# Minimum area of the disc to collect all light

A concave mirror of radius of curvature $$R$$ has a circular outline of radius $$r$$.

A circular disc is to be placed normal to the axis at the focus so that it collects all the light that is reflected from the mirror from a beam parallel to the axis.for $$r << R$$.

The area of the disc has to be atleast $$\text{__________ }$$.

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