Investigate a light ray passing into and out of a rectangular glass block. Measure angles to the normal and relate ray directions to a change in speed at the boundary.Original labelled apparatus schematic; not to scale. Follow the stated measurements and safety instructions.
Use a ray box with its slit/grating attachment to make a single narrow beam, glass block, plain paper, sharp pencil, ruler and protractor. Keep the block on a traced outline so its position can be checked.
Do not look into the beam; use a ray box safely and let a hot lamp cool. Handle the block gently because chipped glass can cut.
The incident ray approaches the block; the refracted ray travels inside it; the emergent ray leaves it. The normal is an imaginary line at right angles to the surface where the ray enters or leaves.
Tracing and measuring
Draw the block outline, then direct a narrow ray at an angle to the normal into one face. Mark two well-separated points along the incident ray and two along the emergent ray.
Remove the block after marking. Join the points with thin ruled lines, extending them to the outline; join entry and exit points to reconstruct the path inside the block.
Draw a normal at each boundary using a right angle. Measure incidence i and refraction r between the relevant ray and normal, not between the ray and block surface.
Repeat for several incidence angles, replacing the block exactly on its outline. Keep the same block, light colour and entry face and record angles clearly in degrees.
At air-to-glass entry, the ray slows and bends towards the normal. At glass-to-air exit, it speeds up and bends away from the normal.
When a ray enters straight along the normal (normal incidence), it does not change direction, although its speed and wavelength change. Frequency stays the same as light crosses the boundary.
For a rectangular block with parallel faces and air on both sides, the emergent ray is parallel to the incident ray but shifted sideways (laterally displaced). This does not mean its internal direction is unchanged.
If the examination supplies a refractive-index relationship, use the measured angles to the normal. Do not replace the required tracing practical with an unsupported formula-only answer.
Precision, errors and conclusions
A narrow single beam, sharp pencil and widely spaced ray marks make direction more precise. Thick lines or marking close together increase angular uncertainty.
Centre the protractor accurately on the entry point and align its baseline with the normal. Read the correct scale and avoid parallax.
Do not move the block between incident and emergent marks. A misplaced outline gives an incorrect reconstructed internal ray; repeats with the same misplacement can remain biased.
Use a dim enough room to see the ray but retain safe visibility. Reduce stray light and check the slit makes one clear ray rather than several overlapping ones.
Repeat angle measurements, compare spread and select a useful range. At very small incidence angles, the bending can be comparable with measurement uncertainty.
Support the conclusion with measured angle pairs and the ray diagram. Reflection may also occur at the surface; distinguish the transmitted ray from a reflected ray.
Exam skills: planning, precision and evaluation
State what you change (the independent variable), what you measure (the dependent variable) and what you keep the same (control variables). Explain how you keep each control variable constant, rather than just saying “make it fair”.
Accuracy means how close a result is to the true value. Precision means how close repeated measurements are to each other. Resolution is the smallest change an instrument can show. More digits on a display do not automatically mean a more accurate result.
Repeat measurements for each condition, calculate a mean and describe how spread out the results are. This helps assess and reduce the effect of random errors. Repeating cannot fix an error that pushes results consistently in one direction (a systematic error), such as measuring ray angles from the surface instead of the normal.
Repeatability means getting similar results when the same person repeats the same method with the same equipment. Reproducibility means getting similar results when someone else, or different suitable equipment, repeats the experiment. Results can be consistent but still inaccurate.
Check that instruments read zero correctly and are calibrated where needed. Read scales at eye level: looking from an angle can give a wrong reading (parallax error). Choose suitable ranges, measurement intervals and scale divisions (resolution).
Write down the original readings straight away in a table, with units in the headings. Use decimal places that match the instrument’s resolution. Keep the original data and round only when needed. Do not discard a result just because it differs from your prediction.
An anomalous result does not fit the pattern of the other results. Repeat that measurement and check the method. Only leave it out of a mean if you have a clear reason; state which result you excluded and why.
For continuous variables, plot the independent variable on the horizontal axis and the dependent variable vertically. Use sensible scales, units and a best-fit line or curve; do not automatically join every point or force the graph through zero.
Find the gradient of a straight best-fit line using a large triangle: vertical change ÷ horizontal change. For a curve, draw a tangent to estimate the gradient at one point. Explain what the gradient shows in this experiment, include its units and use measured values to support your conclusion.
Uncertainty describes the possible range around a measurement. For one reading on a scale, half the smallest division is a useful classroom estimate unless the question says otherwise. If you subtract two readings, both have uncertainty. Percentage uncertainty = absolute uncertainty ÷ measured value × 100. Follow the method specified in the question.
Use results as evidence and then explain what they mean. A pattern linking variables (a correlation) does not prove that one causes the other. If the ranges of repeat results overlap, a claimed difference may be less convincing. Keep conclusions within the range tested and suggest an improvement that tackles a specific error.
Mr Kidman's Classroom