Two thin equiconvex lenses each of focal length 0 . 2 m are placed coaxially with their optic centres 0 . 5 m apart. Then the focal length of the combination is

Question

Two thin equiconvex lenses each of focal length 0 . 2 m are placed coaxially with their optic centres 0 . 5 m apart. Then the focal length of the combination is, - 0 . 4 m, 0 . 4 m, - 0 . 1 m, 0 . 1 m

MARKS App · Accepted Answer

Equivalent focal length ( F ) of two lenses separated by distance d is given by 1 F = 1 f 1 + 1 f 2 - d f 1 f 2 = 1 0.2 + 1 0.2 - 0.5 0.2 0.2 = 5 + 5 - 0.5 × 5 × 5 = 10 - 12.5 = - 2.5 F = - 1 2.5 = - 0.4 m

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