devarshi-dt-logo

Question:

An object of height 2.5cm is placed at a distance of 15cm from the optical centre 'O' of a convex lens of focal length 10cm. Draw a ray diagram to find the position and the size of the image formed. Mark optical center 'O', principal focus F and height of the image in the diagram.

Solution:

Given : u = -15cm, F = +10cm, ho = 2.5cm
Using lens formula : 1/v - 1/u = 1/F
∴ 1/v - 1/(-15) = 1/10
⟹ 1/v = 1/10 - 1/15 = (3-2)/30 = 1/30
⟹ v = +30cm
Also, m = hi/ho = v/u
∴ hi/2.5 = 30/-15
hi = -5cm (- sign shows that image formed is inverted)