Power of Combination of Lenses
Power of Combination of Lenses
Power of Lens:
The power of a lens is defined as the reciprocal of its focal length.
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.
.
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:
Let us take the case of two lenses of focal length and
. The power of these lenses be
and
. Then
This equation is often used in deciding the power of lens to be prescribed for a person suffering a vision defect
Illustration: If by using two lenses of power +1.5 D and +1.0 D in contact the person can see properly, then find power of the lens prescribed to the person.
Solution: Given power of two lenses
then power of the combination will be:
A combination of lens power +2.50 D is prescribed to the person.
Uses of Combination of Lenses:
1. The use of combination of lenses increases the sharpness of image, the image produced is also free from many defects.
2. The additive property of the powers of the lenses can be used to design lens systems to minimise certain defects in images produced by a single lens. Such a lens system, consisting of several lenses in contact, is commonly used in designing of camera lenses and the objectives of microscopes and telescopes.
Which of the following are correct ? (a) 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 (b) The S.I. unit of power is diopter (D). (c) The combination of lens is used in designing of camera lenses and the objectives of microscopes and telescopes. | |||
| Right Option : D | |||
| View Explanation | |||
Combination of lens is used in _____________________. | |||
| Right Option : D | |||
| View Explanation | |||
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 | |||
| View Explanation | |||
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