If the magnification of a virtual image formed of an object 10cm from a convex lens is 3, then the focal length of the lens is

JAMB 1980

If the magnification of a virtual image formed of an object 10cm from a convex lens is 3, then the focal length of the lens is

A. 10cm

B. 15cm

C. 16cm

D. 19cm

E. 20cm

Correct Answer: Option B

Explanation

V = 3u, U = 10cm

V = 10 × 3 =30cm

Since, we are dealing with a vertual image, V = -ve,

F = UV/(U+V)

= (10×(−30))/(10−30)

= 15cm

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