Electromagnetic waves in matter and the refraction core practical

A ray is a line representing the direction in which light travels. Arrowheads show its direction. To describe how it bends, draw a normal at the point where it meets a boundary. This line is perpendicular to the surface. Measure the angle of incidence, $i$, between the incoming ray and the normal, and the angle of refraction, $r$, between the transmitted ray and the normal.

Light travels more slowly in glass than in air. When it enters glass at an angle, it bends towards the normal, so $r<i$. When it leaves glass for air, it speeds up and bends away from the normal, so $r>i$ at that boundary.

The opposite faces of a rectangular block are parallel. The bending at the second face reverses the change in direction at the first, so the emerging ray is parallel to the original incoming ray, but usually displaced sideways. If the ray enters along the normal, it changes speed but does not change direction at either face.

Core practical: trace and measure the light path

The aim is to investigate light travelling from air into glass and from glass into air. Use a ray box with a single slit, low-voltage power supply, rectangular glass block, plain paper, ruler and protractor. The slit produces one narrow ray whose path can be marked accurately. Ray boxes can become hot enough to cause burns, so avoid touching hot parts.

1. Place the paper on the desk and set up the ray box to shine a single ray across it. Place the glass block on the paper and draw around it.
2. Shine the ray into one face at an angle. Mark small crosses along the incoming and emerging rays. Use at least two well-separated marks on each ray so that its direction can be reconstructed.
3. Remove the block. Use a ruler to join the marks and extend the ray lines to the block outline. Join the entry and exit points to show the path inside the glass.
4. Draw a normal at each boundary. Measure $i$ and $r$ at entry, then measure $i$ and $r$ at exit. At exit, the incoming ray is the one travelling inside the glass.
5. Replace the block exactly within its outline and repeat for several different entry angles.
6. Finally, aim the ray at right angles to the surface. Observe that it travels straight through without bending.

Keep the material and block the same while changing the entry angle. This allows you to investigate how angle of refraction depends on angle of incidence, rather than mixing the effects of angle and material.

Recording results and recognising the pattern

Record four angles for each trial, keeping the entry and exit measurements in separate pairs:

Plot a scatter graph with angle of incidence on the horizontal axis and angle of refraction on the vertical axis, both in degrees. Plot the air-to-glass and glass-to-air results on the same axes, distinguish the two sets, and draw a separate smooth curve of best fit for each. Each curve should follow the overall trend rather than joining successive measurements with straight segments.

Data for Illustrative entry and exit angle patterns

The graph shows the pattern to look for. Its straight joins are only guides to the pattern, not examples of smooth best-fit curves. In both directions, increasing the angle of incidence increases the angle of refraction: there is a positive correlation. For air to glass, the refracted angle is smaller than the incident angle; for glass to air, it is larger. For example, the air-to-glass point $(30°,19°)$ represents bending towards the normal. The corresponding exit measurement is approximately $(19°,30°)$, representing bending away from it.

Use your ray traces as well as the graph: the incoming and emerging rays should be parallel. Together, these observations support the conclusion that light changes direction at the boundaries because it travels at different speeds in air and glass.

To evaluate accuracy, consider how precisely you could locate the ray centres and read the angles. A narrow ray, small crosses, well-separated marks and thin ruler lines make the path easier to reconstruct. Centre the protractor on the boundary point and align its zero with the normal. Repeating measurements helps reveal inconsistent readings.

Higher tier: the wavelength matters

A substance does not necessarily interact with all electromagnetic wavelengths in the same way. It may transmit one range, absorb another and reflect another. For example, glass transmits visible light but can absorb ultraviolet radiation. The amount absorbed depends on the type of glass and the wavelength, so being transparent to visible light does not mean being transparent to every electromagnetic wave.

Refraction also depends on wavelength. Different wavelengths can travel at different speeds in the same material and therefore bend by different amounts. A glass prism separates white light into its constituent colours by this process, called dispersion. Violet light is refracted more than red light, spreading the colours into a spectrum.

Core practical recall

Single-slit ray box and power supply; rectangular glass block; paper, ruler and protractor.

Trace block → mark incoming and emerging rays → remove block and join marks → join entry to exit → draw both normals → measure both pairs of angles → repeat at different angles and at normal incidence.