Q25 of 284 Page 262

A convex lens of power S D and a concave lens of power 7.5 D are placed in contact with each other. What is the:

(a) power of this combination of lenses?


(b) focal length of this combination of lenses ?

Power of convex lens = 5 D

Power of concave lens = -7.5 D


(a) Power of the combination of the lens = P1+ P2 = 5 – 7.5= -2.5 D


(b) P = 1/f


f = 1/P = 1/ -2.5 = -40 cm


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23

An object of height 4 cm is placed at a distance of 1S cm in front of a concave lens of power, -10 dioptres. Find the size of the image.

 24

An object of height 4.2S mm is placed at a distance of 10 cm from a convex lens of power +S D. Find (i) foe length of the lens, and (ii) size of the image.

 26

A convex lens of focal length 2S cm and a concave lens of focal length 10 cm are placed in close contact with one another.

(a) What is the power of this combination?


(b) What is the focal length of this combination?


(c) Is this combination converging or diverging ?

 27

The power of a combination of two lenses X and Y is S D. If the focal length of lens X be 1S cm :

(a) calculate the focal length of lens Y.


(b) state the nature of lens Y.