When the object is at distances u₁ and u₂ the images formed by the same lens are real and virtual respectively and of the same size. Then focal length of the lens is :
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Question

When the object is at distances u₁ and u₂ the images formed by the same lens are real and virtual respectively and of the same size. Then focal length of the lens is :

G1 Physics · Accepted Answer

Derived solution: Step 1: Use the thin-lens formula 1/f = 1/v - 1/u with m = v/u. For lens material/medium changes use the lens-maker relation with the relative refractive index; powers of thin lenses in contact add. Step 2: Substitute the values given in the question and apply the geometry/sign convention shown. Step 3: On simplification the result is (u₁ + u₂)/2. Therefore the correct option is B.