An eye specialist prescribes spectacles having a combination of a converging lens of focal length 25 cm and a diverging lens of focal length of 20 cm. The power of the combination of two lenses (in dioptre) is

-1.0

The correct answer is Option (2) → -1.0

Given:

Focal length of converging lens = $f_1 = +25 \text{ cm} = +0.25 \text{ m}$

Focal length of diverging lens = $f_2 = -20 \text{ cm} = -0.20 \text{ m}$

Power of each lens:

$P_1 = \frac{1}{f_1} = \frac{1}{0.25} = +4 \text{ D}$

$P_2 = \frac{1}{f_2} = \frac{1}{-0.20} = -5 \text{ D}$

Total power of combination:

$P = P_1 + P_2 = 4 + (-5) = -1 \text{ D}$

The power of the combination is $-1 \text{ dioptre}$.