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Define focal length of a mirror and hence relate focal length and radius of curvature of a mirror. |
Answer» Solution : Let us consider a concave MIRROR of pole P, center of curvature C and FOCUS F. Let the focal length f and radius of curvature R. AM is incident ray, MF is reflected ray and MC is normal From `DeltaMCP, tan THETA= (MP)/(CP)` From `Delta MFP, tan 2THETA = (MP)/(FP)` Since `theta` is very SMALL `tan theta ~~ theta and tan 2theta ~~ 2 theta` `therefore = (MP)/(CP)and 2 theta = (MP)/(FP) or theta = (MP)/(2FP)` `therefore CP= 2FP therefore R= 2f or r = (R )/(2)` |
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