C.3 · IB Physics
Total internal reflection and optical fibres
Send a ray from glass into air and watch how the light divides between reflected and refracted rays as the angle grows past the critical angle. Switch to fibre mode to see light guided along a core by repeated total internal reflection.
Try this
- With n₁ = 1.50, raise θ₁ slowly: watch the reflected ray brighten until it carries all the light at θc ≈ 41.8°.
- Set n₁ = 2.00 and check that the critical angle drops to 30.0°; explain why high-index materials trap light more easily.
- In fibre mode, move n₂ closer to n₁ and find the largest entry angle α that still keeps the light trapped; compare it with the acceptance angle readout.
Common error: writing sin θc = n₁/n₂ upside down; a critical angle exists only when light goes from the higher to the lower index, so sin θc = n₂/n₁ must be less than 1.