Try this
Material: Convex lens, screen, metre scale, stand for the lens etc.
Method:
Keeping the screen fixed, obtain a clear image of a distant object like a tree or a building with the help of the lens on the screen. Measure the distance between the screen and the lens with the help of the metre scale. Now tum the other side of the lens towards the screen. Again obtain a clear image of the distant object on the screen by moving the lens forward or backward. Measure the distance between the screen and the lens again.
What ls this distance between the lens and the screen called?

Explanation

The rays incident on a convex lens from a distant object always converge at its principal focus. Hence, the distance between the lens and the screen is called focal length.