What is meant by the power of a lens? Give its $SI$ unit. When two or more lenses are placed in contact,what will be their combined power?

(N/A) The power of a lens is defined as the measure of the degree of convergence or divergence of light rays falling on it. It is mathematically defined as the reciprocal of its focal length in meters.
i.e.,$P = 1 / f$ (where $f$ is in meters).
The $SI$ unit of power is the dioptre $(D)$.
When two or more thin lenses are placed in contact,the total power $(P)$ of the combination is the algebraic sum of the individual powers of the lenses:
$P = P_1 + P_2 + P_3 + ...$