Question

A convex lens has a focal length of 10 cm. What is the power of the lens in diopter ?

• A. 1D
• B. 2D
• C. 5D
• D. 10D

Answer 1

Answer:

D. 10D

• The Power (P) of a lens is defined as the reciprocal of its focal length (f) measured in meters.

• The formula is represented as: P = 1 / f(m).

• The SI unit of lens power is the Diopter (D), where 1 Diopter = 1 m⁻¹.

• Given focal length (f) = 10 cm.

• Convert the focal length to meters: 10 cm = 0.1 m.

• Apply the power formula: P = 1 / 0.1.

• Result: P = +10 D.

• Sign Convention: Convex lenses always have a positive (+) focal length and, consequently, a positive power. Conversely, concave lenses have a negative (-) focal length and negative power.

• Relation to Curvature: A lens with a shorter focal length has a higher power because it deviates light rays more sharply.

• Combined Lenses: When multiple thin lenses are placed in contact, the total power is the algebraic sum of individual powers (P = P₁ + P₂ + ...).