Question

An object placed 20 centimetres in front of the convex mirror is found to have an image

15 centimetres behind the mirror. Find the focal length of the mirror
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Answer 1

Answer:

) 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

Explanation:

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Answer 2

Answer:

(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