Question

The power of a combination of two lenses X and Y is 5 D . If the focal length of lens X be 15 cm :
a) calculate the focal length of lens Y.
b) State the nature of lens Y .

Answer 1

Focal length of lens X = 15 cm = 0.15 m

Power of lens X
= 1/0.15
= 100/15
= 20/3
= +6.66 D

Power of Combination of lenses X & Y
= power of lens X + power of lens Y

5 = 6.66 + Power of lens Y

Power of lens Y
= 5-6.66
= -1.66 D
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(a) Focal length = 1/Power

f = 1/(-1.66)
f = -0.6 m
f = -60 cm
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(b) As the focal length of lens Y is negative hence the nature of lens is Diverging lens, that is Concave lens.
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Answer 2

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