Lesson focus: A refraction investigation maps paired angles while controlling block position, boundary shape and measurement alignment.
Collect and interpret incidence and refraction data and evaluate a glass-block method.
Build on earlier learning
RefractionThe previous lesson predicted how rays bend. This lesson tests the relationship with measured angles.
Practical
Refraction through a glass block
- Draw around a rectangular glass block and draw a normal.
- Shine a ray into the block at an angle and mark the ray in and out.
- Join the marks to show the ray inside the block.
- Measure the angles of incidence and refraction.
Record: A ray diagram and a table showing that the ray bends towards the normal on entering glass.
Refraction through a block
Light slows down as it enters glass or water, so it bends towards the normal. It speeds up as it leaves and bends away, coming out parallel to the ray that went in.
Controls
At 0° the ray goes straight through: it slows down but does not change direction.
You will be successful when you can…
- Plan a ray-box refraction investigation.
- Identify independent, dependent and control variables.
- Interpret incidence–refraction data.
- Explain limitations including thick rays, parallax and block replacement.
Words that carry the science
- angle of incidence
- The angle between the incident ray and normal.
- angle of refraction
- The angle between the refracted ray and normal.
- ray box
- A source used with a slit to produce a narrow beam for ray experiments.
- repeatability
- How closely repeated measurements agree under the same conditions.
- resolution
- The smallest scale interval an instrument can distinguish.
- lateral displacement
- The sideways shift between parallel incident and emergent rays.
Measure refraction without hiding uncertainty
01Trace the block before collecting data
Place the transparent block on paper and trace its boundary. Draw a normal at the chosen entry point, then direct a narrow ray at a measured incidence angle.
Mark two points on each visible ray before removing the block. Joining separated points gives a more reliable direction than tracing a thick glowing line freehand. Avoid looking into the source.
02Change one angle and control the geometry
The independent variable is angle of incidenceThe angle between a light ray hitting a surface and the normal (a line at right angles to the surface). and the dependent variable is angle of refractionThe angle between a light ray after it bends into a new material and the normal (a line at right angles to the surface).. Keep block material, face, entry point and wavelength or colour of light consistent.
Replace the block exactly on its outline for repeats. Small position or rotation changes alter the true boundary and create systematic differences.
03The relationship is not simple equality
For air entering glass, the refraction angle is smaller than the incidence angle, but the two do not differ by one fixed number. The change depends nonlinearly on angle and material.
At higher incidence angles, the difference usually grows. KS3 evidence should describe this pattern without inventing a constant subtraction rule.
Interpret paired angle data
Incidence angles 20°, 40° and 60° produce refraction angles 13°, 25° and 35°.
- Compare each refracted angle with its incident angle.
- Each refracted angle is smaller, supporting bending toward the normal.
- Calculate differences: 7°, 15° and 25°.
- Notice the difference is not constant and becomes larger across this range.
How to make the reasoning visible: The data support refraction toward the normal but do not support subtracting one fixed angle.
Audit a glass-block data set
The values are illustrative. Each angle is measured from the normal.
| Incidence / ° | Refraction / ° trial 1 | Trial 2 | Difference between repeats |
|---|---|---|---|
| 10 | 7 | 7 | 0° |
| 25 | 16 | 17 | 1° |
| 40 | 25 | 24 | 1° |
| 55 | 33 | 34 | 1° |
| 70 | 39 | 51 | 12° |
Describe the main pattern and identify the result requiring investigation.
Suggest a reason high-angle measurements may be difficult and an improvement.
Check your understanding
Answer from memory, then check against the suggested answer.
What is changed deliberately?
What is measured?
Why trace the block outline?
A student measures angles from the block surface rather than the normal. What happens to the data?
Explain why repeating a poorly aligned block can produce precise but inaccurate data.
Exam Question Practice
Exam Question Practice
Mark points shown after marking
A student investigates refraction. She traces a broad ray, moves the block between readings and measures the angles from the surface of the block.
Identify three problems with this method.
(3)Suggest two improvements.
(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., connect each procedural flaw to the measurement it weakens.