Quick answer: Light is reflected, refracted or absorbed when it meets matter. This grade 7 science lab on reflection of light gives the aim, the equipment, the safety points, a step-by-step method, sample results and 8 analysis questions; the answers are on a separate page and the lab sheet is a free printable PDF.
Does light really bounce off a mirror at the same angle that it arrives? In this lab you measure angles of incidence and reflection with a ray box, then see how a clear block bends the ray, so that you can test the law of reflection and describe refraction.
Level: Grade 7 science (Year 8 · Class 7) · Curriculum: MS-PS4-2
Aim
To test the law of reflection with a plane mirror and to see how a ray bends when it enters a clear block.
Equipment
- Ray box, or a torch with a slit made from card
- Small plane mirror with a block to stand it up
- Rectangular block of clear plastic or glass
- Sheet of plain A4 paper, pencil and ruler
- Protractor
Safety
Use a low-power ray box or torch and never shine a laser pointer into anyone’s eyes. Handle the glass or mirror by the edges and tell your teacher if a piece is chipped or cracked.
Method
- Draw a straight line across the paper and stand the mirror on it. Draw the normal at right angles to the mirror at its centre.
- Shine a narrow ray along the paper to hit the mirror at the normal, at an angle of incidence of 15°. Mark two dots on the incident ray and two on the reflected ray.
- Remove the mirror, join the dots and measure the angle of reflection from the normal.
- Repeat for angles of incidence of 30°, 45°, 60° and 75°.
- Replace the mirror with the block. Direct a ray into it at 20°, 40° and 60° and draw the ray inside the block. Measure the angle of refraction each time.
Results
Sample measurements for reflection:
| Angle of incidence i (°) | 15 | 30 | 45 | 60 | 75 |
|---|---|---|---|---|---|
| Angle of reflection r (°) | 16 | 29 | 45 | 61 | 74 |
Sample measurements for refraction into the block:
| Angle of incidence i (°) | 20 | 40 | 60 |
|---|---|---|---|
| Angle of refraction r (°) | 13 | 25 | 35 |
Analysis questions
Question 1 Differences
Calculate i – r for each reflection reading.Question 2 Plotting r against i
Describe the graph of r against i and calculate its gradient from the first and last points.Question 3 Conclusion
State a conclusion about the angles of incidence and reflection.Question 4 Uncertainty
A protractor can be read to about ±1°. Explain why the small differences do not disprove the law.Question 5 Refraction results
Calculate i – r for each refraction reading and describe the pattern.Question 6 Prediction
Predict the angle of refraction for an angle of incidence of 50° by reading between the readings at 40° and 60°.Question 7 Light speed
Explain, using speed, why the ray bends towards the normal in the block.Question 8 Improvement
Suggest two improvements that would make the angle readings more reliable.
