Which part carries light via total internal reflection?

• A. Cladding
• B. Core
• C. Buffer
• D. Jacket

Answer: (B)

Light is guided in an optical fiber by keeping the signal inside a region that has a higher refractive index than its surroundings. The central region, the core, has a higher refractive index than the surrounding cladding, so light traveling in the core hits the boundary at angles that cause total internal reflection. This reflection at the core–cladding boundary keeps the light confined and propagating along the fiber rather than escaping. The cladding provides that lower-index boundary necessary for total internal reflection, but it is the core that actually carries the light signal. The buffer and jacket are protective layers and do not participate in guiding the light.