Question

An object is placed first at infinity and then at 20 cm from the object side focal plane of a convex lens. The two images thus formed are 5 cm apart the focal length of the lens is
A. 5 cm
B. 10 cm
C. 15cm
D. 20 cm

Answer 1

Correct Answer - B
`(1)/(v)-(1)/(-(20+f))=(1)/(f)`
`(1)/(v)=(1)/(f)-(1)/(20+f)=(20)/(f(20+f))`
`v=(f(20+f))/(20)`
`(f20+f)/(20)-f=5`
`f^2=100impliesf=10cm`