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What is the effective focal length of a system in air?

1. 1/F1 = 1/f + 1/f = 2/f
2. F1 equals half of f
3. Filling glycerin converts it into a concave lens with focal length (-f)
4. Dividing equation (i) by equation (ii) gives F1/F2 = 1/2

Correct answer: F1 equals half of f

Solution

Option B is correct because the equivalent focal length in air for two identical lenses in contact is given by the formula F1 = f/2, where f is the focal length of each lens.

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