Question

A light ray enters a prism through the side PR. PQR is a cross section of the prism. The emergent ray lies along the boundary OQ. ∠P = 90° and PQ = PR. The ray diagram has been given.

iv. What is the speed of light in the prism? (Speed of light in air is $3\times {10}^{8}m/s$)

[1 mark]

• A. $3\times {10}^{8}m/s$
• B. $2.1\times {10}^{8}m/s$
• C. $1.4\times {10}^{8}m/s$
• D. $1.2\times {10}^{8}m/s$

Answer 1

The correct option is B $2.1\times {10}^{8}m/s$

$Refractiveindex=\frac{1}{\sin \left(criticalangle\right)}=\frac{1}{\sin {45}^{\circ }}=\sqrt{2}$


The refractive index of the medium is 1.41.

$Speedoflightinmedium=\frac{Speedoflightinair}{Refractiveindexofmedium}$

= $\frac{3\times {10}^8}{1.41}m/s$

= $2.1\times {10}^{8}m/s$