Question

Explain an experimental method to determine the focal length of a convex lens by obtaining the image of a distant object. Why this method cannot be used to determine the focal length of a concave lens?

Answer 1

1. Experiment Method:

Parallel rays from a distant object pass through a convex lens and converge at its focal plane to form a sharp, real, and inverted image. To find the focal length, place a metre scale horizontally with the '0' mark touching a white wall. Move the lens along the scale until a clear image of a distant object forms on the wall. The distance between the lens and the wall gives the focal length (f).

Ray Diagram Explanation:

Light rays coming from infinity pass through the convex lens. After refraction, these parallel rays converge and meet precisely at point (Q) on the wall. This distance from the optical center (O) to the wall is equal to the focal length (f).

2. A concave lens is a diverging lens that spreads light rays outward. It forms a virtual image that cannot be caught on a physical wall.