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Define power of lens, obtain its equation and write its SI unit. |
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Answer» <P> Solution :Power of a lens is a measure of the convergence or divergence, which a lens introduces in the light falling on it.Clearly a lens of shorter focal length bends the incident light more, while converging it in case of a convex lens and diverging it in case of a concave lens. The power P of a lens is defined as the tangent of the ANGLE by which it converges or diverges a beam of light falling at unit distant from the optical centre.  From figure `TAN delta=h/f` If h=1 `tan delta=1/f` and for small value of `delta" "tan delta=delta` `therefore delta=1/f` `therefore` Power P=`1/f` The SI unit for power of a lens is dioptre (D) `therefore1D=1m^(-1)` The power of a lens of focal length of 1 metre is one dioptre. Power of a lens is POSITIVE for a converging lens and negative for a diverging lens. Thus, when an optician prescribes a corrective lens of power + 2.5 D, the required lens is a convex lens of focal length + 40 cm. `f=1/P=(1)/(2.5)m` `thereforef=(1000)/(25)cm=+40 ` cm A lens of power of – 4.0 D means a concave lens of focal length f = `(1)/(-4)`m = - 25 cm By using lens maker.s formula, `P = 1/f = (n-1)(1/R_1-1/R_2)`power of lens can be obtained. |
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