A converging lens has a focal length of 25\text{ cm}. What is the power of the lens in dio… — JAMB UTME

A converging lens has a focal length of $25\text{ cm}$ . What is the power of the lens in dioptres?

Answer: 4 D

The power of the lens is

4\text{ D}

The power

P

of a lens in dioptres is defined as the reciprocal of its focal length

f

expressed in meters (

P = \frac{1}{f}

). Expressing

25\text{ cm}

in meters gives

0.25\text{ m}

. Substituting this value yields

P = \frac{1}{0.25\text{ m}} = 4\text{ D}

Convert the focal length from centimeters to meters

f = 25\text{ cm} = 0.25\text{ m}

The unit of dioptre (

D

) is defined as reciprocal meters (

m^{-1}

), so focal length must be in meters.

Apply the lens power formula

P = \frac{1}{f}

and compute the value

P = \frac{1}{0.25} = 4\text{ D}

The power of a converging lens is positive and equal to the reciprocal of its focal length in meters.

Power of a Lens

Question