Question

Calculate the focal length of a convex lens which produces a virtual image at a distance of 50 cm of an object placed 20 cm in front of it.

Answer 1

Given:
Object distance, u = $-$20 cm
Image distance, v = $-$50 cm (Image is virtual.)
Applying lens formula, we get:
1/v $-$ 1/u = 1/f (f = focal distance)
1/($-$50) $-$ 1/ ($-$20) =1/f
or, 1/f = (-2+5)/100
or, 1/f =3/100
or, f = 100/3 = + 33.3 cm
Thus, the focal length of convex lens is 33.3 cm.