Chapter 30, Problem 24A

To determine

The reason why a magnifying glass has often been called a “burning glass”.

Explanation of Solution

Introduction:

A converging lens behave as a magnifying glass when an object is kept at the distance less than the focal length.

Magnifying glass is a converging lens which produces a virtual magnified image of an object placed at a distance less than the focal length of the lens. However, for any object placed beyond the focal point of the converging lens, it converges the incident rays at a point forming a real image. Since, the sun is at large distance from the earth, the rays from the sun incident on any converging lens are parallel in nature and lens converges the parallel incident rays on the lens at focal point on the other side of the incident rays. Any surface kept at the focal point of the lens acts as a screen and the since the rays from the sun have enough energy, sunrays converged at a point on the surface of the substance can heat it. Thus, named as “burning glass”

Conclusion:

Converging lens converges the parallel incident rays from the sin on the focal point and thus can burn the surface of any substance kept at the focal point of the lens.