Question

9.
1. ₹ 30 प्रति दर्जन की दर से पेन्सिल बेचने पर दुकानदार को
₹10 का लाभ होता है, तो उसका प्रतिशत लाभ होगा
[
INV 2017, 2009]
(1120​

Answer 1

The approximate focal length of a convex lens can be determined by using the principle that a beam of parallel rays incident from a distant object is converged in the focal plane of the lens.

In an open space, against a white wall, a metre scale is placed horizontally with its 0 cm end touching the wall.

By moving the convex lens to and fro along the scale, focus a distant object on wall. The image which forms on the wall is very near to the focus of the lens and the distance of the lens from the image is read directly by the metre scale. This gives the approximate focal length of the lens.