Question

A student focused the image of acandle flame on a white screen using a convex lens he noted down the position of candle, screen and the lens as under. Position of candle=12cm , position of convex lens=50cm, position of screen=88cm what is the focal length of convex lens

Answer 1

Subtract 50 from each value given so that the convex lens is placed on the origin
u = - 12 - 50 = - 62 cm
v = 88 - 50 = 38 cm
1/v - 1/u = 1/f
1/38 - 1/-62 = 1/f
1/38 + 1/62 = 1/f
62/2356 + 38/2356 = 1/f
100/2356 = 1/f
f = 2356/100 = 23.56 cm
Focal length of the Lens is 23.56 cm.