Science required practicals
How do you do the light required practical?
Shine a narrow ray from a ray box into a glass block on paper, mark the rays with crosses, then draw them in and measure the angles of incidence and refraction from the normal with a protractor. Repeat with blocks of other materials, and with different surfaces for reflection. For a mirror, the angle of reflection equals the angle of incidence. This practical is on separate Physics only, not Combined Science.
Last updated 23 September 2026 · Written and fact-checked by the GCSEwiz team
What does the light practical investigate?
It looks at two things light does when it meets a surface. Reflection: how different surfaces reflect a ray of light. Refraction: how much a ray changes direction when it passes into different transparent materials, such as glass and water.
It is a required practical on AQA GCSE Physics only. If you are taking Combined Science: Trilogy, it is not on your list, so you do not need it.
What equipment do you need?
The main kit is a ray box that gives a single narrow ray, plus blocks and surfaces to shine it at.
- A ray box with a single-slit cover, and a power supply
- A rectangular glass block, plus blocks of other materials, such as acrylic or a clear box of water
- A plane mirror, and other surfaces to compare, such as white paper
- A protractor, a ruler and a sharp pencil
- Sheets of plain white paper
What is the method?
Each ray is marked on paper, then drawn in and measured, so careful drawing is most of the practical. Work in a dimmed room so the ray shows up clearly.
- Put the glass block on a sheet of paper and draw round it with a sharp pencil.
- Take the block off. Draw a normal: a line at 90° to one long side, where you want the ray to go in. Put the block back.
- Shine the ray at the point where the normal meets the block, at an angle to the normal.
- Mark the ray going in, and the ray coming out of the far side, with two small crosses on each, placed as far apart as you can.
- Take the block off and join the crosses. Then join the point where the ray went in to the point where it came out: that line is the ray inside the block.
- Measure the angle of incidence and the angle of refraction from the normal with the protractor.
- Repeat for several angles of incidence, then with the other blocks, using the same angles.
- For reflection, stand the mirror on a line drawn on the paper, draw a normal, shine the ray at the point where the normal meets the mirror, and measure both angles. Then try the other surfaces and note how clear the reflected ray is.
What are the variables?
You change the angle at which the ray meets the surface, and measure the angle at which it leaves. To compare materials, you change the block and keep everything else the same.
| Variable | In this practical |
|---|---|
| Independent variable | The angle of incidence (and, when you compare, the material of the block or the type of surface) |
| Dependent variable | The angle of refraction, or the angle of reflection for the mirror |
| Control variable | The same set of angles of incidence for every material, so the results can be compared |
| Control variable | A narrow ray from the same ray box and slit |
| Control variable | The point where the ray enters the block, on the normal you drew |
How do you work out and present the results?
There is no equation to use here: you compare angles, recorded in a table for each material. Light bends towards the normal as it goes from air into the block, and away from the normal as it comes back out. A ray that meets the block along the normal (at 0°) goes straight through without changing direction. The ray that comes out of a rectangular block should be parallel to the ray that went in, which is a handy check on your drawing.
Example: at an angle of incidence of 40°, a ray entering glass refracts to about 25°. Entering water, it refracts to about 29°. The glass bent the ray more, because light slows down more in glass than in water.
Plot the angle of refraction (y-axis) against the angle of incidence (x-axis) for each material on the same axes. Each line is a smooth curve through the origin, and the lower the curve, the more that material bends light. For the mirror, the angle of reflection against the angle of incidence is a straight line through the origin with a gradient of 1, because the two angles are equal.
For the other surfaces, describe what you see. A smooth, shiny surface gives one clear reflected ray (specular reflection). A rough surface such as paper scatters the light in many directions (diffuse reflection), so there is no single ray to measure.
How do you make it accurate and safe?
Use a narrow ray, because with a wide beam it is hard to tell where the middle of the ray is. Put the crosses far apart: a small slip in placing two crosses close together swings the whole line. Draw the normal with a protractor so it really is at 90°, and always measure angles from the normal, never from the surface. Keep the block still while you mark the rays.
On safety: the ray box gets hot, so don't touch it while it is on and let it cool before you pack away. In a dimmed room, keep bags and stools out of the way. Check the glass block for chipped edges and handle it carefully.
What do exam questions ask about it?
Questions ask you to describe the method, or to complete a ray diagram and measure its angles. Higher tier questions can ask you to explain refraction using wave fronts and the change in speed as light enters a new material.
Here is one we wrote in the exam style: "Explain why the student marks each ray with two crosses placed far apart, rather than close together. (2 marks)" A good answer says a small error in placing a cross changes the angle of the line much less when the crosses are far apart, so the measured angles are more accurate.
Common mistakes
- Measuring angles from the surface of the block. Angles are always measured from the normal.
- Drawing the normal by eye. Use a protractor to make it a true 90°.
- Using a wide beam. Fit the single-slit cover to get a narrow ray.
- Letting the block move while you mark the rays. Hold it still, and if it slips, line it up with its outline again before you carry on.
- Expecting a ray along the normal to bend. At 0°, the angle of refraction is also 0°.