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 will happen if you use a concave lens in this experiment?

Explanation

If `""_(g)n_(a)` =1.5 and `R_(1)=R_(2)=R`, then f=R . If a concave lens is used in the above experiment , no image will be formed on the screen.