Required Practical: Light

This lesson covers the Required Practical on investigating the reflection of light by different types of surface and the refraction of light by different substances, as required by the AQA GCSE Physics specification (4.6.1). You must know the equipment, method, variables, how to record results, and how to analyse them.

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Overview

AQA requires you to investigate:

1. Reflection of light — verifying the law of reflection using a ray box and plane mirror.
2. Refraction of light — investigating how light refracts as it passes through a glass block.

These experiments test your understanding of how light behaves at boundaries and your ability to take accurate angular measurements.

Exam Tip: This required practical is frequently examined. You may be asked to describe the method, draw the ray diagram, identify sources of error, or interpret results presented in a table or graph. Practise drawing ray diagrams accurately using a ruler and protractor.

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Experiment 1: Investigating the Reflection of Light

Equipment

Equipment	Purpose
Ray box (single slit)	Produces a narrow beam of light
Plane mirror	Reflects the light ray
Protractor	Measures angles of incidence and reflection
Ruler	Draws straight lines and the normal
Plain white paper	Placed on the bench to trace rays
Pencil	Marks the positions of rays on the paper

Method

1. Place a sheet of plain white paper on the bench.
2. Place the plane mirror vertically on the paper. Draw around the base of the mirror to mark its position.
3. Draw the normal — a line perpendicular to the mirror surface at the point where you want the light to hit.
4. Use the ray box to direct a narrow beam of light at the mirror along a chosen angle of incidence.
5. Mark the positions of the incident ray and the reflected ray on the paper using a pencil (mark at least two points along each ray, then use a ruler to draw the lines).
6. Use a protractor to measure the angle of incidence (between the incident ray and the normal) and the angle of reflection (between the reflected ray and the normal).
7. Repeat for at least five different angles of incidence (e.g. 10, 20, 30, 40, 50, 60, 70 degrees).
8. Record the results in a table.

Results Table

Angle of Incidence (degrees)	Angle of Reflection (degrees)
10	10
20	20
30	30
40	40
50	50
60	60
70	70

Analysis

• The results should show that the angle of incidence equals the angle of reflection for every measurement.
• Plotting a graph of angle of reflection (y-axis) against angle of incidence (x-axis) should give a straight line through the origin with a gradient of 1.
• Any deviations from this pattern are due to measurement error, not a failure of the law of reflection.

Exam Tip: If asked to plot a graph of this data, draw a straight line of best fit through the origin. The gradient should be exactly 1. If your points do not lie exactly on the line, this is due to measurement errors (parallax, imprecise protractor reading), not because the law of reflection is wrong.

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Experiment 2: Investigating the Refraction of Light

Equipment

Equipment	Purpose
Ray box (single slit)	Produces a narrow beam of light
Rectangular glass block (or semicircular block)	Refracts the light ray
Protractor	Measures angles of incidence and refraction
Ruler	Draws straight lines and the normal
Plain white paper	Placed on the bench to trace rays
Pencil	Marks the positions of rays

Method

1. Place a sheet of plain white paper on the bench.
2. Place the glass block on the paper and draw around it to mark its position.
3. Draw the normal at the point where the light will enter the glass block (perpendicular to the surface of the block).
4. Use the ray box to direct a narrow beam of light at the glass block at a chosen angle of incidence.
5. Mark the position of the incident ray (before it enters the block), the point where the ray enters the block, the point where it exits the block, and the emergent ray (after it leaves the block).
6. Remove the glass block. Use a ruler to join the entry and exit points — this shows the path of the ray through the glass.
7. Draw the normal at the point of entry and use a protractor to measure the angle of incidence and the angle of refraction.
8. Repeat for at least five different angles of incidence.
9. Record all results in a table.

Results Table

Angle of Incidence (degrees)	Angle of Refraction (degrees)
10	7
20	13
30	19
40	25
50	31
60	35
70	39

Analysis

• The angle of refraction is always less than the angle of incidence when light enters a denser medium (air to glass). This is because the light slows down and bends towards the normal.
• As the angle of incidence increases, the angle of refraction also increases — but not by the same amount.
• The emergent ray (leaving the glass block) is parallel to the incident ray but is displaced sideways (laterally shifted).

graph TD
    subgraph "Refraction Through a Glass Block"
        A["Incident ray in air"]
        B["Normal at entry surface"]
        C["Refracted ray in glass - bent towards normal"]
        D["Normal at exit surface"]
        E["Emergent ray in air - bent away from normal"]
        F["Emergent ray is parallel to incident ray but shifted sideways"]
    end
    A --> C --> E

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Variables

Variable Type	Reflection Experiment	Refraction Experiment
Independent	Angle of incidence	Angle of incidence
Dependent	Angle of reflection	Angle of refraction
Control	Same mirror, same ray box, same distance	Same glass block, same ray box position

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Sources of Error

Source of Error	Effect	How to Reduce
Parallax error when reading the protractor	Inaccurate angle measurement	Read the protractor with your eye directly above the scale
Thick light ray from the ray box	Difficult to mark the exact centre of the ray	Use a narrow slit; mark the centre of the ray consistently
Glass block not placed exactly on the outline	Exit point is in the wrong position	Draw around the block carefully; do not move it during the experiment
Drawing rays inaccurately	Wrong angle measured	Use sharp pencil and ruler; mark at least two points along each ray
Protractor not centred correctly	Angle read incorrectly	Align the protractor centre point exactly on the intersection of the ray and normal

Exam Tip: When describing how to reduce errors, be specific. Do not just say "be more careful." Instead, say "measure the angle with the eye directly above the protractor to avoid parallax error" or "use a very narrow slit on the ray box to produce a thin, well-defined ray."

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Safety Precautions