Question

The powers of lenses used in the spectacles of a person are -2.5D and +2D respectively.State the focal length and nature of these lenses. ​

Answer 1

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For concave lens Power is -ve

P = 1/f

f = 1/-2.5

= -0.4m = -40cm

For Convex Lens Power is +ve

F = 1/2

= 0.5m = 50cm

Answer 2

Explanation:

power (p)= -2.5 D

focal length (f)=?

p=1/f

-2.5=1/f

by cross multiplication,

f=1/-2.5

f= -0.4 m

lens is concave.

power (p)=2D

focal length (f)=?

p=1/f

by cross multiplication

2=1/f

f=1/2

f=0.5m

lens is convex.