Concepts of Physics

IIT JEE Physics (1978-2018: 41 Years) Topic-wise Complete Solutions

Image formed by a Convex Lens

Introduction

In this activity we will use the convex lens of a magnifying glass. First find the approximate focal length (\(f\)) of the lens. Then fix it vertically on a stand. Draw a long straight line on a table and place the lens stand on it. The principal axis of the lens should be parallel to and exactly above the line on the table. On one side of the stand mark the points \(F_1\) and \(2F_1\) on the line, at distance \(f\) and \(2f\) respectively from the lens. Similarly, mark \(F_2\) and \(2F_2\) on the other side of the stand.

Apparatus

a convex lens, screen, lens stand, candle, scale

Procedure

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Make a small screen of stiff paper and fix it on the stand. The screen should be vertical, with its centre at about the same height as the principal axis of the lens. Place the screen stand on the line drawn on the table.

Light a candle and place it on the line such that the lens is between the candle and the screen. The flame of the candle should be at about the same height as the principal axis of the lens. Move the screen back and forth till you see a sharp image of the flame on the screen. Repeat this by placing the candle at different positions. In each case note the size of the image. Try to see the image properties for different locations of the object. In which cases are the image not formed on the screen?