Notes by Dushyant Kumar
Class- X
Physics
POWER OF A LENS
The power of a lens is a measure of the degree by which it can bend light rays.
If a lens can bend light rays more, it has more power. And if a lens can bend light rays less, it has less power.
The power of a lens depends on its focal length.
A convex lens is also called the converging lens. Because it has the power to converge light rays towards the principal axis at the principal focus.
A convex lens has +ve focal length. So, its power is also +ve.
A concave lens is also called the diverging lens. The reason is it has the power to diverge light rays away from the principal axis.
A concave lens has -ve focal length. So, its power is -ve.
More thickness ↑ Less thickness ↓
Less focal length ↓ Larger focal length ↑
More power ↑ Less power ↓
The power of a lens is the reciprocal of its focal length in metres.
Where, P = power of the lens
f = focal length (in metres)
If the focal length is given in cm, then first change it into m . Or,
Use this formula----
A lens of short focal length has more power.
A lens of long focal length has less power.
Power of combination of lenses
When you have more than one lens placed side by side, just add the powers of individual lenses to find the power of the combination.
P = P1 + P2 + P3 +.......
The combination of lenses is used to increase the sharpness of the image free from the defects.
Be sure to put a proper sign before the value of the power of a lens.
The power of a convex lens is always taken +ve. Like +1D, +2.5D, +2.0D etc.
The power of a concave lens is always taken -ve. Like -1D, -1.5D, -2.5D etc.
1 dioptre is the power of a lens whose focal length is 1 m.
Dioptremeter is the instrument used to measure the power of a lens.
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