Question

In an optical fiber, the light beam propagates due to which one of the following?

• A. Simple reflection of light at a boundary between two media
• B. Refraction of light in the medium
• C. Total internal reflection at the boundary of the fiber
• D. Scattering of light in the medium

Hint:

Total Internal Reflection is only possible when light moves from a "Denser" medium to a "Rarer" medium. This is why the Core ($n_1$) must always have a higher refractive index than the Cladding ($n_2$).

Answer 1

The Correct Option is C

1. The Structure of the Fiber: An optical fiber consists of a "Core" with a higher refractive index ($n_1$) surrounded by a "Cladding" with a lower refractive index ($n_2$).

2. Total Internal Reflection (TIR): When light traveling in the core hits the boundary with the cladding at a shallow angle (specifically, an angle of incidence greater than the critical angle), it does not pass through into the cladding. Instead, 100% of the light is reflected back into the core. $$\theta_c = \sin^{-1}\left(\frac{n_2}{n_1}\right)$$

3. Propagation Mechanism: This process of Total Internal Reflection occurs repeatedly as the light travels down the fiber, zig-zagging back and forth but always staying contained within the core. This allows the fiber to "pipe" light even around bends and curves without losing the signal to the outside environment.