Question

A real image one fifth of object at a distance of 18cm from a mirror what is the focal length and nature

Answer 1

Explanation:

a real image is 51th the size of the object, and it is formed at a distance of 18cm from the mirror. To find the focal length of the mirror and the nature of the mirror. magnification, m=−51 as the image formed is real hence it will be inverted. Since focal length is negative, therefore it is a concave mirror.

Answer 2

Answer:

Explanation:

Height of image=1/5×height of object

h'=1/5×h

h'×5=h

5h'=h

Image distance= -18cm

According to magnification formula,

-v/u=h'/h

-(-18)/u= -h'/5h'

18/u= -1/5

u= -18×5

u= -90cm

According to mirror formula,

1/f=1/v+1/u

1/f=1/(-18)+1/(-90)

1/f= -1/18×5/5+(-1/90)

1/f= -5/90-1/90

1/f= (-5-1)/90

1/f= -6/90

f= -90/6

f= -15cm.

The mirror is concave as the focal length is negative.