Power of Combination of Lenses


 
 
Concept Explanation
 

Power of Combination of Lenses

Power of Lens: 

The power of a lens is defined as the reciprocal of its focal length.

P=frac{1}{f} 

Here SI unit for the focal length is meters. The S.I. unit of power is diopter(D).

Power of Combination of Lens:

When two or more thin lenses are kept in contact with each other such that they have the same principal axis, then the power of the combination is treated as a single thin lens.The power of this combination is equal to the algebraic sum of powers of individual lenses i.e.

    P=P_1+P_2+.......P_n.

When two or more thin lenses are used in combination, the equivalent focal length (f) can be calculated as the reciprocal of the resultant power of the combination (P). The focal length F of the equivalent single lens is given by:

F=frac{1}{P}

Let us take the case of two lenses of focal length f_1 and f_2 . The power of these lenses be P_1 and P_2. Then

P=P_1+P_2

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Sample Questions
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Question : 1

The SI unit of power is ____________________ .

Right Option : C
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Explanation
Question : 2

Power of a lens is defined as ______________________.

Right Option : B
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Explanation
Question : 3

Which of the following are correct  ?

(a) The SI unit for the focal length is kilometers.

(b) The SI unit of power is diopter (D).

(c) The power of this combination is equal to the algebraic sum of powers of individual lenses .

Right Option : B
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Explanation
 
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