d(object and screen) = D = u + v
and d = v – u
Thus, d + D = 2v
∴ v = [(d + D) / 2] and u = [(D – d) / 2]
∴ m1 = (I1 / O) = (v/u) = [{D + (d/2)} / {D – (d/2)}] = [(D + d) / (D – d)]
m2 = (I2 / O) = (u/v) = [{D – (d/2)} / {D + (d/2)}] = [(D – d) / (D + d)]
∴ (I1 / I2) = [(D + d)2 / (D – d)2]
Hence, (I2 / I1) = [(D – d)2 / (D + d)2].