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
cm
3​

Answer 1

Answer:

4)80 cm

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Explanation:

although i have chosed without knowing it

Answer 2

The focal length of the mirror in water medium will be 20 cm (1).

Given:

Focal length of the mirror $=20cm$

Refractive index of water $=\frac{4}{3}$

To find:

Focal length of the mirror when placed in water.

Explanation:
In mirrors, the ray of light incident on the surface is reflected back at the same angle and does not enter the medium of the mirror hence, no deviation in the path of light is observed.

We know, the focal length of the mirror is halve the radius of curvature of the lens.

                  $f=\frac{R}{2}$

This is to say that the incident ray or the angle of incidence does not depend upon the medium in which the mirror is placed.

The incident ray hits the periphery of the mirror and gets reflected back both of which lie inside the water medium.

In lenses, on the other hand, the incident ray enters the material of the lens, which is primarily glass (refractive index = 1.5) and when leaves the glass, faces water of refractive index 1.33.

This change in the refractive indices at the interfaces of the two mediums results in the refraction or deviation or change in path of the refracted light from original.

Thus, the focal length of lenses depends on the refractive index of water but mirrors does not.

Hence,

The focal length of the mirror $=20cm$                                      

Final answer:

Hence, the focal length of the mirror in water medium will be 20 cm (1).