Question

A slab of glass, of thickness 6 cm and refractive index $\mu =1.5$ is placed in front of a concave mirror as shown in the figure. If the radius of curvature of the mirror is 40 cm and the reflected image coincides with the object, then the distance of the object from the mirror is :

• A. 30 cm
• B. 22 cm
• C. 42 cm
• D. 38 cm

Answer 1

The correct option is C 42 cm

Shift due to glass slab =

$t[1-\frac{1}{\mu }]=\left(6\right)[1-\frac{2}{3}]=\left(6\right)\left[\frac{1}{3}\right]=2cm$

Since radius of curvature $=40cm$

Object must be placed on a distance of $42cm$ so that due to shift they will be at 40 cm and both coincide.