Question

size of image of as an object by a mirror
having a focal length of 20 com is observed to
be reduced to of the size at what distance
the object has been placed from the mirror
what is nature of the image and the
mirror
​

Answer 1

Answer:

-80 cm(concave),-40 cm(convex)

Explanation:

I think your ques is -

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 and size?

Ans -

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.

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

=> 1/20= -3/4 + 1/4 =-2/4

therefore, u= -40 cm.