Question

A point object O is kept at origin. When a concave mirror M1 placed at x = 6 cm, image is formed at infinity. When M1 is replaced by another concave mirror M2 at same position, image is formed at x = 30 cm, then ratio of the focal length of M1 to that of M2 is (A) 3/4 (B) 4/3 (C) 5 (D) 1/5

Answer 1

When M1 is placed, the object is placed at 6cm the image formed is at infinity

Therefore, Focal length of M1 = F1 = -6cm

In the second case,

u = -6cm

v = -30cm

Using mirror formula,

1/f = 1/v + 1/u

f = uv/(u+v) = 180/(-36) = -5cm

F2 = -5cm

Therefore,

Ratios of focal length = -6:-5 = 6:5

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Answer 2

Answer:

c

Explanation: