A7LAM CSCA ACADEMYCSCA Academy / Physics / Optics
CSCA Physics · Lesson 13 of 16Reflection and refraction
Réflexion et réfraction · الانعكاس والانكسار
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Light travels in straight lines until it meets a surface. Then it bounces back (reflection) or bends into the new material (refraction).
Key ideas
- Reflection: angle of incidence = angle of reflection, both measured from the normal.
- Refraction: n₁ sin θ₁ = n₂ sin θ₂.
- Refractive index n = c/v > 1; entering a denser medium, light bends towards the normal.
- Total internal reflection: going to a less dense medium beyond the critical angle C, with sin C = 1/n.
Formulas to know
- n₁ sin θ₁ = n₂ sin θ₂
- n = c / v
- sin C = 1 / n
Worked example
Light enters glass (n = √3) from air at 60° to the normal. Angle of refraction?
- Law: sin 60° = √3 · sin r
- Solve: sin r = (√3/2) / √3 = 1/2
- Angle: r = 30°
Answer: r = 30°
Practice questions
1. Critical angle for glass with n = 2?
30° ✓ · 45° · 60° · 90°
sin C = 1/2.
2. Speed of light in water (n = 4/3)?
4 × 10⁸ m/s · 2.25 × 10⁸ m/s ✓ · 3 × 10⁸ m/s · 1.33 × 10⁸ m/s
v = c/n = 3×10⁸ × 3/4.
3. Total internal reflection can happen when light goes from:
air to glass · glass to air ✓ · air to water · vacuum to air
Only from the denser medium to the less dense one.