Question

A concave mirror has a focal length of 20 cm. Find the position or positions of an object for which the imagesize is double of the object-size.

Answer 1

The positions are 10 cm or 30 cm from the concave mirror.

Comments

explain 1st case briefly

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Case 1: When image is real
v/u=-2x/x=-2
v= -2u
1/v+1/u=1/f=1/20
=> 1/u-1/2u=1/20
=>1/2u=1/20
=>u=10 cm