Question

The numerical aperture of a step index fiber in (refractive index = 1), is 0.39. The diameter of the core is $200\mu \text{m}$.

Two experiments are conducted in which light is launched into the fiber from a uniformly distributed planar source kept 5 mm away from the tip. In the first experiment $\left(E_1\right)$ both the source and the fiber are in air. In the second experiment $\left(E_2\right)$ both the source and the fiber are in water. Neglecting absorption in the medium between the source and the tip, the ratio of the amount of light coupled into the fiber in $\left(E_1\right)$ to the amount of light coupled into the fiber in $\left(E_2\right)$ is closest to

• A. 1.4
• B. 0.5
• C. 1.0
• D. 1.9

Answer 1

The correct option is C 1.0

The amount of light that can be gathered is depicted by the numerical aperture (N.A.) of the fiber, and N.A. is independent of the surrounding medium so that answer is 1.0.