When an object is placed at a distance of 25 cm from a mirror, the magnification is m1. The object is moved 15 cm away with respect to the earlier position, magnification becomes m2, If m1m2=4, the focal length of the mirror is
u1=−25cm,u2=−(25+15)=−40cm
m1=−v1u1
m2=−v2u2
m1m2=v1v2×u2u1
4=v1v2.4025
v1=2.5v2
1u1+1v1=1f⇒1−25+12.5v2=1f....(1)
1u2+1v2=1f⇒1−40+1v2=1f....(2)
Divinding (2) by 2.5 , 1−100+12.5v2=1f......(3)
(1)−(3)⇒1−25+1100=1f−12.5f
−3100=1.52.5f
f=−20cm