Question

Focal length of a mirror in vacuum is 20 cm. Focal length of the same mirror in water of refractive index 4/3 is
1) 20 cm
2) 15 cm
3) 10 cm
4)80/3cm​

Answer 1

Answer:

20cm focal length is same for the mirror

Answer 2

Given:

Focal length of a mirror in vacuum is 20 cm.

To find:

Focal length of the same mirror in water of refractive index 4/3.

Solution:

First of all , we need to know the definition of focus and focal Length:

• Focus is a point on the principal axis at which paraxial rays after reflection from a mirror meet.

• Focal length is the length between the pole of the mirror and the focus of the mirror.

Now, focal length of a mirror is independent of the medium in which the mirror has been kept.

$\sf{f = \dfrac{radius \: of \: curvature}{2} }$

$\boxed{ \implies \sf{f \propto { \mu}^{0} }}$

Here $\mu$ refers to the refractive index of the medium in which the mirror has been placed.

So, the focal length of the mirror in water will be same as that in vacuum and will be equal to 20 cm.