Question

A concave mirror has a radius of curvature of 40 cm. It is at the bottom of a glass that has water filled up to 5 cm (see figure). If a small particle is floating on the surface of water, its image as seen from directly above the glass is at a distance d from the surface of water. The value of d is dose to: (Refractive index of water = 1.33)

Options

• 11.7 cm

• 6.7 cm

• 13.4 cm

• 8.8 cm

Answer 1

8.8 cm

Explanation:

There is water between the object and the mirror.

Applying mirror formula,

`1/v + 1/u = 1/f`

Or `1/v + 1/(-5) = 1/(-20)`

Or `1/v = 1/5 - 1/20`

`1/v = (4 - 1)/20`

`1/v = 3/20`

Or v = `20/3` cm

This distance in water is equivalent to distance in air

= `20/3 xx n`

= `20/3 xx 1.33`

= 8.8 cm