What is meant by power of a lens ? What is one dioptre ?

Explanation

Power of a lens is its ability to converge or diverge the light rays. A lens of small focal length converges or diverges the rays through a large angle and hence possess more power.
Power of a lens in SI is measured by dioptre. Power of a lens in dioptre (D) is the reciprocal of focal length of lens in metres.
`P=1/f`
where P is in dioptre and f in metres.
Powers are added algebraically. If lenses of power `P_(1),P_(2),P_(3)....` are placed in contact, their combined power is :
`P=P_(1)+P_(2)+P_(3)+....`