Question

6. A concave mirror of radius 30cm is placed in water. Its focal length in air and water differ by
a) 15 b) 20 c) 30 d) 0

​

Answer 1

Answer:

Focal length of concave mirror in air and water will differ by:

d) 0

Explanation:

Given:

Radius of concave mirror = 30 cm

To find:

When placed in water, the focal length of concave mirror in air and water will differ by = ?

Solution:

First of all, let us have a look at the formula for relation between focal length and radius of spherical mirrors.

$f =\dfrac{R}{2}$

where R is the radius of spherical mirror.

and  f  is the focal length of spherical mirror.

We are given that, R = 30 cm

$f =\dfrac{30}{2} = 15\ cm$

Impact of medium on the focal length of Spherical Mirror:

Spherical mirrors do not have any impact on their focal length depending on the medium in which they are placed.

It means, they will have same focal length in any medium as that of focal length in air.

$\therefore$ when placed in water, focal length, $f'$= 15 cm

So, the difference in focal length = $f'-f$ = 15 - 15 = 0 cm

Hence, the answer is:

Focal length of concave mirror in air and water will differ by:

d) 0