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As shown in figure, a plano-convex lens of focal length 20 cm is silvered at its plane surface and it is made reflecting, then find new focal length of system. |
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Answer» 20 cm For first refraction from curved surface `therefore(1)/(f_1)`then reflection from SILVERED plane surface `therefore(1)/(F_M)`and then second refraction from curved surface `therefore(1)/(f_1)` where `f_1`= FOCAL length of plano convex lens = 20 cm `F_m `= focal length of plane mirror =`infty` Now, equivalent focal length, F can be obtained as follows. `1/F=(1)/(f_1)`[Plano convex lens] `+(1)/(F_M)` [Plane mirror] `+(1)/(f_1)`[Plano covex lens] `1/F=(2)/(f_1)+(1)/(f_m)=(2)/(20)+(1)/(infty)=(1)/(10)` `therefore`F=10 cm [Note that here `f_1` is positive. If it was concave `f_1` would be negative] |
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