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The focal length of a convex lens is 20 cm. What is its power in diopters?
+5 D
5 D
+0.5 D
0.5 D
+5 D
The power of a lens is defined as the reciprocal of its focal length in meters. For a convex lens, the focal length is positive, which makes its optical power positive according to the sign convention.
The power of a lens is defined as the reciprocal of its focal length in meters. For a convex lens, the focal length is positive, which makes its optical power positive according to the sign convention.
Think of a magnifying glass focusing sunlight onto a paper: a stronger lens bends the light more sharply over a shorter distance, meaning it has a higher power.
P equals 1 over f (Power over Focal length)
P=f1тАЛ тАФ Power of a lens where f is in meters
P=f100тАЛ тАФ Power of a lens where f is in centimeters
Optical power measures the degree of convergence or divergence of light rays by a lens. Since power P=f┬а(in┬аmeters)1тАЛ, a shorter focal length results in a higher bending capability and thus a higher power.
The SI unit of lens power is the diopter (D).
A convex lens has a positive focal length and positive power.
A concave lens has a negative focal length and negative power.
Useful for calculating combined power of lenses in optical instruments.
Assumes thin lens approximation for absolute accuracy.
Spectacles and eyeglasses design
Microscopes and telescopes
Given f=+20┬аcm=+0.2┬аm, P=0.21тАЛ=+5┬аD.
Option B lacks the positive sign, making Option A the accurate choice since convex lenses have positive power.
A is correct тАФ The power of the convex lens with a focal length of +20 cm is calculated as +5 D.
Always double-check sign conventions for convex and concave lenses when calculating power in competitive exams.
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