Question

1. A ray of light travelling in air falls on the surface of a glass slab at an angle 45° with the normal. The refractive index of glass is 1.5. Calculate the angle of refraction.
2. A point object is placed at a distance of 18 cm from a convex lens on its principal axis. Its image is formed on the other side of the lens at 27 cm. Calculate the focal length of the lens. [5 MARKS]

Answer 1

Each problem: 2.5 Marks

1. Angle of incidence = ${45}^0$

Refractive index of glass = n = 1.5

$n=\frac{Sini}{Sinr}$

Sin r = $\frac{Sin{45}^0}{1.5}=0.4728$

$r={28}^0$

2. According to the sign convention, the object is placed on the left side of the lens, therefore object distance is negative i.e. u = - 18 cm. Since the image is formed on the other side, the image-distance is positive i.e., n = + 27 cm.

Using lens formula, $\frac{1}{v}-\frac{1}{u}=\frac{1}{f}$, we have

$\frac{1}{+27}-\frac{1}{-18}=\frac{1}{f}$

$f=10.8cm$