Question

The core of optical Fiber has refractive index:

• A. Less than cladding
• B. Equal to cladding
• C. Greater than cladding
• D. Zero

Hint:

The refractive index difference $\Delta = \frac{n_{\text{core}} - n_{\text{cladding}}}{n_{\text{core}}}$ is typically kept very small (around $1\%$).
This small difference helps reduce modal dispersion while still ensuring total internal reflection.

Answer 1

The Correct Option is C

Step 1: Understanding the Question:
The question asks for the relative difference in refractive indices between the core and the cladding of an optical fiber.

Step 2: Key Formula or Approach:
Optical fibers transmit light signals over long distances using the principle of Total Internal Reflection (TIR).
For total internal reflection to occur, two conditions must be satisfied:
1. Light must travel from a optically denser medium to an optically rarer medium.
2. The angle of incidence must be greater than the critical angle ($\theta_c$), defined as:
\[ \theta_c = \sin^{-1}\left(\frac{n_2}{n_1}\right) \]
where $n_1$ is the refractive index of the denser medium and $n_2$ is the refractive index of the rarer medium.

Step 3: Detailed Explanation:

• An optical fiber consists of a central core surrounded by an outer layer called the cladding.

• To guide light within the core, light must undergo total internal reflection at the core-cladding boundary.

• According to the laws of refraction, total internal reflection can only happen if the core is the denser medium.

• Therefore, the refractive index of the core ($n_{\text{core}}$) must be strictly greater than the refractive index of the cladding ($n_{\text{cladding}}$).

• This difference in refractive index traps the light waves within the core, allowing them to propagate along the fiber through a series of reflections with minimal energy loss.

Step 4: Final Answer:
The core of an optical fiber has a refractive index greater than the cladding.