Question

Write down in four steps how to find out the focal length of a convex lens experimentally in the Lab.

Answer 1

Hint : We know that when the rays from the object incident on the lens are parallel, then the image is formed at the focus. So we have to use a distant source as the object, which can be approximately assumed to be at infinity. Since a concave lens forms a real image, so the image is obtainable on the screen.

Complete step by step answer
For the experimental determination of the focal length of a convex lens, we need to have a horizontal bench, a convex lens fitted on a lens holder, a screen, and a measuring scale.
Following are the steps to find out the focal length of a convex lens:
-We need to arrange the lens and the screen on the horizontal table with the lens facing a distant object, such a tall building or a tree.
-We need to arrange the relative position of the lens and the screen so that a sharp image of the distant object is obtained on the screen.
-With the help of the measuring scale, we need to measure the distance between the lens and the screen, which is equal to the focal length of the lens.
-On repeating the above steps for other distant objects to get two more readings and taking the mean of these three readings we get the final average focal length of the convex lens.

Note
The screen and the lens must be normal to the bench. The distant object must be well illuminated for obtaining a clear image. The bench used should be perfectly horizontal.