Three thin lenses of powers +2.5 D, +2.0 D and −0.5 D are placed in contact. What is the power of the combination?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → 4.0 D
When thin lenses are placed in contact, the power of the combination is the algebraic sum of their powers, with the sign of each power kept (convex lens positive, concave lens negative).
Powers: \(P_1=+2.5\,\mathrm{D}\), \(P_2=+2.0\,\mathrm{D}\), \(P_3=-0.5\,\mathrm{D}\)
\(P=P_1+P_2+P_3=2.5+2.0-0.5\)
\(P=+4.0\,\mathrm{D}\)
So the combination acts as a convex lens of power 4.0 D.