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If a lens has a power of +2.5 Diopters, what is its focal length?
20 cm
40 cm
50 cm
25 cm
40 cm
The focal length of a lens is inversely proportional to its optical power when measured in meters. Given a power of +2.5 Diopters, the focal length is calculated as f=P1тАЛ=2.51тАЛ=0.4extm, which equals 40extcm.
The focal length of a lens is inversely proportional to its optical power when measured in meters. Given a power of +2.5 Diopters, the focal length is calculated as f=P1тАЛ=2.51тАЛ=0.4┬аm, which equals 40┬аcm.
Think of reading glasses: a stronger prescription (higher Diopters) bends light more sharply, meaning it focuses rays closer and has a shorter focal length, whereas a weaker lens focuses further away.
P=f1тАЛ тАФ Power of a lens where f is in meters
f=P1тАЛ тАФ Focal length formula
Optical power (P) measures the degree of convergence or divergence of light rays by a lens, expressed in Diopters (D) when the focal length (f) is in meters. A positive power indicates a convex (converging) lens, which bends incoming parallel rays inward to a real focal point.
Power of a lens is expressed in Diopters (D) when focal length is in meters (m).
A positive power (+2.5┬аD) denotes a converging (convex) lens.
A negative power denotes a diverging (concave) lens.
Correction of hypermetropia (farsightedness) using convex lenses with positive power.
Optical instruments such as microscopes and cameras.
Unit of power is Diopter (mтИТ1).
Option A (20┬аcm) corresponds to a power of +5.0┬аD.
Option C (50┬аcm) corresponds to a power of +2.0┬аD.
Option D (25┬аcm) corresponds to a power of +4.0┬аD.
B is correct тАФ The focal length for a lens of power +2.5┬аD is 40┬аcm.
Always ensure the unit of focal length is in meters before applying the power formula P=f1тАЛ, then convert to centimeters if required by the options.