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Light guidance in an optical fiber can be understood by considering a structure comprising of thin solid glass cylinder of refractive index n_(1) surrounded by a medium of lower refractive index n_(2) . The light guidance in the structure takes place due to successive total internal reflections at the interface of the media n_(1) and n_(2) as shown in the figure . all rays with the angle of incidence i less than a particular value of i_(m) are confined in the medium of refractive index n_(1) . The numerical aperture (NA) of the structure is defined as "sin"i_(m) If two structures of same cross - sectional area, but different numerical apertures NA_(1) and NA_(2)(NA_(2) lt NA_(1)) are joined longitudinally, the number aperture of the combined structure is |
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Answer» `implies i_(m2) lt i_(m1)` Hence if the combination can be placed both ways i.e., `1^(ST)`STRUCTURE & then `2^(nd)` structure and then reversedalso , then the CONDITION of TIR is satisfied for lower `i_(m)` then it can satisfied for all other LESS angler as WELL. Hence `NA_(2)` will be the numerical aperture of the combined structure . |
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