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# CBSE Class 12-science Answered

<div> <p class="MsoNormal"><span style="font-size: 14.0pt; mso-bidi-font-size: 11.0pt; line-height: 115%;">In a plano-convex lens, if the Convex side is perfectly covered with a silver foil, how to calculate the focal length of the system.</span></p> </div>
Asked by Malavika Umesh | 05 May, 2015, 12:55: PM
When light from an object falls on the plano convex lens silvered at the convex surface, there are three optical processes involved:

First: Light gets refracted at the plane surface of the lens,

Second: Light gets reflected from the silvered convex surface which now acts as a concave mirror, and

Third: Light after reflection undergoes refraction from the lens back to air.

Hence, the total focal length of the system will be given as a cumulative effect of the three optical processes involved.

here, f is the focal length of the lens and fm is the focal length of the mirror.

This is how we can calculate the focal length of the system.

This expression holds true for any lens with any surface mirrored.
Answered by Romal Bhansali | 06 May, 2015, 06:31: PM

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