Question

what is meant by power of a lens write is its SI unit a student uses a lens of focal length 40 cm and another of minus 20 CM write the nature and power of each lens​

Answer 1

Power of lens is defined as the ability of eye lens to converge and diverge the rays of light

Its SI unit is dioptre

The student using the eye lens of 40cm is using a convex lens and the power is 1/0.4m=2.5D

The student using the eye lens of -20 cm is using a concave lens and the power is 1/0.2m=5D