A2 Daily A Level Physics question
In a lab test, a step-index optical fibre with core refractive index 1.50 and cladding 1.40 carries light by repeated internal reflections at the core boundary. A small chip in the cladding is covered by an index‑matching gel of refractive index 1.48 that now directly contacts the core over a short section. For rays that were just barely guided before the chip, which statement must be true at the gel‑contact section?
Answer
The correct answer is C.
Correct: C — They will refract out of the core because increasing the outside index increases the critical angle toward 90°, shrinking the range of internal angles that reflect. A Higher outside index actually weakens total internal reflection (it does not make the boundary more reflective), so marginal rays are more likely to leak. B Increasing the outside refractive index makes the critical angle larger, not smaller, so this reverses the trend and mispredicts continued guidance. C As the outside index approaches the core’s, the critical angle moves toward 90° so only extremely grazing rays could reflect, meaning previously just‑guided rays now refract out. D The condition for total internal reflection depends on both media at the boundary, so changing the external medium does affect guidance.