1.

A concave mirror gives a real image magnifies 4 times. When the object is moved 3 cm the magnification of the real image is 3 times. Find the focal length of mirror.

Answer» We know that, ` m=(v)/(u)=(f)/(u-f)`
In first case, ` (f)/(u-f)=4`
`F=4u-4f` and `5f=4u`
In `2^(nd)` case, `(f)/((u+3)-f)=+3`
`F=3u+9-3f`
`:. 4u-4f=3u+9-3f`
`u=f+9=(4u)/(5)+9`
`u=45 cm` and `f=36 cm`


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