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Q.
If image formed by both mirror coincide then what is the focal length of convex mirror?
Ray Optics and Optical Instruments
Solution:
Image should be formed $4 \,cm$ behind the mirror
$\Rightarrow \frac{1}{v}+\frac{1}{u}=\frac{1}{f} $
$\Rightarrow \frac{1}{4}+\frac{1}{-20}=\frac{1}{f} $
$\Rightarrow f=5\, cm$
