The radii of curvature of both the surfaces of a lens are equal. How will its focal length and power change if one of the surfaces of the lens is made plane ?
Text Solution
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Let `R_(1) = R_(2) = R` From lens maker's formula `(1)/(f) = (mu - 1)((1)/(R_(1)) - (1)/(R_(2)))` `= (mu -1)((1)/(R_(1)) + (1)/(R_(2))) = (2(mu -1))/(R )` When one surface is made plane, `R_(2) = oo` `:. (1)/(f') = (mu -1)((1)/(R )) = (mu -1)/(R )` Clearly `f' = 2f`, i.e., focal length becomes twice. As `P prop (1)/(f)`, power becomes half.
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