Two thin lenses of power +3D and -1D are held in contact with each other. Focal length of the combination would be:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → 50 cm
The power of a lens is the reciprocal of its focal length in metres, \( P = 1/f \). For thin lenses in contact, the powers add algebraically.
\( \displaystyle P = P_1 + P_2 = (+3\,\mathrm{D}) + (-1\,\mathrm{D}) = +2\,\mathrm{D} \)
\( \displaystyle f = \frac{1}{P} = \frac{1}{2\,\mathrm{D}} = 0.5\,\mathrm{m} \)
\( \displaystyle f = 0.5\,\mathrm{m} = 50\,\mathrm{cm} \)
The positive sign shows that the combination acts as a converging lens.
Note: A similar question was also asked in CUET (UG) 2023 on 21-05-2023 (Slot 3).