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

Answer: (B)

$8 \,cm$

Answer 2

Time taken by light to cross the glass slab,
$t =\frac{\text { Distance }}{\text { Speed of light in glass }}$
$=\frac{5 \times 10^{-2}}{\frac{3}{1.6} \times 10^{8}} s =2.67 \times 10^{-10} \,s$
In same time distance travelled by light in air
$=c \times t=3 \times 10^{8} \times 267 \times 10^{-10}$
$=802 \times 10^{-2} m \approx 8 \,cm$