Reflection and Refraction

This lesson covers the behaviour of waves when they meet a boundary — specifically reflection and refraction — as required by the Edexcel GCSE Physics specification (1PH0), Topic 4: Waves. You need to understand the law of reflection, be able to draw ray diagrams for reflection and refraction, and explain why waves change direction when they enter a different medium.

---

Reflection of Waves

Reflection occurs when a wave bounces off a surface and changes direction. All types of waves can be reflected — including light, sound, and water waves.

The Law of Reflection

The law of reflection states:

The angle of incidence equals the angle of reflection.

i = r

Where:

• i = angle of incidence (the angle between the incoming ray and the normal)
• r = angle of reflection (the angle between the reflected ray and the normal)

The Normal Line

The normal is an imaginary line drawn perpendicular (at 90°) to the reflecting surface at the point where the incident ray hits. All angles are measured between the ray and the normal — not between the ray and the surface.

Drawing a Reflection Ray Diagram