Question

size of image of an object formed by a mirror having a focal length of 20cm, is observed to be reduced to 1/3rd of its size. at what distance of the object has been placed from the mirror? what is the nature of the image and the mirror ?

Answer 1

CASE 1:- m=-1/3(in case of concave mirror)

f= -20 cm
m= -1/3 =-v/u
v=u/3

1/f= 1/v +1/u .

=> -1/20 = 3/u + 1/ u= 4/u

therefore.,, u = -80 cm.

CASE 2:-

M=+1/3 (IN CASE OF CONVEX MIRROR)

f=+20 cm.

m=+1/3 =-v/u

v=-u/3.

i/f=1/v+1/u.
=> 1/20= -3/4 + 1/4 =-2/4

therefore, u= -40 cm....
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