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 ?

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Solution

a)- For converging lenses, the focal length is always positive, while diverging lenses always have negative focal lengths.
so A is converging and B is diverging lens.

b) Pa = $\frac{1}{20} = 0.05 D$

Pb = $\frac{1}{- 10} = - 0.1 D$

c)-P=Pa+Pb
P=0.05-0.1=-0.05D

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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 ?

a)- For converging lenses, the focal length is always positive, while diverging lenses always have negative focal lengths. so A is converging and B is diverging lens. b) Pa = 120=0.05D Pb = 1−10=−0.1D c)-P=Pa+Pb P=0.05-0.1=-0.05D

a)- For converging lenses, the focal length is always positive, while diverging lenses always have negative focal lengths.
so A is converging and B is diverging lens.

b) Pa = $\frac{1}{20} = 0.05 D$

Pb = $\frac{1}{- 10} = - 0.1 D$

c)-P=Pa+Pb
P=0.05-0.1=-0.05D