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# A Convex Lens of Power 5 D and a Concave Lens of Power 7.5 D Are Placed in Contact with Each Other. What is the : (A) Power of this Combination of Lenses? (B) Focal Length of this Combination of Lenses? - CBSE Class 10 - Science

ConceptRefraction at Spherical Surfaces and by Lenses Power of a Lens

#### Question

A convex lens of power 5 D and a concave lens of power 7.5 D are placed in contact with each other. What is the :
(a) power of this combination of lenses?
(b) focal length of this combination of lenses?

#### Solution

(a) Power of convex lens P1 = + 5 D
Power of concave lens P2 = - 7.5 D
The combined power is the algebraic sum of the individual powers of the lenses.
∴ Power of the combination P = P1 + P2
P = 5 D - 7.5 D = - 2.5 D
Therefore, the power of the combination of the lenses is - 2.5 D.

(b) The power and focal length are related as:

p=1/f

-2.5=1/f

∴ f=-1/2.5=-0.4m=-40 cm

Therefore, the focal length of the combination of the lenses is 40 cm. The negative sign shows that the combination of the two lenses causes it to behave like a concave lens.

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#### APPEARS IN

Solution A Convex Lens of Power 5 D and a Concave Lens of Power 7.5 D Are Placed in Contact with Each Other. What is the : (A) Power of this Combination of Lenses? (B) Focal Length of this Combination of Lenses? Concept: Refraction at Spherical Surfaces and by Lenses - Power of a Lens.
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