Question

If speed of light travelling c from air to a medium decreases by $40\%$, find the refractive index of the medium with respect of air?

• A. $2.5$
• B. $1.67$
• C. $1.3$
• D. $1.25$

Answer 1

Answer: (B)

$1.67$

Speed of light in air is $c$. Let the refractive index of medium w.r.t air $n$.

Speed of light in medium decreases by $40$% of $c$.

Thus speed of light in medium $v=0.6c$

Using $n=\frac{c}{v}$

$\therefore$ $n=\frac{c}{0.6c}$

$⟹$ $n=\frac{10}{6}=1.67$