Question

A ray of light is incident normally on a glass slab of thickness $5$ cm and refractive index $1.6$. The time taken to travel by a ray from source to surface of slab is same as to travel through glass slab. The distance of source from the surface is :

• A. $4$ cm
• B. $8$ cm
• C. $12$ cm
• D. $16$ cm

Answer 1

The correct option is B $8$ cm

Let, $T_1$=time taken to reach from source to surface

$T_2$=time taken to travel through glass slab

Speed in glass $v=\frac{c}{\mu }$ where $\mu =1.6$

Now $S_1$=distance of source from the surface

and $S_2$=thickness of slab=distance traveled in slab

$T_1=T_2$

$⟹\frac{S_1}{c}=\frac{S_2}{v}=\frac{S_2\mu }{c}$

$⟹S_1=1.6\times 5cm$

$⟹S_1=8cm$

Hence, answer is option-(B).