Question

Light coming out of an optical fiber is incident on a plane perpendicular to the fiber axis and 50 mm away from the end of the fiber. The light coming out creates a circular spot that can at most be of $20\text{mm}$ diameter. Neglecting the diameter of the fiber, the numerical aperture of the fiber is, approximately,

• A. 0.20
• B. 0.40
• C. 0.14
• D. 0.34

Answer 1

The correct option is A 0.20



Clearly, $\theta ={\tan }^{-1}\left(\frac{10}{50}\right)={11.45}^{\circ }$

N.A. $=\sin \theta =\sin \left({11.45}^{\circ }\right)=0.19\simeq 0.2$