Question

The image of an object placed on the principal axis of a concave mirror of focal length 12 cm is formed at a point which is 10cm more distance form the mirror than the object. The magnification of the image is
A. 1.5
B. 2
C. 2.5
D. 3

Answer 1

Correct Answer - A
`u=-x,v=-(10+x),f=-12`
`(1)/(v)+(1)/(u)=(1)/(f)implies(1)/(-(10+x))+(1)/(-x)=(1)/(-12)`
`(x+10+x)/((10+x)x)=(1)/(12)`
`24x+120=x^2+10x`
`x^2-14x-120=0`
`(x-20)(x+6)=0`
`x=20cm`
`u=-20cm`, `v=-(10+20)=-30cm`
`m=-(v)/(u)=-(-30)/(-20)=(3)/(2)=1.5`