How is numerical aperture defined, and how does it relate to the fiber's acceptance angle?

• A. NA equals the square root of the sum of the core and cladding indices.
• B. NA = n0 sin θa; relates to the maximum external angle that can be guided.
• C. NA is the difference between the core and cladding indices.
• D. NA is the cladding index minus the core index.

Answer: (B)