Topic 6: Power of a Lens (imp)

Power of a Lens

The power of a lens is a measure of its ability to converge or diverge light rays.

Definition

Power of a lens is defined as the reciprocal of its focal length in metres.

P = 1 / f

where,
P = power of lens (in dioptre)
f = focal length (in metre)

Unit of Power

The SI unit of power of a lens is dioptre (D).

1 dioptre = power of a lens having focal length of 1 metre.

Sign Convention for Power

Convex lens: Positive power (+)
Concave lens: Negative power (−)

Examples

Convex lens of focal length +0.5 m → P = +2 D
Concave lens of focal length −0.25 m → P = −4 D

Combination of Lenses (Brief)

When two or more thin lenses are placed in contact, their resultant power is the algebraic sum of individual powers.

P = P₁ + P₂

Q1. Power of a lens is given by:
A) f
B) 1/f
C) f²
D) 1/f²

Answer: B


Q2. SI unit of power of a lens is:
A) metre
B) metre⁻¹
C) dioptre
D) second

Answer: C


Q3. A lens having focal length of 0.5 m has power:
A) 0.5 D
B) 1 D
C) 2 D
D) 5 D

Answer: C


Q4. Power of a concave lens is:
A) Positive
B) Negative
C) Zero
D) Infinite

Answer: B


Q5. If focal length of a lens is −0.25 m, its power is:
A) −0.25 D
B) −2 D
C) −4 D
D) +4 D

Answer: C


Q6. Power of a convex lens is always:
A) Negative
B) Zero
C) Positive
D) Infinite

Answer: C


Q7. When two lenses of power +2 D and −1 D are placed together,
the resultant power is:
A) +1 D
B) −1 D
C) +3 D
D) −3 D

Answer: A


Q8. Which quantity must be converted into metres before
calculating power of a lens?
A) Height of object
B) Height of image
C) Focal length
D) Image distance

Answer: C

Chapter 13: Refraction of Light

Topic 6: Power of a Lens – Quick Revision

1. Power of Lens

Ability of a lens to converge or diverge light.

2. Formula

P = 1 / f (f in metre)

3. Unit

Dioptre (D)

4. Sign Convention

Convex lens → positive power
Concave lens → negative power

5. Combination of Lenses