A convex lens produces a real inverted image of an object on a screen placed from the object. There are two positions of the lens between object and screen that form sharp images on the screen (displacement method). If the distance between the two lens positions is , what is the focal length of the lens?
Answer and explanation
Correct answer: D.
- Step 1: In the displacement method, the focal length is related to the fixed distance between object and screen and the lens separation by .
- Step 2: Here and .
- Step 3: Compute: .
Answer
.
Why not the others: is ; uses in denominator; is the lens displacement.
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