsilon }_0(1-\frac{N{e}^2}{{\epsilon }_{0}m{\omega }^2})}}$
$\therefore$ $v_p>c$ but $v_g<c$ in ionised medium.