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)
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
Memory Aid
- Denser medium (light must be in the denser material)
- Angle is greater than critical angle
- No refraction occurs
- Goes back by total reflection