An object moves toward a concave mirror at 2 cm/s. At an instant the real image moves at 18 cm/s and the object-image separation is 40 cm. Find |f| in centimetres.
Physics · Geometrical optics · spherical mirror
JEE Mains NEET

Question

An object moves toward a concave mirror at 2 cm/s. At an instant the real image moves at 18 cm/s and the object-image separation is 40 cm. Find |f| in centimetres.

G1 Physics · Accepted Answer

Speed ratio 9 gives |m|=3, hence v=3u. Separation=2u=40 gives u=20, v=60 and f=uv/(u+v)=15 cm.