Two lenses of power 3D and -1D are kept in contact. The focal length of the combined lens would be:
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → 50 cm
For thin lenses in contact, the powers add algebraically, and the focal length in metres is the reciprocal of the power in dioptres.
\( \displaystyle P = P_1 + P_2 = 3\,\mathrm{D} + (-1\,\mathrm{D}) = 2\,\mathrm{D} \)
\( \displaystyle f = \frac{1}{P} = \frac{1}{2\,\mathrm{D}} = 0.5\,\mathrm{m} = 50\,\mathrm{cm} \)
Note: A similar question was also asked in CUET (UG) 2023 on 15-06-2023 (Slot 2).