Lesson focus: A reflection investigation tests an equality by measuring paired angles and evaluating uncertainty rather than seeking a perfect table.
Plan, analyse and evaluate a ray-box investigation of the law of reflection.
Build on earlier learning
ReflectionThe previous lesson stated the reflection law. This lesson asks whether measured evidence supports it.
Practical
The law of reflection
- Draw a line for the mirror and a normal at 90° to it.
- Shine a narrow ray at the mirror where the normal meets it.
- Measure the angles of incidence and reflection from the normal.
- Use the model on this page to test more angles.
Record: A table of angle of incidence against angle of reflection.
The law of reflection
Change the angle of incidence. Both angles are measured from the normal.
Controls
You will be successful when you can…
- Identify variables in a reflection investigation.
- Describe safe, precise ray-box and protractor use.
- Compare incidence and reflection data.
- Handle anomalies and uncertainty transparently.
Words that carry the science
- independent variable
- The angle of incidence deliberately changed.
- dependent variable
- The angle of reflection measured.
- control variable
- A condition kept consistent, such as mirror position or ray width.
- parallax
- An apparent shift caused by viewing a mark or scale from different positions.
- anomaly
- A result that does not fit the main pattern and should be investigated.
- line of equality
- A graph line where dependent and independent values are equal.
Test the law with paired angle measurements
01The method must locate one point of incidence
Place a plane mirror on a marked line and direct a narrow ray to a chosen point. Draw the normalAn imaginary line drawn at right angles (90°) to a surface where a ray of light hits it. at 90° through that point before measuring either angle.
Change the incidence angle across a suitable range. Measure both angles from the normal and keep mirror position, surface and ray width consistent. Never look directly into a bright ray source.
02Measurements have finite uncertainty
A thick pencil line, broad light ray, misaligned mirror or tilted protractor can shift an angle. Repeats reveal random variation, while the protractor resolution limits sensible precision.
Reading directly above the protractor centre reduces parallaxA reading mistake made when you look at a scale from an angle instead of straight on.. Recording whole degrees does not imply the true angle is exact.
03Evidence should approach equality
If the law is supported, reflection angle should increase one-for-one with incidence angle. A graph of reflection against incidence should lie close to a straight line of equalityA line on a graph where both values are the same. through the origin.
An unexpected point should be checked against the diagram and repeated. Keep the raw value and state any reason for excluding it from a mean or fit.
Judge an unexpected result
At incidence 40°, a group measures reflection angles 39°, 40° and 54°.
- Compare each value with the predicted 40°.
- The 39° and 40° readings agree within a small measurement difference.
- The 54° value is separated from both repeats and the expected pattern.
- Retain it, check the setup and repeat before deciding whether a procedural error caused it.
How to make the reasoning visible: The 54° reading is a possible anomalyA result that does not fit the pattern and should be checked.; it should be investigated, not silently erased.
Analyse illustrative reflection data
These illustrative results come from a ray-box experiment.
| Incidence / ° | Reflection / ° trial 1 | Trial 2 | Judgement |
|---|---|---|---|
| 10 | 10 | 11 | Supports equality |
| 25 | 24 | 25 | Supports equality |
| 40 | 40 | 54 | One possible anomaly |
| 55 | 54 | 55 | Supports equality |
| 70 | 69 | 70 | Supports equality |
Describe the overall relationship and support it with two values.
Suggest one cause of the 54° result and a linked improvement.
Check your understanding
Answer from memory, then check against the suggested answer.
What is the independent variable?
What is measured as the dependent variable?
How should a possible anomaly be handled?
A graph of reflection angle against incidence angle has gradient 0.98 and intercept 1°. Does it broadly support the law?
Explain how a mirror consistently tilted by 2° from its marked line could create systematic error.
Exam Question Practice
Exam Question Practice
Mark points shown after marking
A student investigates reflection using a broad ray of light. She holds the mirror by hand and records one pair of angles for each setting.
Identify three problems with this method.
(3)Suggest how to improve the method and judge the results.
(2)Total for this question: 5 marks
Command word: For evaluateA command word: weigh up the good points and bad points, then give your overall judgement., pair every limitation with its effect and a practical improvement.