Question

16. A concave mirror gives a real image magnified 4 times. When the object is moved 3
magnification of the
real image is 3 times. Find the focal length of mirror.
A) 18 cm
B) 24 cm
C) 36 cm
D) 48 cm
​

Answer 1

Answer:

f = 36 cm

Explanation:

m = 4, v = 4u

when u’ = (u + 3), then m = 3

- 1/4u - 1/u = 1/f  ⇒ f = 4u/5

-1/3(4 + 3) - 3/3(4 + 3) = 1/f  ⇒ f = -3(4 + 3)/3 4u/5 = 3(4 + 3)/3

⇒ 164 = 15(u + 3) ⇒ u = 45

1/-4 x 45 - 4/4 x 45 = 1/f

f = 36 cm

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