A convergent lens is placed 40 ~cm to the right of a diverging lens of focal length 15 ~cm . A parallel beam of light enters the divergent lens from the left and the beam is again parallel when it emerges from the convergent lens. The focal length of the convergent lens is

Step-by-step solution

We are given with following arrangement, From diagram it is clear that focus of lens 1, point F₁ must be focus of lens 2 also. In this way, a parallel beam, of light becomes again paralkel after passing through system of lenses. Hence, distance F₂ L₂ , F₂ L₂=15+40=55 ~cm So, focal length of convex lens is 55 ~cm .