Question

A convex lens has a focal length of 10cm. At what distance from the lens should the object be placed so that it forms real and inverted image 20cm away from the lens? What would be the size of the image formed if the object is 2cm high? With the help of ray diagram show the formation of the image.

Answer 1

Focal length=f=10cm

Image distance=v=20cm

Using sign convention,

f is positive,v is also positive

Lens formula,

1/f=1/v-1/u

1/10=1/20–1/u

1/u=1/20–1/10

1/u=(10–20)/200

1/u=-10/200

u=-20

Object distance=20cm(minus indicates that the lens is converging lens)

hi=height of image and ho=height of object

hi/ho=-v/u

hi/2=-20/20

hi=2cm