• Tags: power, combination, dioptre
• Difficulty: Moderate

Power of a lens: P = 1/f (in dioptres when f is in metres). Converging lens has positive power; diverging lens has negative power.

For two thin lenses in contact: P = $P_{1}$ + $P_{2}$, or equivalently 1/f = 1/f_{1} + 1/f_{2}.

For two thin lenses separated by distance d: 1/f = 1/f_{1} + 1/f_{2} - d/(f_{1}·f_{2}), or P = $P_{1}$ + $P_{2}$ - d·$P_{1}$·$P_{2}$.

JEE frequently tests the separated lens formula. Remember d must be in metres when using dioptres.