1
You visited us 1 times! Enjoying our articles? Unlock Full Access!
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?

Open in App
Solution

## (a) Power of convex lens P1 = + 5 D Power of concave lens P2 = - 7.5 DThe combined power is the algebraic sum of the individual powers of the lenses.∴ Power of the combinationP = P1 + P2P = 5 D - 7.5 D = - 2.5 DTherefore, the power of the combination of the lenses is - 2.5 D.(b) The power and focal length are related as:P=1f−2.5=1f∴f=−12.5=−0.4m=−40cm.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.

Suggest Corrections
12
Join BYJU'S Learning Program
Related Videos
Power of Accommodation and Defects
PHYSICS
Watch in App
Explore more
Join BYJU'S Learning Program