A concave mirror initially gives magnification −4. The object is moved away until magnification is −2. If the object-image separation decreases by 54 cm, find |f| in centimetres.
Physics · Geometrical optics · spherical mirror
JEE Mains NEET

Question

A concave mirror initially gives magnification −4. The object is moved away until magnification is −2. If the object-image separation decreases by 54 cm, find |f| in centimetres.

G1 Physics · Accepted Answer

For M=4, separation=v−u=f[(M+1)−(1+1/M)]=f(M−1/M)=15f/4. For M=2 it is 3f/2. Decrease=(15/4−3/2)f=9f/4=54, so f=24 cm.