Two thin lenses of powers $+5\ \text{D}$ and $-2\ \text{D}$ are placed in contact. What is the focal length of the combination?

1. Powers of lenses in contact add: $P = P_1 + P_2$. 2. Substitute: $P = (+5) + (-2) = +3\ \text{D}$. 3. Focal length is $f = \dfrac{1}{P} = \dfrac{1}{3} \approx 0.33$ m. 4. Adding the magnitudes (7 D) would give $0.14$ m, a trap that ignores the sign. 5. The value $3$ m mistakes the power for the focal length. _Source: Samacheer Kalvi (TN SCERT) Class 10 Science, Unit 2 Optics "Power of a Lens", p.30_