Question

An object of height 4 cm is placed at a distance of 30 cm from the optical centre 'O' of a convex lens of focal length 20cm.Draw a ray diagram to find the position and size of the image formed. Mark optical Centre 'O' and principal focus 'F' on the diagram, also find the approximate ratio of size of the image to the size of the object.

Answer 1

1/v-1/u=1/f

1/v=1/f+1/u=1/20-1/30

3-2/60=1/60

h'/h=v/u=h'/4=60/-30=h'=-8 cm

Ratio=Height of image / Height of object

=-8/4=-2:1 or

m= - 2

So the image formed is real, inverted and enlarged.