Total internal reflection traps light in a medium.

The critical angle is the incident angle (in the higher-index medium) for which the refracted ray emerges at 90° along the boundary.

• What “90° refracted” means
• Which side has the higher index
• Finding the critical angle from indices
• Common setup mistakes

Total internal reflection occurs only when light tries to go from higher index to lower index, and the incident angle exceeds the critical angle.

• High-to-low index requirement
• Incident angle must be larger than critical
• What happens below critical angle
• How to justify using Snell’s law

Optical fibers guide light using repeated total internal reflection. Fiber design uses a core (higher index) and cladding (lower index) to confine light.

• Core vs cladding idea
• Why confinement is robust
• Qualitative acceptance angle intuition
• Loss mechanisms (intro)

Practice finding critical angles, testing TIR conditions, and interpreting fiber-style diagrams.

• Critical angle calculation drills
• “Does TIR occur?” decision questions
• Index-direction reasoning practice
• Fiber-guiding concept checks
• Exam-style TIR problem sets