Red Seal Electrician Practice
Signalling and communicationQuestion 45 of 1506

What keeps light inside the core of an optical fibre?

  1. A metal shield around the glass core reflects the light back inward
  2. Total internal reflection, caused by glass layers with different refractive indexes
  3. Electric charge on the core surface repels the light pulses inward
  4. Magnetic fields from the cable sheath bend the light back into the core of the fibre
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Correct answer: Total internal reflection, caused by glass layers with different refractive indexes

The cladding has a different refractive index from the core, so light stays in the core.

Source: Lessons in Electric Circuits IV: Digital (Kuphaldt), 14.5: Optical Data Communication

Practice signalling and communication

Questions last reviewed: 18 October 2026