Practical: measuring the refractive index of glass revision notes

Edexcel International GCSE Physics (4PH1), chapter 12: Light waves, specification point 3.19. Free lesson.

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Measure how light refracts through a glass block at a range of incidence angles, then use a graph to find the refractive index. Using several readings and a line of best fit makes the result more reliable.

How to measure the angles

A normal is a line at 90° to the surface. Measure both the angle of incidence, i, and the angle of refraction, r, from the normal. The refracted ray is measured inside the glass block. Use a rectangular glass block, a ray box with a narrow slit, white paper, a ruler and a protractor.

How to plot the results

Work out sin i and sin r for each pair of readings using a calculator. Plot sin i on the vertical axis and sin r on the horizontal axis. Sine values are ratios, so the axes have no units. Plot the sines, not the angles themselves: the angles produce a curve, while the sine values should form a straight line through the origin. Draw a line of best fit through the points. The gradient of this line is the refractive index, n.

How to find the refractive index

Find the gradient using a large triangle on the line of best fit. Calculate the rise divided by the run, using the differences in the values on each axis. The gradient equals the refractive index, which has no unit.

Why use several readings and a graph?

A single reading can be spoiled by one badly drawn ray or a small measurement error. A line of best fit uses all six readings, so small errors are averaged out. It also makes an anomaly, a result that does not fit the pattern, easier to spot. A straight line through the origin checks that sin i ÷ sin r is constant. Take extra care with large angles. The beam spreads out along the face of the block, so a small slip can change the measured angle a lot. A thin ray, a sharp pencil and careful protractor work improve the measurements.

How to improve a one-reading method

If a method uses one angle and a wide ray, improve both the ray and the number of readings. Use a narrower slit so the ray is thin and its middle is clear. Use a range of incidence angles, plot sin i against sin r, and take the gradient.