Question

If you don’t know the focal length of a lens, you can estimate it by holding it just above the floor until overhead ceiling lights get focused onto the floor. The ceiling lights are 7.62 meters above the lens, and the lens has focal length 0.158 cm.

What height h off the floor should the lens be to make a clear image of the ceiling lights on the floor?

Answer 1

Ceiling light is 7.62 meters above the lens (u = -7.62 m). Focal length is 0.158 cm (f = 0.158 cm). Since object distance is very large as compared to focal length (7.62 cm >> 0.158 cm), we can take object distance as infinity. Now we know that the image of an object placed at infinity, gets formed at the focus. Hence we need to place the lens at a height equal to its focal length, h = 0.158 cm off the floor to make a clear image of the ceiling lights.