This topic summary covers Knowledge Organiser: Reflection and Refraction 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 13 of 13 in this topic. Use this topic summary to connect the idea to the wider topic before moving on to questions and flashcards.
Knowledge Organiser: Reflection and Refraction
Key Terms
- Normal: perpendicular line to surface at point of contact
- Specular: reflection from smooth surface → clear image
- Diffuse: reflection from rough surface → scattered rays
- Critical angle: angle beyond which TIR occurs
- TIR: total internal reflection when angle > critical angle
Key Rules
- i = r (law of reflection)
- Fast to slow → bends toward normal
- Slow to fast → bends away from normal
- TIR: denser medium + angle > critical angle
- Hit at 90° → no bending, but speed changes
TIR Applications
- Optical fibres (data transmission)
- Endoscopes (medical imaging)
- Diamonds (sparkle)
- Binoculars / periscope prisms
Exam Tips
- Always draw the normal first in ray diagrams
- Angles measured from normal, not surface
- State BOTH TIR conditions for full marks
- "Slow to fast bends AWAY" — away = opposite to where it came from
Key Equations
- Angle of incidence = angle of reflection (reflection law)
- n = sin(i) ÷ sin(r) (Snell's law — refractive index)
- n = c ÷ v (refractive index = speed in vacuum ÷ speed in medium)
- sin(c) = 1 ÷ n (critical angle from refractive index)
Common Mistakes
- Measuring angles from the surface instead of the normal: All angles of incidence, reflection, and refraction are measured from the normal (perpendicular to the surface) — not from the surface itself
- Getting refraction direction wrong: Light bends towards the normal when entering a denser medium (slow); it bends away from the normal when entering a less dense medium (fast)
- Stating only one condition for TIR: Both conditions are needed — light must travel from a denser to a less dense medium AND the angle of incidence must exceed the critical angle
- Confusing reflection and refraction in diagrams: In reflection, the ray bounces back on the same side; in refraction, the ray passes through into the new medium at a changed angle
- Forgetting frequency is unchanged during refraction: When light refracts, its speed and wavelength change — but its frequency stays constant
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.
Quick recall flashcards
Law of reflection
Angle of incidence = Angle of reflection
What is refraction?
Change in direction when light enters a different medium (due to speed change)