Define power of a lens. What is its unit? One student uses a lens of focal length 50 cm and another of -50 cm. What is the nature of the lens and its power used by each of them?
Power of a lens: The power of a lens is defined as reciprocal of its focal length. P = 1f f= focal length (in metre) The SI unit of power is ‘dioptre’. It is denoted by the letter D. Here, f= 50 cm = 0.5 m Power P = 1/f=1/0.5 = +2D f = – 50 cm = -0.5 m Power P = 1/f=1/0.5 = -2D
