Board note: Total internal reflection, the critical angle and Snell's law are examined by Edexcel, OCR and Eduqas. AQA (8463/8464) does not include these terms — AQA's refraction content is qualitative wavefront-bending only (Higher tier), and AQA's reflection content covers ray diagrams without total internal reflection.
What it is: When light reflects COMPLETELY inside a denser medium instead of refracting out.
Two conditions required:
- Light travelling from a denser → less dense medium (e.g., glass to air)
- Angle of incidence > critical angle
Applications:
- Optical fibres — light bounces along the inside by TIR, carries data at the speed of light
- Endoscopes — doctors see inside the body without surgery
- Diamonds — sparkle due to a low critical angle (about 24°, compared with about 42° for glass) and many TIR reflections inside
- Binoculars/periscopes — use glass prisms where TIR redirects the light path
Quick Check: A ray of light hits a mirror at 40° to the surface. What is the angle of reflection?
The angle to the surface is 40°, so the angle of incidence (measured from the normal) = 90° − 40° = 50°. Therefore the angle of reflection = 50°.
This key facts covers Total Internal Reflection within Reflection & Refraction for GCSE Physics. Revise Reflection & Refraction in Waves for GCSE Physics with 17 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 5 of 13 in this topic. Use this key facts to connect the idea to the wider topic before moving on to questions and flashcards.
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.