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Question:

An object of height 4.0cm is placed at a distance of 30cm 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.

Solution:

Object distance u = -30 cm
Focal length of convex lens f = 20 cm
Height of object ho = 4 cm
Using lens formula :
1/v - 1/u = 1/f
→ 1/v - 1/(-30) = 1/20
→ v = 60 cm
Magnification m = hi/ho = v/u
→ hi/4 = 60/(-30)
→ hi = -8 cm
Ratio of size of image to that of object = |hi|/|ho| = 8/4 = 2