Question

A beam of monochromatic green light of wavelength λ 532 mn (measured in air) is incident on the core of an optical fiber with refractive index ncore- 1.48, as shown. The core is surrounded by a cladding of refractive index nc!adding- = 1.39. Cladding Core Air 17) What must be the maximum incident angle θ of the beam at the air-core interface, as shown in the figure, such that light cannot escape through the cladding of the optical fiber?
I understand that you use sin(theta1)=n2/n1sin(theta2) to but what I do not understand is why theta 2 is 90 degrees. Please explain in depth would be very helpful.

Answer 1

You can see from figure attached the angle of refraction 90⁰ the refracted ray passes parallel and coinciding the wall of fiber. For any other refraction angle less than as shown in example just sayfor 60⁰

incitehe

the ray will come outside of the fiber glass and for all other angle >=90⁰ it will remain outside.

And the minimum angle is thus calculated for 90⁰ refraction angle.