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  4. A plano-convex lens (f = 20 cm) is silvered at plane surface. Now focal length will be

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Q. A plano-convex lens $(f = 20 cm)$ is silvered at plane surface. Now focal length will be

COMEDKCOMEDK 2010Ray Optics and Optical Instruments

Solution:

The focal length of the plano-convex lens is 20 cm with the plane surface being silvered.
So, the net power of the lens is given by
$ P = \frac{1}{f} \times 2 = \frac{2}{f}$
Thus, the net focal length is $ \frac{f}{2}=\frac{20}{2}=10\, cm $