Question

what is meant by power of a lens.write si unit.A student uses a lens of of focal lens 40 cm and another of -20 cm.write nature and power of each lens

Answer 1

Power is reciprocal of focal length. It's SI unit is Dioptre. Nature of lens of focal length 40 cm is a convex lens and its power is 100/40=2.5D . Nature of lens of focal length -20 cm is concave lens and its power is 100/-20= -5D