• In this figure I is the image of the object O.
• From ΔOPI,

           δ = ∝ + β                    …(1)

• For paraxial rays ∝ and β are very small.
• So, ∝ ≈ tan ∝, and β ≈ tan β
• From ΔOPL

           ∝ ≈ tan ∝ = PL / OL

or       ∝ = h / (−u)                  …(2)

• From ΔPIL,

           β ≈ tan β = PL / LI

or      β = h / v                        …(3)

• From equations (1), (2) and (3), we get

        …(4)

• For thin lens, lens formula is

                            …(5)

• From equations (4) and (5), we get

• Since the value of δ does not depend on ∝ and β.
• Therefore all the rays incident at P would suffer the same deviation.