Question

A lens is placed between a source of light and a wall. if it forms images of area a1 and a2 on the wall for its two different positions then the area of the source of light is

Answer 1

According to question

Area of first image = a1
and
Area of second Image = a2

Thus area of the source light  = resultant of the area of the two images
                                               = root [ a1^2 + a2^2 + 2a1a2 ]
                                               = root [(a1+a20^2]
                                               = a1+a2

Thus the area of the source image is a1 + a2