Chapter 3.2 • Interactive Lesson
Reflection & Refraction of Light
Cambridge IGCSE Physics 0625 • Core + Supplement
🔦
The Hook: Why Do Mirrors Work?
When you look in a bathroom mirror, you see yourself at the same distance behind the mirror as you are in front of it. When a straw in water looks bent. When fibre optic cables carry internet signals at light speed. All these phenomena follow two fundamental laws: Reflection and Refraction.
🎯 The Why: Real-World Applications
- • Periscopes & Telescopes: Use plane mirrors (Reflection)
- • Fibre Optic Communications: Internet cables use Total Internal Reflection
- • Medical Endoscopes: Doctors see inside your body using light pipes
- • Eyeglasses & Contact Lenses: Correct vision using Refraction
Part 1: REFLECTION (Core Content)
Baby Steps: What Happens When Light Hits a Mirror?
- 1.A light ray travels in a straight line until it hits the mirror surface
- 2.We draw an imaginary line perpendicular to the mirror called the Normal
- 3.We measure the Angle of Incidence (i) between the incoming ray and the Normal
- 4.The light bounces off at the Angle of Reflection (r)
- 5.KEY LAW: The angle of incidence ALWAYS equals the angle of reflection (i = r)
🔬 Interactive Lab 1: The Mirror Experiment
✓ LAW OF REFLECTION: Angle i = Angle r → 30° = 30°
📐 Properties of Plane Mirror Images (Core):
- • Virtual: Cannot be projected on a screen (light rays don't actually meet)
- • Upright: Same orientation as the object
- • Same Size: Image height = Object height
- • Laterally Inverted: Left becomes right (ambulance writes backwards)
- • Same Distance: Image distance behind mirror = Object distance in front
Part 2: REFRACTION (Core + Supplement)
Baby Steps: What Happens When Light Enters Glass?
- 1.Light travels at different speeds in different materials (fastest in vacuum, slower in glass)
- 2.When light crosses a boundary (air → glass), it changes speed
- 3.This speed change causes the light ray to bend (change direction)
- 4.If light slows down (air → glass), it bends TOWARDS the Normal
- 5.If light speeds up (glass → air), it bends AWAY from the Normal
🔬 Interactive Lab 2: Snell's Law Simulator
Common values: Water ≈ 1.33, Glass ≈ 1.5, Diamond ≈ 2.42
🧮 The Formula Triangle (Supplement)
Refractive Index (n):
n = c / v
n = sin i / sin r
c = speed of light in vacuum (3 × 10⁸ m/s)
v = speed of light in material
i = angle of incidence
r = angle of refraction
v = speed of light in material
i = angle of incidence
r = angle of refraction
Part 3: TOTAL INTERNAL REFLECTION (Supplement)
Baby Steps: When Light Can't Escape
- 1.When light goes from dense → less dense (glass → air), it bends AWAY from Normal
- 2.As we increase the angle of incidence, the refracted ray bends more and more
- 3.At a special angle called the Critical Angle (c), the refracted ray runs along the boundary (r = 90°)
- 4.If i > c, light CANNOT escape → it reflects back inside (Total Internal Reflection)
- 5.Formula: n = 1 / sin c (for glass → air)
🔬 Interactive Lab 3: Critical Angle & TIR
Critical Angle for n = 1.50:
c = 41.8°
Calculated using: sin c = 1 / n = 1 / 1.50 = 0.6667
🌐 Application: Optical Fibres
Why Optical Fibres Use TIR:
- • Light enters fibre at an angle greater than critical angle
- • Undergoes Total Internal Reflection at each bounce
- • No light escapes → No signal loss over long distances
- • Used in: Internet cables, medical endoscopes, telephone networks
- • Advantage over copper wires: Faster, immune to electromagnetic interference
⚠️ EXAMINER'S TRAPS
❌ TRAP 1: Angles measured from the surface
Always measure angles from the Normal, NOT from the mirror/boundary surface!
❌ TRAP 2: "Light slows down, so it bends away"
When light slows down (air → glass), it bends TOWARDS the Normal. When it speeds up (glass → air), it bends AWAY.
❌ TRAP 3: Confusing n = c/v with n = sin i / sin r
Both are correct! Use c/v when given speeds. Use sin i / sin r when given angles.
❌ TRAP 4: "TIR happens at the critical angle"
At critical angle, r = 90° (ray runs along boundary). TIR happens when i > critical angle.
❌ TRAP 5: Forgetting TIR only works dense → less dense
TIR can ONLY occur when light travels from a denser medium to a less dense medium (glass → air). NOT air → glass.
✅ SYLLABUS CHECKLIST (Cambridge 0625)
✓Core 3.2.1: Understand reflection at plane surfaces
✓Core 3.2.1: Know angle i = angle r
✓Core 3.2.1: Describe plane mirror image properties
✓Core 3.2.2: Understand refraction at boundaries
✓Core 3.2.2: Know meaning of critical angle
✓Supp 3.2.2: Define refractive index n = c/v
✓Supp 3.2.2: Use n = sin i / sin r
✓Supp 3.2.2: Use n = 1 / sin c
✓Supp 3.2.2: Understand TIR conditions
✓Supp 3.2.2: Describe optical fibre use
🎓 Key Exam Takeaways
- • Reflection: Use Law of Reflection (i = r) with angles from Normal
- • Refraction: Remember "slow down = bend towards, speed up = bend away"
- • Refractive Index: Three equations: n = c/v, n = sin i / sin r, n = 1 / sin c
- • TIR: Only dense → less dense, only when i > critical angle
- • Units: Speeds in m/s, angles in degrees (or radians with calculator warning)
- • Mark Scheme Keywords: "Virtual image," "Towards Normal," "Total Internal Reflection," "Optical fibre"
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