3.2.5Waves
Core Practical: Investigating Snell’s law — Edexcel International GCSE Science Double Award 4SD0 - Physics Revision Notes & Topical Questions
5 min read 7 parts Lesson 13 of 14
Section 1 of 7Aim and variables
Aim
- Aim To investigate how the angle of refraction depends on the angle of incidence for light passing into a rectangular glass or Perspex block,
- and so find the refractive index of the material.
Variables
- Variables Independent variable: the angle of incidence, i.
- Dependent variablethe angle of refraction, r.
Control variables
- Control variables Use the same block throughout, the same narrow beam width and light of the same colour (frequency and wavelength).
- Always measure angles from the normal, not from the surface of the block.
Section 2 of 7Equipment and its purpose
| Equipment | Purpose |
|---|---|
| Ray box with single slit | Gives a narrow beam of light that is easy to mark |
| Rectangular glass or Perspex block | The transparent material that refracts the light |
| Sheet of plain paper | Holds the marks showing the ray paths and the outline of the block |
| Sharp pencil and ruler | Mark points and draw straight lines accurately |
| Protractor (resolution 1°) | Measure the angles of incidence and refraction |
| Set square | Draw the normal at exactly 90° to the block face |
Section 3 of 7Method
1
Place the block on the paper and draw carefully around its outline with a sharp pencil.
2
Draw a dashed normal at right angles to the long side of the outline, at the point where the ray will enter.
3
Use the protractor to mark a line at the first angle of incidence, for example 10°, and note it.
4
Switch on the ray box and aim the beam along this line towards the point on the normal.
5
Mark a point on the ray near the ray box, the point where it enters the block, the point where it leaves the block, and a point on the emerging ray about 5 cm from the block.
6
Remove the block and join the points with three straight lines. Draw the ray inside the block from entry to exit.
7
Measure the angle of incidence and the angle of refraction between each ray and the normal, then replace the block and repeat for at least five further angles, for example up to 70°.
Section 4 of 7Processing the results
1Record
- Record Put i and r in a table, with units in the column headings (degrees).
2Calculate sines
- Calculate sines Use a calculator in degree mode to find sin i and sin r for each row.
3Plot a graph
- Plot a graph Plot sin i on the y-axis against sin r on the x-axis and draw a straight line of best fit through the origin.
4Find the gradient
- Find the gradient The gradient is the refractive index, n = sin i ÷ sin r.
- A glass block usually gives a value of about 1.5.What the symbols mean: n = refractive index (no unit) · i or θ1 = angle of incidence · r or θ2 = angle of refraction
- (both measured from the normal)
- A glass block usually gives a value of about 1.5.
Section 5 of 7Errors, improvements and safety
Evaluating the practical
Systematic error
- Systematic error If the normal is not drawn at exactly 90° every angle is shifted in the same direction.
- Use a set square to draw it.
Random error
- Random error Points marked at the edge of a wide beam are inaccurate.
- Use a sharp pencil, a narrow beam and mark in the middle of it.
Resolution
- Resolution A protractor with 1° divisions limits the accuracy, so use a finer one, or repeat and average.
Safety
- Safety The ray box bulb gets hot and can burn skin, so cool any burn under running water for at least five minutes.
- Do not look directly at the lamp, and keep liquids away from the electrical equipment.
Section 6 of 7Key terms
| Term | Meaning |
|---|---|
| Refraction | The change in direction of a wave when it passes from one medium into another with a different speed. |
| Normal | A line drawn at right angles to a boundary at the point where the ray meets it. |
| Angle of incidence | The angle between the incoming ray and the normal. |
| Angle of refraction | The angle between the refracted ray and the normal. |
| Refractive index | A measure of how much a material slows and bends light; n = sin i ÷ sin r. |
Section 7 of 7Exam technique
1
Measuring angles from the surface of the block instead of from the normal. Both angles are always taken from the normal.
2
Plotting the graph the wrong way round. The specification asks for sin i on the y-axis and sin r on the x-axis so that the gradient equals n.
3
Calculating sines with the calculator in radian mode.
4
Marking points at the edge of the beam instead of its centre, which makes the lines inaccurate.
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