Two thin convex lenses of focal length f₁ and f₂ are placed in contact with each other. The equivalent power of the lens combination is .

Question

Accepted Answer

(3) (f₁ + f₂)/(f₁ × f₂). Given: two thin convex lenses of focal lengths f₁ and f₂, placed in contact. Idea: when thin lenses touch, the second simply carries on bending rays the first has already bent, so their powers add: P = P₁+P₂. P = 1/(f₁)+1/(f₂) Over a common denominator, P = (f₂+f₁)/(f₁ f₂). The reciprocal of that, (f₁ f₂)/(f₁+f₂), is the equivalent focal length -- useful, but not what was asked for. So the equivalent power is (f₁+f₂)/(f₁× f₂).

Two thin convex lenses of focal length f₁ and f₂ are placed in contact with each other. The equivalent power of the lens combination is ________.

Step-by-step solution

Why the other options are wrong