Question

19. An object is placed at 12 cm distance from the pole of concave mirror and the image formed at 4 cm from the
pole. Find the focal length of the mirror? Is the focal length of the mirrors are same in all mediums?
Guess. ​

Answer 1

Solution: By applying 1/f = 1/v + 1/u we get

→ 1/f = 1/(- 4) + 1/(- 12)

→ 1/f = - 1/4 - 1/12

→ 1/f = (- 3 - 1)/12

→ 1/f = - 4/12

→ Focal Length = - 3

Hence focal length of mirror is 3 cm (negative sign shows focal length of concave mirror is at negative position)

1. No, focal length is not same in all mediums
2. It is inversely proportional to refractive index of medium.

Answer 2

Answer:

Explained

Explanation:

u (object) = -12 cm

v(image) = -4 cm

so the focal length is

1/v-1/u = 1/f

or -1/4 -1/12

or -3-1/12

or -4/12 = 1/f

or f = 12/-4

or f = -3 cm

no it depends upon the medium

the refractive index of the surrounding medium

and the thickness of the lens also