Grade: 10
Subject: Physical Sciences
Term: 2
CAPS Type: Informal Practical Investigation
Topic: Geometrical Optics, Reflection of Light
Aim
To investigate the relationship between the angle of incidence and the angle of reflection using a plane mirror.
Learning Outcome
- Define reflection of light.
- Identify the incident ray, reflected ray and normal.
- Measure angles correctly using a protractor.
- Investigate the relationship between the angle of incidence and angle of reflection.
- Verify the law of reflection.
- Distinguish between regular and diffuse reflection.
Time Allocation
Approximately 45 to 60 minutes.
How Does It Work?
Reflection occurs when light bounces off a surface.
When light strikes a smooth reflective surface such as a plane mirror, the light reflects at a predictable angle.
Angle of Incidence = Angle of Reflection
The angle of incidence is measured between the incident ray and the normal.
The angle of reflection is measured between the reflected ray and the normal.
The normal is an imaginary line drawn at 90° to the mirror surface.
Scientific Background
A plane mirror is a flat reflective surface that produces regular reflection.
When light strikes the mirror:
- The incident ray travels towards the mirror.
- The reflected ray travels away from the mirror.
- The incident ray, reflected ray and normal all lie in the same plane.
- The angle of incidence is equal to the angle of reflection.
This relationship is called the law of reflection.
Important Definitions
| Term | Definition |
|---|---|
| Incident Ray | The incoming ray of light striking the mirror. |
| Reflected Ray | The ray of light bouncing off the mirror. |
| Normal | An imaginary line drawn at 90° to the mirror surface. |
| Angle of Incidence | The angle between the incident ray and the normal. |
| Angle of Reflection | The angle between the reflected ray and the normal. |
| Plane Mirror | A flat reflective surface. |
Regular vs Diffuse Reflection
| Type of Reflection | Surface Type | Behaviour of Light |
|---|---|---|
| Regular Reflection | Smooth surface | Reflected rays remain parallel. |
| Diffuse Reflection | Rough surface | Reflected rays scatter in different directions. |
Hypothesis
If the angle of incidence increases, then the angle of reflection will increase by the same amount.
Variables
Independent Variable
Angle of incidence.
Dependent Variable
Angle of reflection.
Control Variables
- Same mirror.
- Same ray box.
- Same paper.
- Same measurement method.
- Same mirror position.
Apparatus
- Ray box or light box
- Single slit ray plate
- Plane mirror
- White paper
- Pencil
- Ruler
- Protractor
- Adhesive tape
Safety Precautions
- Do not shine the light directly into anyone’s eyes.
- Handle the mirror carefully to avoid breakage.
- Keep the work area neat and dry.
- Switch off the ray box when not in use.
Important Note
Angles must always be measured from the normal, not from the mirror surface.
Experimental Setup
Preparing the Ray Diagram
- Place a sheet of white paper on the desk.
- Draw a straight horizontal line near the centre of the page.
- This line represents the mirror position.
- Mark a point near the centre of the line.
- Place the centre of the protractor on the marked point.
- Draw a dotted line at 90° to the mirror line.
- This dotted line is the normal.
Setting the Incident Ray
- Place the protractor on the normal.
- Measure an angle of 45° from the normal.
- Mark two small points along this angle.
- Use a ruler to join the points to form the incident ray.
- Draw an arrow showing the direction of light towards the mirror.
Method
- Place the back edge of the plane mirror directly on the mirror line.
- Set up the ray box using the single slit ray plate.
- Direct the thin light ray exactly along the incident ray.
- Observe the reflected ray.
- Mark two small points along the reflected ray while the mirror remains in place.
- Remove the mirror and ray box carefully.
- Use a ruler to join the reflected ray points.
- Draw an arrow showing the direction of the reflected light.
- Use a protractor to measure the angle of incidence and the angle of reflection.
- Record the results in the table.
- Repeat the investigation using at least four different angles of incidence.
Results Table
| Trial | Angle of Incidence (°) | Angle of Reflection (°) |
|---|---|---|
| 1 | ||
| 2 | ||
| 3 | ||
| 4 |
Sample Results
| Trial | Angle of Incidence (°) | Angle of Reflection (°) |
|---|---|---|
| 1 | 20 | 20 |
| 2 | 35 | 35 |
| 3 | 45 | 45 |
| 4 | 60 | 60 |
Ray Diagram Requirements
- Incident ray
- Reflected ray
- Normal
- Angle of incidence
- Angle of reflection
- Direction of light travel
- Mirror line
What Learners Should Observe
- The light reflects off the mirror surface.
- The reflected ray leaves the mirror at the same angle as the incident ray.
- Increasing the angle of incidence also increases the angle of reflection.
- The two angles remain equal in every trial.
Analysis of Results
Compare the angle of incidence and the angle of reflection for each trial.
Learners should find that:
Angle of Incidence = Angle of Reflection
This confirms the law of reflection.
Accuracy and Reliability
Improving Accuracy
- Use a thin light ray.
- Draw sharp pencil lines.
- Measure angles carefully.
- Keep the mirror fixed.
- Use a properly aligned protractor.
Improving Reliability
- Repeat the experiment several times.
- Use different angles of incidence.
- Compare repeated measurements.
Sources of Experimental Error
- Thick light rays.
- Incorrect protractor placement.
- Poor line drawing.
- Moving the mirror accidentally.
- Parallax error when reading angles.
- Unclear reflected rays.
Important Notes
- Angles must always be measured from the normal.
- The normal must be drawn at 90° to the mirror.
- A thinner light ray improves measurement accuracy.
- Performing the experiment in a slightly dim room improves ray visibility.
- Plane mirrors produce regular reflection.
Conclusion
The investigation shows that the angle of incidence is equal to the angle of reflection.
The experiment therefore confirms the law of reflection.
Questions for Learners
- What is reflection?
- What is the law of reflection?
- What is the normal?
- Why are angles measured from the normal?
- What happened when the angle of incidence increased?
- What type of reflection occurs on a smooth mirror surface?
- What happens during diffuse reflection?
- Why is a plane mirror used in this experiment?
- Why should the mirror remain fixed?
- How could the reliability of the experiment be improved?
Common Mistakes
- Measuring angles from the mirror surface instead of the normal.
- Drawing the normal incorrectly.
- Using thick light rays.
- Forgetting to label rays.
- Moving the mirror during the experiment.
- Incorrect protractor alignment.
- Drawing untidy ray lines.
Teacher Notes
- Learners often measure angles from the mirror instead of the normal.
- Demonstrate proper protractor use before the practical begins.
- Encourage learners to use sharp pencils and rulers carefully.
- A single slit ray gives the best results.
- This experiment prepares learners for future refraction investigations.
Teacher Tip
Demonstrate one complete ray tracing before learners begin independently. This reduces incorrect measurements and improves learner confidence.
Expected Results
The angle of reflection should always be equal to the angle of incidence.
This relationship confirms the law of reflection.
Extension Activity
Ask learners to investigate reflection using:
- Different mirror angles
- Curved mirrors
- Rough surfaces
- Multiple mirrors
Learners can compare regular reflection and diffuse reflection.
Real-World Application
Reflection is important in mirrors, telescopes, periscopes, vehicle mirrors, cameras, laser systems, optical instruments and security mirrors.