Question

A convex lens forms a real and inverted image of a needle at a distance of 25cm from the lens .if the image is of the same size as that of the needle,then where should the needle be placed in front of the lens.also calculate the power of the lens,

Answer 1

v=25 cm

given that image is same size as that of object so object distance will be equal to image distance

u= -25 cm

lens formula

1/f=1/v-1/u

1/f=1/25+1/25

f=25/2

power of lens P=1/f

P=100/(25/2)

P=200/25= + 8 D

hope this helps

Answer 2

Explanation:

Object distance,v=25cm

Image distance,u= -25cm

Using lens formula,

1/f=1/v-1/u

1/f= 1/25+ 1/25

1/f=2/25

f= 25/2 cm

Power of lens,P=1/f

P=100/f

=100/(25/2)

=200/25

=+8 D