Question

a) A ray of light travelling in the 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.
b) 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

part a): 2 Marks
part b): 3 Marks

a) Angle of incidence = ${45}^0$

Refractive index of glass = n = 1.5

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

Sin r = $\frac{Sin{45}^0}{1.5}=0.47$

$r={28}^0$

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

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

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

$f=10.8cm$