What you'll learn
When a wave meets a boundary between two materials, it can bounce back, bend, or pass through — and these behaviours explain everything from mirrors to lenses to why a straw looks bent in water. For AQA GCSE Physics you need to understand reflection and refraction, use the correct terms and rules, draw ray diagrams, and explain why refraction happens in terms of the wave changing speed. This guide covers the law of reflection, the difference between specular and diffuse reflection, how and why waves refract, and how to draw the ray diagrams that examiners ask for. By the end you should be able to describe both behaviours precisely, draw the diagrams, and explain refraction in terms of wave speed.
Key terms and definitions
Reflection — When a wave bounces back off a surface or boundary.
Refraction — When a wave changes direction as it passes from one material into another because its speed changes.
Normal — A construction line drawn at right angles (90°) to a surface at the point where a ray meets it.
Angle of incidence — The angle between the incoming ray and the normal.
Angle of reflection — The angle between the reflected ray and the normal.
Angle of refraction — The angle between the refracted ray and the normal.
Specular reflection — Reflection from a smooth surface, producing a clear image.
Diffuse reflection — Reflection from a rough surface, scattering the light in different directions.
Core concepts
Reflection and the law of reflection
When a wave hits a surface, it can be reflected. All angles are measured from the normal, the line drawn at 90° to the surface. The law of reflection states:
angle of incidence = angle of reflection
So if a ray hits a mirror at 30° to the normal, it reflects at 30° to the normal on the other side. This rule applies to all waves, including light and sound.
Specular and diffuse reflection
The type of reflection depends on the surface:
- Specular reflection happens at a smooth surface, such as a mirror. All the parallel rays reflect in the same direction, so a clear image forms.
- Diffuse reflection happens at a rough surface, such as paper. The surface is uneven, so parallel rays reflect in many different directions and scatter. No clear image forms, but the object can still be seen.
In both cases the law of reflection still holds at each point — it is the roughness of the surface that scatters the rays in diffuse reflection.
Refraction
Refraction happens when a wave passes from one material into another and changes speed. This change in speed usually causes the wave to change direction. For light passing into a denser material, such as from air into glass or water:
- The wave slows down.
- It bends towards the normal.
When light passes from a denser material into a less dense one, such as from glass back into air:
- The wave speeds up.
- It bends away from the normal.
If a wave meets the boundary along the normal (at 90° to the surface), it still changes speed but does not change direction, because there is no angle to bend.
Why refraction happens
Refraction is caused by the change in the wave's speed as it enters a new material. Think of the wavefront reaching the boundary at an angle: one side of the wave enters the new material and slows down before the other side does. This makes the wave "swing round" and change direction, in the same way a trolley turns if one wheel hits a rough patch before the other. The key phrase in any answer is that the wave changes speed, and this causes the change in direction.
Wavelength, frequency and speed in refraction
When a wave refracts, its speed and wavelength change, but its frequency stays the same. The frequency is set by the source and does not change when the wave enters a new material. Since wave speed = frequency × wavelength, if the speed decreases and the frequency is constant, the wavelength must decrease too.
Transmission and absorption at a boundary
When a wave meets a boundary, reflection and refraction are not the only possibilities. A wave can also be transmitted (pass through and carry on) or absorbed (its energy is transferred to the material). Which of these happens depends on the wavelength of the wave and the materials involved. For example, a mirror mainly reflects light, clear glass mainly transmits and refracts it, and a black surface mainly absorbs it, warming up. Often more than one of these happens at once — a glass window both reflects a little light (so you see a faint reflection) and transmits most of it. Being able to say that a wave may be reflected, transmitted or absorbed at a boundary is useful in longer answers.
Drawing ray diagrams
Ray diagrams are a common exam requirement. To draw refraction at a boundary:
- Draw the boundary between the two materials.
- Draw the normal as a dashed line at 90° to the boundary where the ray meets it.
- Draw the incident ray arriving at the boundary.
- Draw the refracted ray bending towards the normal if entering a denser material, or away if entering a less dense material.
- Label the angle of incidence and angle of refraction, both measured from the normal.
Always use a ruler, draw the normal as a dashed line, and add arrows to show the direction the wave travels.
Everyday examples of refraction
Refraction explains many everyday observations. A swimming pool looks shallower than it really is because light from the bottom refracts as it leaves the water, so the bottom appears raised. A straw in a glass of water looks bent at the surface for the same reason. Lenses in glasses, cameras and the eye all work by refraction, bending light to form a focused image. Rainbows form because different colours of light refract by slightly different amounts as they pass through raindrops, splitting white light into a spectrum. Linking the physics of refraction to these familiar examples is a common way for exams to test understanding.
Worked examples
Example 1: Using the law of reflection
A ray of light strikes a mirror at an angle of incidence of 40°. What is the angle of reflection? By the law of reflection, angle of incidence = angle of reflection, so the angle of reflection is 40°, measured from the normal.
Example 2: Predicting the bend
A ray of light passes from air into a glass block. State whether it bends towards or away from the normal, and why. It bends towards the normal, because glass is denser than air, so the light slows down as it enters, causing it to change direction towards the normal.
Example 3: Explaining why a straw looks bent
Explain why a straw in a glass of water appears bent at the surface. Light from the part of the straw underwater refracts as it passes from water into air, bending away from the normal. This changes the apparent direction the light comes from, so the straw appears bent at the water surface even though it is straight.
Example 4: Frequency during refraction
When light passes from air into glass, what happens to its frequency, speed and wavelength? The speed decreases and the wavelength decreases, but the frequency stays the same, because frequency is set by the source and does not change when the wave enters a new material.
Common mistakes and how to avoid them
The most common error is measuring angles from the surface instead of from the normal. Every angle in this topic — incidence, reflection and refraction — is measured from the normal, which is drawn at 90° to the surface. Measuring from the surface gives the wrong value.
Students often forget to draw the normal at all, or draw it solid. Draw it as a dashed line at right angles to the boundary, and measure every angle from it.
Another mistake is saying refraction happens "because the material is denser". The direct cause is that the wave changes speed; the density change is why the speed changes. Always mention the change in speed.
A frequent error is saying the frequency changes during refraction. It does not — frequency stays the same, while speed and wavelength change.
Finally, do not forget that a ray hitting the boundary along the normal does not bend, even though it still changes speed.
Exam technique for "Reflection, refraction and wave behaviour"
Ray diagrams are commonly worth several marks, so practise drawing them neatly with a ruler: boundary, dashed normal, incident ray, refracted or reflected ray, and labelled angles measured from the normal.
For explanation questions, use the phrase "the wave changes speed as it enters a different material, which causes it to change direction". For a denser material, add that it slows down and bends towards the normal.
When explaining everyday effects, such as a bent straw or a swimming pool looking shallower than it is, link them to refraction bending the light as it leaves the water. For reflection questions, always state that angle of incidence equals angle of reflection, and distinguish specular from diffuse reflection when asked about surfaces.
Quick revision summary
- All angles are measured from the normal, drawn at 90° to the surface.
- Reflection: angle of incidence = angle of reflection; smooth surfaces give specular reflection (clear image), rough surfaces give diffuse reflection (scattered).
- Refraction: a wave changes direction because it changes speed on entering a new material.
- Into a denser material, light slows and bends towards the normal; into a less dense material, it speeds up and bends away.
- During refraction, speed and wavelength change but frequency stays the same.
- A wave meeting a boundary along the normal changes speed but does not change direction.