What is meant by power of a lens? Give its SI unit. When two or more lenses are placed in contact, what will their combined power?
Power of a lens is defined as the ability of the lens to bend the rays of light incident on it. i.e., P = 1/f
The SI unit of power is called a dioptre. If $P_{1}$, $P_{2}$ are the powers, then P of the combination is given by
P = $P_{1}$ + $P_{2}$.