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
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → -1.0
The power of a lens is \( P = 1/f \) with \( f \) in metres: positive for a converging lens, negative for a diverging one. Powers of lenses in contact add.
\( \displaystyle P_1 = +\frac{1}{0.25} = +4\,\mathrm{D} \)
\( \displaystyle P_2 = -\frac{1}{0.20} = -5\,\mathrm{D} \)
\( \displaystyle P = P_1 + P_2 = 4 - 5 = -1\,\mathrm{D} \)