Question

An object placed 20 cm in front of a mirror is found to have an image 15 cm (a) in front of it, (b) behind the
mirror. Find the focal length of the mirror and the kind of mirror in each case.
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Answer 1

Explanation:

(a) u = -20cm, v = -15cm

We know that

1/v + 1/u = 1/f

⇒ 1/-15 + 1/-20 = 1/f

⇒ 1/f = - (1/15) – 1/20 = (-4 – 3)/60 = - (7/60)

∴ f = - (60/7) cm

The mirror is a concave mirror.

(b) u = -20cm, v = 15cm

We know that

1/v + 1/u = 1/f

⇒ 1/15 + 1/-20 = 1/f

⇒ 1/f = 1/15 – 1/20 = (4 – 3)/60 = 1/60

∴ f = 60 cm

The mirror is a convex mirror.

Answer 2

Answer:

20

Explanation:

1/f=1/v-1/u

substituting the values