Question

List four precautions a student should observe while determining the focal length of a given convex lens by obtaining the image of a distant object on a screen.

Answer 1

Hint: In this question we have been asked to list four precautions while determining the focal length of a convex lens. Therefore, to solve this question we shall first discuss the process of obtaining focal length by obtaining the image on the screen. In doing so, we shall discuss the necessary precautions that must be taken when obtaining the image on the screen.

Complete answer:
While trying to determine the focal length of the convex lens the lens is placed facing to the distant object as shown in the figure. Later, we shall place the lens and screen properly in the holder. Now, the position of the screen should be such that a sharp image of a distant object is obtained on it. Now, we know that the focal length of the convex lens will be given by the difference between the position of the lens and the screen. Now, in order to get accurate reading, repeat the process a couple of times by varying the position of distant objects.

Now, from the above process, we can say that the steps where we must take precautions are:
1) While placing the distant object, the object should be clearly visible.
2) The image on the screen should be sharp and well defined.
3) While measuring all the distances the scale should be kept parallel to the ground.
4) The meter should be properly placed between screen and centre of convex lens.

Note:
Focal length of a lens is a measure of how much the lens can diverge or converge the rays of light. It is the distance between the lens and the screen on which the image is obtained. A positive focal length indicates that the light rays will be converged by the lens. Similarly a negative focal length indicates divergence of the rays.