CBSE Class 10CBSE
Share
Notifications

View all notifications

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

Login
Create free account


      Forgot password?

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.

  Is there an error in this question or solution?

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.
S
View in app×