GCSE Physics revision

Reflection of waves (physics only)

Waves in air, fluids and solids

AQA 4.6.1.3

Your specification

AQA student objectives

Learning pathway

All · Most · Some

ALL 🎯

Construct ray diagrams to illustrate the reflection of a wave at a surface.

MOST 🎯🎯

Apply the specified scientific knowledge of reflection of waves to a relevant example.

SOME 🎯🎯🎯

Analyse an unfamiliar example of reflection of waves using the specified scientific ideas.

Revision summary

Key knowledge

Read on screen, then print for Cornell-style active revision.

What Happens When a Wave Meets a Boundary?

When a wave arrives at a boundary between two materials (mediums), one of three things can occur: absorption, transmission, or reflection.
Absorption is when the wave's energy is transferred into the energy stores of the material it hits.
Transmission is when the wave enters and passes through the material, often leading to refraction.
Reflection is when the wave bounces off the surface and never enters the material at all.
Which of the three outcomes occurs depends on the wavelength of the wave and the properties of both materials involved.

1. What Happens When a Wave Meets a Boundary?AQA

Notes

When a wave arrives at a boundary between two materials (mediums), one of three things can occur: absorption, transmission, or reflection.
Absorption is when the wave's energy is transferred into the energy stores of the material it hits.
Transmission is when the wave enters and passes through the material, often leading to refraction.
Reflection is when the wave bounces off the surface and never enters the material at all.
Which of the three outcomes occurs depends on the wavelength of the wave and the properties of both materials involved.

Introducing Ray Diagrams

Ray diagrams are used to represent and visualise the reflection of waves, such as light, at a boundary.
The diagram typically begins with a boundary line separating two materials, for example air above and a mirror below.
An incoming ray (incident ray) is drawn with an arrow pointing towards the boundary to show the direction of travel.

2. Introducing Ray DiagramsAQA

Notes

Ray diagrams are used to represent and visualise the reflection of waves, such as light, at a boundary.
The diagram typically begins with a boundary line separating two materials, for example air above and a mirror below.
An incoming ray (incident ray) is drawn with an arrow pointing towards the boundary to show the direction of travel.

The Normal Line

The normal is a dashed line drawn perpendicular (exactly 90°) to the boundary at the point where the incoming ray strikes the surface.
The normal is essential because all angles of incidence and reflection are measured from the normal, not from the boundary surface itself.
The point where the incoming ray meets the boundary is called the point of incidence.

3. The Normal LineAQA

Notes

The normal is a dashed line drawn perpendicular (exactly 90°) to the boundary at the point where the incoming ray strikes the surface.
The normal is essential because all angles of incidence and reflection are measured from the normal, not from the boundary surface itself.
The point where the incoming ray meets the boundary is called the point of incidence.

The Law of Reflection

The law of reflection states that the angle of incidence is always equal to the angle of reflection.
The angle of incidence is the angle between the incoming ray and the normal.
The angle of reflection is the angle between the reflected ray and the normal.
For example, if the angle of incidence is 45°, the angle of reflection must also be 45°.
In an exam, you may be required to measure these angles accurately using a protractor.

4. The Law of ReflectionAQA

Notes

The law of reflection states that the angle of incidence is always equal to the angle of reflection.
The angle of incidence is the angle between the incoming ray and the normal.
The angle of reflection is the angle between the reflected ray and the normal.
For example, if the angle of incidence is 45°, the angle of reflection must also be 45°.
In an exam, you may be required to measure these angles accurately using a protractor.

Drawing a Complete Ray Diagram

To draw a ray diagram: draw the boundary, add the incoming ray with an arrow, draw the normal as a dashed perpendicular line at the point of incidence, measure the angle of incidence, then draw the reflected ray at an equal angle on the other side of the normal.
The reflected ray is drawn on the opposite side of the normal to the incoming ray, at the same angle.

5. Drawing a Complete Ray DiagramAQA

Notes

To draw a ray diagram: draw the boundary, add the incoming ray with an arrow, draw the normal as a dashed perpendicular line at the point of incidence, measure the angle of incidence, then draw the reflected ray at an equal angle on the other side of the normal.
The reflected ray is drawn on the opposite side of the normal to the incoming ray, at the same angle.

Specular Reflection

Specular reflection occurs when light reflects off a perfectly smooth, flat surface such as a mirror.
Because the surface is flat, all the normals point in exactly the same direction, so all incoming light rays are reflected in the same direction.
Specular reflection produces a clear, sharp image, which is why you can see your reflection in a mirror.

6. Specular ReflectionAQA

Notes

Specular reflection occurs when light reflects off a perfectly smooth, flat surface such as a mirror.
Because the surface is flat, all the normals point in exactly the same direction, so all incoming light rays are reflected in the same direction.
Specular reflection produces a clear, sharp image, which is why you can see your reflection in a mirror.

Diffuse (Scattered) Reflection

Diffuse reflection (also called scattered reflection) occurs when light reflects off a rough surface, such as paper.
Although the incoming rays are parallel, the bumpy surface means the normals point in many different directions, causing reflected rays to scatter in all directions.
Because the reflected light scatters, you cannot see a clear image of yourself in rough materials like paper.
Importantly, the law of reflection still applies at every point on a rough surface — it is the varying direction of the normals that causes scattering, not a violation of the law.

7. Diffuse (Scattered) ReflectionAQA

Notes

Diffuse reflection (also called scattered reflection) occurs when light reflects off a rough surface, such as paper.
Although the incoming rays are parallel, the bumpy surface means the normals point in many different directions, causing reflected rays to scatter in all directions.
Because the reflected light scatters, you cannot see a clear image of yourself in rough materials like paper.
Importantly, the law of reflection still applies at every point on a rough surface — it is the varying direction of the normals that causes scattering, not a violation of the law.

Key Comparison: Specular vs Diffuse Reflection

The key difference between specular and diffuse reflection is the smoothness of the reflecting surface, not a difference in the law of reflection.
Smooth surfaces (e.g. mirrors) → all normals aligned → specular reflection → clear image formed.
Rough surfaces (e.g. paper) → normals in different directions → diffuse reflection → no clear image formed.

8. Key Comparison: Specular vs Diffuse ReflectionAQA

Notes

The key difference between specular and diffuse reflection is the smoothness of the reflecting surface, not a difference in the law of reflection.
Smooth surfaces (e.g. mirrors) → all normals aligned → specular reflection → clear image formed.
Rough surfaces (e.g. paper) → normals in different directions → diffuse reflection → no clear image formed.