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If two structures of same cross-sectional area, but different numerical apertures NA 1 and NA 2 (NA 2 < NA 1 ) are joined longitudinally, the numerical aperture of the combined structure is

1. 2 1 2 1 NA NA NA NA +
2. NA 1 + NA 2
3. NA 1
4. NA 2

Correct answer: NA 2

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