Reflection & Refraction

IGCSE Edexcel Physics
3.14–3.22 Laws of reflection, refraction, Snell's law, critical angle and TIR
Key Concepts: Reflection: angle of incidence = angle of reflection (measured from normal). Refraction: $n = \sin i / \sin r$ (Snell's law). Critical angle: $\sin c = 1/n$. Total internal reflection occurs when angle of incidence > critical angle.

Section A — Reflection

1. State the two laws of reflection. [2]
2. A ray of light strikes a plane mirror at 35° to the mirror surface. State the angle of incidence and the angle of reflection. [2]
3. Describe the image formed in a plane mirror. State three characteristics. [3]

Section B — Refraction and Snell's Law

4. Explain what happens to the speed and direction of light when it passes from air into glass. [3]
5. Write Snell's law and define the refractive index $n$. [2]
6. Light enters glass from air at an angle of incidence of 40°. The refractive index of the glass is 1.5. Calculate the angle of refraction. [3]
7. A ray of light travels from glass ($n = 1.6$) to air and is refracted at 60°. Calculate the angle of incidence inside the glass. [3]

Section C — Critical Angle and Total Internal Reflection

8. Define the critical angle. [2]
9. Calculate the critical angle for glass with a refractive index of 1.5. [2]
10. Describe the conditions needed for total internal reflection and give one application. [3]

Total marks: 25

Mark Scheme

1. The incident ray, reflected ray and normal all lie in the same plane [1]; angle of incidence equals angle of reflection [1] [2]
2. Angle of incidence = 90° − 35° = 55° [1]; angle of reflection = 55° [1] [2]
3. Any three: virtual; same size as object; same distance behind mirror; laterally inverted; upright [3]
4. Speed decreases as light enters the denser medium (glass) [1]; direction changes — light bends toward the normal [1]; because the wavefronts slow down at the boundary [1] [3]
5. $n = \sin i / \sin r$ [1]; $n$ = refractive index of the medium (ratio of speed of light in vacuum to speed in medium) [1] [2]
6. $\sin r = \sin 40° / 1.5 = 0.6428 / 1.5 = 0.4285$ [1]; $r = \sin^{-1}(0.4285) \approx 25.4°$ [2] [3]
7. $n \sin i = \sin r$; $1.6 \sin i = \sin 60° = 0.866$ [1]; $\sin i = 0.866/1.6 = 0.541$ [1]; $i \approx 32.8°$ [1] [3]
8. The critical angle is the angle of incidence (in the denser medium) at which the refracted ray travels along the boundary [1]; i.e. the angle of refraction = 90° [1] [2]
9. $\sin c = 1/n = 1/1.5 = 0.667$ [1]; $c = \sin^{-1}(0.667) \approx 41.8°$ [1] [2]
10. Light must be in the denser medium [1]; angle of incidence must be greater than the critical angle [1]; application: optical fibres (for endoscopes / telecommunications) or prisms in binoculars [1] [3]