A lens is placed between a source of light and a wall. It forms images of area A1  and A2 on  the wall for its two different positions. The area of the source of light is

Let say the length and breadth of object is a and b .Area of object  = a × b   In first position of displacement method,   Area of Image1 , A1= m1a×m1b → (1)In second  position of displacement method,   Area of Image2 , A2= m2a×m2b → (2) Multiply equation 1 and 2  A1A2 = (m1m2)2 (ab)2Also, in displcement method , m1m2 = 1   ⇒A1A2 = (ab)2⇒ab  =A1A2 ⇒Area of object = A1A2