Power Of A Lens
Light of Class 10
POWER OF A LENS
The power of a lens is the degree of convergence or divergence of light rays achieved by a lens.
The power of a lens is defined as the reciprocal of its focal length in meters. It is denoted by the letter P.
where P = power of a lens
and f = focal length of the lens in meters.
The S.I. unit of power is dioptre. It is denoted by the letter ‘D’.
One dioptre is the power of a lens whose focal length is 1 metre.
The power of a lens is measured by an instrument called dioptre meter.
A convex lens has positive focal length so the power of a convex lens is positive.
A concave lens has a negative focal length, so the power of a concave lens is negative.
Since the power of a lens is inversely proportional to its focal length P = 1/f(in meters) Therefore, a lens of short focal length has more power than a lens of long focal length (i.e. shorter the focal length, more is the power and vice-versa).
Power of a combination of lenses
If a number of lenses are placed in close contact, then the power of the combination of lenses is equal to the algebraic sum of the powers of individual lens.
i.e. P = P1 + P2 + P3 + ………
Where P = power of combination of lenses.
P1, P2, P3, ……………… = Powers of individual lens placed close to each other.
Ex. A convex lens produces an inverted image magnified three times of an object placed at a distance of 15 cm from it. Calculate focal length of the lens.
Solution: Here, magnification, m = –3
Object distance, u = –15 cm
And, image distance, v = ?
Now, for a lens, magnification,
m = v/u
-3 = v/-15
v = –3 × (–5)
Thus, image distance, v = +45 cm.
Also, for a lens:
Thus, focal length, f = + 11.25 cm.
- Some Important Terms Related With Light
- Reflection Of Light
- Some Important Terms Used In The Reflection Of Light
- Laws Of Reflection
- Plane Mirror
- Some Important Terms Related To Spherical Mirrors
- Mirror Formula
- Linear Magnification Produced By Mirrors
- Refraction Of Light
- Optical Density
- Spherical Lenses
- Principal Focus And Focal Length Of A Convex Lens
- Principal Focus And Focal Length Of A Concave Lens
- Sign Convection For Refraction By Spherical Lenses
- Lens Formula
- Linear Magnification Produced By Lenses
- Some Important Conclusions
- Power Of A Lens
- Solved questions
- Exercise 1
- Exercise 2
- Exercise 3
- Exercise 4 (Subjective)