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What do you mean by power of a lens ? Define its unit. |
Answer» Solution : Power of a lens is a measure of the degree of convergence and divergence of light FALLING on it. MATHEMATICALLY, the power (P) of a lens is defined as the tangent of the angle by which it CONVERGES or diverges a light beam falling at UNIT distant from the optical centre C. As shown in Fig. 9.40, `tan delta = h/f` and if h=1, then `tan delta = 1/f`, Thus, power `P = 1/f` The SI unit of power is 1 dioptre (1 D). 1 dioptre is the power of a lens of focal length 1 m. Power of a converging (convex) lens is taken as +ve and that of a diverging (concave) lens as -ve. |
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