when an object is placed 20 cm From a concave mirror , a real image magnified three times of fromed.Find: where must the object be placed to give a virtual image three times the height of the object?
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Explanation:
Given :-
u = -20 cm, m = -3, for the real image
(a) We know that
m = - (v/u)
∴ m = -3 = - (v/(-20))
⇒ v = -60 cm
We have
1/v + 1/u = 1/f
⇒ 1/(-60) + 1/(-20) = 1/f
⇒ 1/f = - (1/60) – 1/20 = (-1 – 3)/60 = - (1/15)
∴ f = - 15 cm
(b) For virtual image m = 3, and f = -15 cm
We know that
m = -(v/u)
∴ m = 3 = - (v/u) ⇒ v = -3u
We have,
1/v + 1/u = 1/f
⇒ 1/(-3u) + 1/u = 1/(-15)
⇒ (-1 + 3)/3u = - (1/15)
⇒ u = - (2 × 15)/3 = -10 cm
So object should be placed 10 cm from the concave mirror.
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