Light and Sound

IGCSE Edexcel Physics
3.14–3.23 Light, Refraction and Sound
Key Concepts: Light waves are transverse and can be reflected and refracted. Reflection: angle of incidence = angle of reflection. Refraction occurs when light enters a different medium. Refractive index can be found from $n = \sin i / \sin r$.

Section A: Reflection and Refraction

1. State that light waves are transverse and can be reflected and refracted. [2]
2. State the laws of reflection. [2]
3. Light travels from air into glass. Use $n = \sin i / \sin r$ to calculate $r$ when $i = 30^\circ$ and $n = 1.5$. [2]
4. Draw a ray diagram for refraction through a rectangular block, showing the normal, incident ray and refracted ray. [3]

Section B: Total Internal Reflection

5. Define critical angle and describe how total internal reflection occurs. [3]
6. Calculate the critical angle for glass with $n = 1.5$ using $n = 1 / \sin c$. [2]

Section C: Applications

7. Explain how optical fibres use total internal reflection. [2]
8. Describe a practical method to find the refractive index of glass using a ray box and a glass block. [4]

Section D: Sound Waves

9. State whether sound is longitudinal or transverse and explain how sound can be reflected and refracted. [3]
Total marks: 23

Mark Scheme

1. Light is transverse; light can reflect and refract [2]
2. Incident ray, reflected ray, and normal are coplanar [1]; angle of incidence equals angle of reflection [1]
3. $\sin r = \sin 30^\circ / 1.5 = 0.333$; $r$ about $19.5^\circ$ [2]
4. Correct ray diagram with normal and bends toward/away from normal [3]
5. Critical angle: angle of incidence giving 90° refraction; TIR occurs at angles greater than critical when travelling from denser to less dense medium [3]
6. $\sin c = 1/1.5 = 0.667$; $c$ about $41.8^\circ$ [2]
7. Light repeatedly reflects inside fibre with no loss at boundary [2]
8. Measure i and r for a range of rays, use n = sin i / sin r; keep block fixed [4]
9. Sound is longitudinal; can reflect (echo) and refract when speed changes in different media [3]