If you want to hit a distant stationary object with arifle, you have to adjust the aiming hole pipe for the mark of corresponding approximate distance. What is the need for it. Why don't you use a single adjustment for firing objects at different distances

A sharp image of a distant object is obtained on a screen by using a convex lens. In order to determine the focal length of the lens, you need to measure the distance between the :

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?