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This lesson covers the reflection of light as required by the Edexcel GCSE Physics specification (1PH0), Topic 5: Light and the Electromagnetic Spectrum. You need to understand the law of reflection, the difference between specular and diffuse reflection, how to draw accurate ray diagrams, and how images are formed in plane mirrors.
When a ray of light hits a surface, it bounces off (reflects). The behaviour of the reflected ray follows a precise rule called the law of reflection.
The law of reflection states:
The angle of incidence is equal to the angle of reflection.
Both angles are measured from the normal — an imaginary line drawn perpendicular (at 90°) to the surface at the point where the light ray hits.
| Term | Definition |
|---|---|
| Incident ray | The incoming ray of light that strikes the surface |
| Reflected ray | The ray of light that bounces off the surface |
| Normal | An imaginary line drawn at 90° to the surface at the point of incidence |
| Angle of incidence (i) | The angle between the incident ray and the normal |
| Angle of reflection (r) | The angle between the reflected ray and the normal |
$$\text{Angle of incidence} = \text{Angle of reflection}$$ $$i = r$$
Exam Tip: A very common mistake is to measure the angle from the surface rather than from the normal. Always measure angles from the normal, not from the mirror surface. If the angle of incidence is measured from the surface, it will be wrong and you will lose marks.
You must be able to draw accurate ray diagrams in the exam. Follow these steps:
flowchart TD
A["Draw the mirror surface"] --> B["Mark the point of incidence"]
B --> C["Draw the normal at 90° to the surface<br/>(dashed line)"]
C --> D["Draw the incident ray"]
D --> E["Measure angle of incidence from normal"]
E --> F["Draw reflected ray at<br/>angle of reflection = angle of incidence"]
F --> G["Add arrows showing direction of light"]
style A fill:#2c3e50,color:#fff
style G fill:#27ae60,color:#fff
Exam Tip: Use a ruler and protractor when drawing ray diagrams. Freehand ray diagrams will lose marks. Make sure all angles are measured from the normal, and label i and r clearly.
Not all surfaces reflect light in the same way. The type of reflection depends on the smoothness of the surface.
Specular reflection occurs when light reflects off a smooth, flat surface (such as a mirror, still water, or polished metal).
Diffuse reflection occurs when light reflects off a rough, uneven surface (such as paper, a brick wall, or frosted glass).
| Feature | Specular Reflection | Diffuse Reflection |
|---|---|---|
| Surface | Smooth and flat | Rough and uneven |
| Reflected rays | Parallel | Scattered in many directions |
| Image | Clear reflection formed | No clear image |
| Examples | Mirror, still water, polished metal | Paper, wall, frosted glass |
Exam Tip: Both specular and diffuse reflection obey the law of reflection at each individual point on the surface. The difference is that on a rough surface, the normals point in different directions at different points, so the reflected rays are scattered overall.
A plane mirror is a flat mirror. When you look into a plane mirror, you see an image of yourself. This image has specific properties that you need to know.
The image formed by a plane mirror is:
Exam Tip: Remember the mnemonic "VULS" — Virtual, Upright, Laterally inverted, Same size. This covers the four key properties examiners expect you to state.
To find the position of the image:
| Feature | Real Image | Virtual Image |
|---|---|---|
| Can be projected onto a screen? | Yes | No |
| Where rays meet | Rays actually converge at the image position | Rays only appear to come from the image position |
| Formed by | Converging lenses, concave mirrors | Plane mirrors, diverging lenses |
A ray of light strikes a plane mirror at an angle of 35° to the mirror surface. What is the angle of reflection?
Step 1: The angle to the mirror surface is 35°. But angles must be measured from the normal.
Step 2: The normal is at 90° to the surface, so the angle of incidence = 90° − 35° = 55°.
Step 3: By the law of reflection, the angle of reflection = the angle of incidence = 55°.
Exam Tip: This is a very common exam trap. If the question gives you the angle to the surface (not the normal), you must subtract it from 90° to find the angle of incidence. Read the question carefully.
An object is placed 25 cm in front of a plane mirror. How far is the image from the object?
Step 1: The image is the same distance behind the mirror as the object is in front. So the image is 25 cm behind the mirror.
Step 2: Distance from object to image = 25 cm + 25 cm = 50 cm.