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The refractive index is 2.33 and the critical angle is 350. Find the numerical aperture.(a) 2(b) 1.9(c) 2.33(d) 12I had been asked this question in an interview for job.This intriguing question comes from Refractive Index and Numerical Aperture in section EM Wave Propagation of Electromagnetic Theory

Answer»

Right answer is (b) 1.9

For EXPLANATION I would say: The numerical aperture is given by NA = N cos θc, where θc is the critical ANGLE and n is the REFRACTIVE INDEX. On substituting for n = 2.33 and θc = 35, we get NA = 2.33 cos 35 = 1.9(no unit).


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