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

G1 Physics · Accepted Answer

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