1.

find the position of an object which when placed in front of a concave mirror of a focal length 20 cm produces virtual image which thrice the size of object​

Answer»

Answer :-

Position of the object is 13.33 CM from the mirror .

EXPLANATION :-

We have :-

→ FOCAL length = 20 cm

→ Size of image is thrice the object .

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From the question, we have :-

• hᵢ = 3hₒ

Now, from the formula of Magnification of spherical mirror :-

m = -(v/u) = hᵢ/hₒ

⇒ -(v/u) = 3hₒ/hₒ

⇒ -(v/u) = 3

⇒ v/u = -3

⇒ v = -3u

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According to sign convention :-

• f = -20 cm

Now, let's calculate the position of the object by using the mirror formula .

1/v + 1/u = 1/f

⇒ 1/(-3u) + 1/u = 1/(-20)

⇒ -1/3u + 1/u = -1/20

⇒ (-1 + 3)/3u = -1/20

⇒ 2/3u = -1/20

⇒ 3u(-1) = 2(20)

⇒ -3u = 40

⇒ u = 40/(-3)

⇒ u = -13.33 cm

[According to sign convention, 'u' is always -ve for a CONCAVE mirror.]


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