PHYSICS · Multi-part · CBSE · Physics · Ray Optics and Optical Instruments · Ray Optics and Optical Instruments
Problem
In a laboratory exercise, an object and a screen are fixed on a common horizontal bench at a distance of 80cm from each other. A thin convex lens is moved between them and two distinct positions of the lens are found where sharp images of the object are obtained on the screen. The two positions of the lens are separated by 20cm.
(a) Draw a neat, labelled ray diagram to show why two lens positions are possible.
(b) Derive an expression for the focal length f of the lens in terms of the fixed object–screen separation D and the distance d between the two lens positions.
(c) Using the values D=80cm and d=20cm, calculate the focal length of the lens.