Two lenses 1and 2 are placed in contact.Focal length of lens 1 is 20cm and of 2 is -10cm . Calculate i) Total Power of combination ii) What is the nature of combination.
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Given: Focal length of first lens = 20cm
Focal length of second lens = -10cm
To find: (a) Total power of combination
(b) What is the nature of combination.
Solution:
(a) The power of the lens is defined as the converging and diverging power of a lens. It is a reciprocal of focal length.
- It is inversely proportional to the focal length.
- The S.I. unit of power is dioptre (D).
P₁ = 1/f(in m)
= 1/0.2 = 50D
P₂ = 1/f
= 1/-0.1 = -10D
The total power of the combination is equal to the summation of the power of both the lens.
P = P₁ + P₂
= 50 - 10 = 40D
Therefore, the total power of the combination is 40D
(b) Since, the total power of the combination is positive, therefore the lens is converging.
- Therefore, the lens is a convex lens.
- The primary purpose of a converging lens is to focus the incoming rays from an object and converge them to form an image.
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