Question

A point source is located at a distance of 20 cm from the front surface of a symmetrical glass biconvex lens. The lens is 5.0 cm thick and the curvature radius of its surfaces is 5.0 cm. How far beyond the rear surface of this lens is the image of the source formed?

Answer 1

As the given lense has significant thickness, the thin lense, formula cannote be used. 

For refraction at the front surface from the formula

On simplifying we get , s' = 30 cm. 

Thus the image l' produced by the front surface behaves as a virtual source for the rear surface at distance 25 cm from it, because the thickness of the lense is 5 cm. Again from the refraction formula at cerve surface

On simplifying we get, s' = 30 cm.

Thus we get a real image l at a distance 6- 25 cm beyond the rear surface (Fig.).