A concave mirror forms a real image with magnification −3. When the object is moved 10 cm away from the mirror, magnification becomes −2. Find |f| in centimetres.
Physics · Geometrical optics · spherical mirror
JEE Mains NEET

Question

A concave mirror forms a real image with magnification −3. When the object is moved 10 cm away from the mirror, magnification becomes −2. Find |f| in centimetres.

G1 Physics · Accepted Answer

For a real image with magnitude M, u=f(1+1/M). Thus u₁=4f/3 and u₂=3f/2. Their difference is f/6=10 cm, giving f=60 cm.