Question

when an object is placed 20 cm from a concave mirror,a real image magnified three times is formed. find focal length of the mirror and where must the object be placed to give a virtual image three times the height of the object​

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When an object is placed 20 cm from a concave mirror a real image magnified three times is formed.

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asked Aug 1, 2019 in Class X Science by navnit40 (-4,945 points)

When an object is placed 20 cm from a concave mirror, a real image magnified three times is formed. Find:

(a) The focal length of the mirror.

(b) Where must the object be placed to give a virtual image three times the height of the object?

reflection of light

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answered Aug 1, 2019 by priya12 (-12,637 points)

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.