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If the focal length of a convex lens is 25 cm, what is its power in diopters?
+2D
+4D
4D
2D
+4D
The power of a lens is defined as the reciprocal of its focal length in meters. For a convex lens with a focal length of 25┬аcm (or 0.25┬аm), the power is calculated as P=0.251тАЛ=+4┬аD because convex lenses have a positive focal length.
The power of a lens is defined as the reciprocal of its focal length in meters. For a convex lens with a focal length of 25┬аcm (or 0.25┬аm), the power is calculated as P=0.251тАЛ=+4┬аD because convex lenses have a positive focal length.
Think of a magnifying glass focusing sunlight onto a point; a stronger magnifying glass (higher power) brings the rays together much faster over a shorter distance.
P = 1/f (Power is inversely proportional to focal length in meters: meters matter!)
P=f(in┬аmeters)1тАЛ тАФ Power of a lens formula
P=f(in┬аcm)100тАЛ тАФ Power formula when focal length is given in centimeters
Lens power measures the degree of convergence or divergence of light rays incident on a lens. A shorter focal length bends light more sharply, resulting in a higher optical power expressed in diopters (D).
The SI unit of optical power is the diopter (D), which is equivalent to mтИТ1.
A convex (converging) lens always has a positive focal length and positive power.
A concave (diverging) lens always has a negative focal length and negative power.
Diopters provide an additive measure of optical power for lenses placed in close contact (P=P1тАЛ+P2тАЛ).
Does not account for lens thickness or higher-order optical aberrations directly.
Prescription eyeglasses and contact lenses for correcting vision defects like hypermetropia.
Optical instruments such as microscopes, telescopes, and cameras.
Option B (+4D) is correct because P=25100тАЛ=+4┬аD.
Option A and D lack the correct sign or value, while Option C misses the positive sign convention required for convex lenses.
B is correct тАФ Option B (+4D) correctly applies the lens power formula with the proper positive sign convention for a convex lens.
Always convert the focal length into meters (or use 100/f in cm) and pay strict attention to sign conventions (+ for convex, - for concave) when solving lens and mirror numericals in RRB exams.
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