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CognitoReflection, refraction and Snell's law
CognitoTotal internal reflection and optical fibres
Key facts & equations
Reflection
- Law of reflection: angle of incidence = angle of reflection (measured from the normal).
- Normal: a line perpendicular to the surface at the point of incidence.
- Specular reflection: smooth surface, parallel reflected rays (e.g. mirror).
- Diffuse reflection: rough surface, rays reflected in different directions (most surfaces).
- In ray diagrams: draw normal first, then measure angles from it (not the surface).
Refraction
- Refraction: change of direction of a wave when it crosses a boundary between two media.
- Caused by a change in wave speed. If medium is denser (optically): wave slows down and bends towards normal.
- Less dense medium: wave speeds up and bends away from normal.
- Frequency does not change on refraction — only speed and wavelength change.
- Snell's law: n1 sin θ1 = n2 sin θ2. Or: n = sin i / sin r (for air→material).
Refractive index
- Refractive index n = speed of light in vacuum / speed in medium. n = c/v.
- n = sin i / sin r (for ray passing from air into material).
- Higher n = slower light in that medium = more bending toward normal.
- n for glass ≈ 1.5. n for water ≈ 1.33. n for air ≈ 1.00.
- Light always bends toward normal when entering a denser medium.
Total internal reflection (TIR)
- Occurs when light travels from a denser to a less dense medium at an angle ≥ the critical angle.
- Critical angle: angle of incidence at which refracted ray is at 90° (travels along boundary).
- sin c = 1/n (for glass-air boundary). E.g. n=1.5 → sin c = 1/1.5 = 0.667 → c ≈ 42°.
- Optical fibres: light travels by TIR along the fibre. Used in communications (data) and endoscopes.
- Diamonds: very high n → small critical angle → TIR at many surfaces → sparkle.
Exam questions — 4 questions · 10 marks · AQA 8463 style
⚡ Show all working — the AI marks against the AQA mark scheme. Method marks awarded for correct working even if the final answer is wrong.
3 marksSnell's law calculationAQA 8463 P2 style 🔢 Calculator
A ray of light travels from air (n=1.00) into glass (n=1.50). The angle of incidence is 40°. Calculate the angle of refraction. (3 marks)
Hint: n1 sin θ1 = n2 sin θ2. sin(40°) ≈ 0.643. Divide by n2 (1.5) to get sin θ2. Use arcsin to find angle.
+30 XP
1 markDirection of refractionAQA 8463 style
A ray of light enters glass from air. Which direction does it bend at the boundary?
A Away from the normal, because light speeds up in glass
B Toward the normal, because light slows down in glass
C It continues in the same direction with no bending
D Away from the normal, because glass is more transparent than air
3 marksCritical angle calculationAQA 8463 Higher style 🔢 Calculator
A glass prism has refractive index 1.5. Calculate the critical angle for total internal reflection at the glass-air boundary. (3 marks)
Hint: sin(critical angle) = 1/n. Calculate 1/1.5, then use arcsin.
+30 XP
1 markOptical fibresAQA 8463 style
How do optical fibres transmit data using light?
A Light is absorbed by the fibre and re-emitted along its length
B Light undergoes total internal reflection repeatedly along the fibre, staying inside it
C The fibre conducts light like a metal conducts electricity
D Light refracts at each reflection and gradually changes direction toward the far end
Module complete! 🎉
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