A ray of light is incident normally on a glass slab to thickness 5 cm and refractive index 1.6. The time…
A ray of light is incident normally on a glass slab to thickness 5 cm and refractive index 1.6. The time taken to travel by a ray from source of light to surface of slab is same as to travel through glass slab. The distance of source from the surface is
5 cm
8 cm
12 cm
24 cm
Solution
$\begin{aligned} & \mathrm{T}_1=\text { time taken to reach from source to surface. } \\ & \mathrm{s}_1=\text { distance of source from surface. } \\ & \mathrm{T}_2=\text { time taken to travel through glass slab } \\ \because \quad & \mathrm{s}_2=\text { distance travelled in slab. } \\ \mathrm{T}_1 & =\mathrm{T}_2 \\ & \frac{\mathrm{~s}_1}{\mathrm{c}}=\frac{\mathrm{s}_2}{\mathrm{v}} \quad \ldots(\because \text { Time }=\text { Distance/velocity }) \\ \therefore \quad & \frac{\mathrm{s}_1}{\mathrm{c}}=\frac{\mathrm{s}_2 \mu}{\mathrm{c}} \quad \ldots(\because \mu=\mathrm{c} / \mathrm{v}) \\ \therefore \quad & \mathrm{s}_1=\mathrm{s}_2 \times \mu=5 \times 1.6=8 \mathrm{~cm}\end{aligned}$