Question

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

Answer 1

As  image could be real or vitural i.e. inverted or erect. 

Image is real inverted. Mirror is concave. 

Answer 2

lets see the answer in cases!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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