This higher tier covers Higher Tier Only: The Critical Angle Formula within Reflection & Refraction for GCSE Physics. Revise Reflection & Refraction in Waves for GCSE Physics with 15 exam-style questions and 15 flashcards. This topic shows up very often in GCSE exams, so students should be able to explain it clearly, not just recognise the term. It is section 10 of 13 in this topic. This section is most useful once the core foundation idea is secure, because it adds the detail that pushes answers higher.
🎓 Higher Tier Only: The Critical Angle Formula
For a material with refractive index n, the critical angle c is given by:
sin c = 1/n
A higher refractive index (denser material, slower light) gives a smaller critical angle. Glass has n ≈ 1.5, so sin c = 1/1.5 = 0.67, giving c ≈ 42°. Diamond has n ≈ 2.4, giving a much smaller critical angle of about 25° — almost any ray inside a diamond is totally internally reflected, which is why diamonds sparkle so brilliantly.
Quick Check: State TWO conditions that must be met for total internal reflection to occur.
1. Light must be travelling from a denser medium to a less dense medium (e.g., glass to air). 2. The angle of incidence must be greater than the critical angle for that boundary.
Practice questions for Reflection & Refraction
According to the law of reflection, the angle of incidence is:
Explain why a ray of light bends when it passes from air into water.