3.5.4Stoichiometry
Investigating the Refraction of Light — Cambridge IGCSE Chemistry 0620 Revision Notes & Topical Questions
5 min read 5 parts Lesson 10 of 28 Unit quiz
Section 1 of 5Experimental Method
A ray box and a block are used to trace the path of light through the material.
1
Place the block on paper and trace around its outline with a sharp pencil, then keep the block nearby for reference.
2
Direct a narrow ray of light at one side face and mark two points on the incident ray, plus the points where the ray enters and leaves the block.
3
Remove the block, join the marked points with straight lines, and draw the normal at the entry point using a set square.
4
Use a protractor to measure the angle of incidence and the angle of refraction, then repeat for several angles and for the second block.
Section 2 of 5Analysing the Results
Angles are converted to sines so that the refractive index can be read from a graph.
Sine graph
- Plot sin i on the vertical axis against sin r on the horizontal axis;
- each block should give a straight line through the origin.
Gradient
- The gradient of the sin i against sin r graph equals the refractive index n of the material, because n = sin i ÷ sin r.
Steeper line
- The glass line is steeper than the Perspex line because glass has the higher refractive index of the two blocks.
Entering the block
- Light entering a block bends towards the normal, so the angle of refraction is smaller than the angle of incidence (i > r).
Leaving the block
- Light leaving the block bends away from the normal, so the angle of refraction is larger than the angle of incidence (i < r).
| Angle of incidence, i | sin i | Angle of refraction, r | sin r |
|---|---|---|---|
| 30° | 0.500 | 19.5° | 0.333 |
| 60° | 0.866 | 35.3° | 0.577 |
Section 3 of 5Evaluating the Experiment
Evaluating the Experiment
Systematic error
- Normal lines drawn at 90° to the block face must be accurate, so use a set square and keep the paper flat.
Random error
- Mark points in the middle of a sharply focused beam with a sharpened pencil to reduce reading uncertainty.
Resolution
- A protractor reading to 1° limits precision, so a protractor with finer divisions gives more reliable angles.
Safety
- Ray boxes can burn skin and light can damage eyes, so never touch the lamp or look directly into the beam.
Section 4 of 5Worked Example
Question
- A student records the data for a glass block in the table.
- Use the two rows to find the refractive index of the glass.
- Step 1: Use n = sin i ÷ sin r for each row; the two values should give the same answer for a straight line.
- Step 2: Find the gradient of sin i against sin r: (0.866 − 0.500) ÷ (0.577 − 0.333) = 0.366 ÷ 0.244.
- Step 3: Calculate the gradient: 0.366 ÷ 0.244 = 1.50, which is the refractive index of the glass.
Answer The refractive index of the glass is 1.50, the gradient of the sin i against sin r graph.
Section 5 of 5Exam Checklist & Tips
3.5.4 – Key Points for the Exam
- Refraction The change in direction of a wave when it crosses a boundary because its speed changes.
- Refractive index A number equal to the speed of light in a vacuum divided by its speed in the material.
- Normal An imaginary line drawn at right angles to the boundary at the point where the ray meets it.
- Angle of incidence The angle between the incident ray and the normal, measured in degrees.
- Sin i against sin r graph A straight-line graph whose gradient gives the refractive index of the material.
Common Mistakes
- Measuring angles from the surface of the block instead of from the normal.
- Cancelling sin i and sin r as if they were ordinary fractions, or using radians instead of degrees on the calculator.
- Saying that light changes colour when it refracts;
- only its speed and wavelength change, while its frequency stays the same.
- Stating that light bends towards the normal when it leaves a denser material.
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