What is the unit of power of a lens? Define the unit of power of a lens.

Two lenses A and B have focal lengths of+ 20 cm and, -10 cm, respectively.

(a) What is the nature of lens A and lens B?

(b) What is the power of lens A and lens B?

(c) What is the power of combination if lenses A and B are held close together?

What do you understand by the power of a lens ? Name one factor on which the power of a lens depend.

A combination of lenses for a camera contains two converging lenses of focal lengths 20 cm and 40 cm and a diverging lens of focal length 50 cm. Find the power and focal length of the combination.

Two lenses A and B have power of (i) + 2 D and (ii) - 4 D respectively. What is the nature and f length of each lens ?