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?

Focal length of lens A = +20 cm = 0.20m

Power of lens A = 1/f = 1/0.20 = +5D


Focal length of lens B = - 10 cm = -0.1 m


Power of lens B = 1/f = 1/-0.1 = -10 D


(a) Lens A is convex and Lens B is concave


(b) Power of lens A = + 5 D; Power of lens B = -10 D


(c) power of combination = P1+ P2 = 5 D -10D = -5D


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