Question

Focal length of two lenses are 20 and - 15 find the focla length and power of combination

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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