Question

size of an image by mirror having a focal length of 20 cm is observed to he reduced to 1/3 of its size at what distancr the object is place from the mirror. what is the nature of the image and the mirror
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Answer 1

Answer:-80cm

Explanation:

Usually Magnification Formula:

m=‐v/u=1/3=v=‐u/3

From Mirror Formula:

1/v+1/u=1/f

From About To Equation:

3/‐u+1/‐u=1/20

u=-80cm

so, the object is placed at the distance of 80 CM in front of the mirror.

Answer 2

Answer:

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.