Core Practical: Investigating Refraction in a Glass Block

This lesson covers the core practical on investigating the refraction of light through a rectangular glass block, as required by the Edexcel GCSE Combined Science specification (1SC0). You need to know the method, how to draw and interpret ray diagrams, how to measure angles accurately, and how to draw valid conclusions.

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Aim

To investigate how the angle of incidence affects the angle of refraction when light passes through a rectangular glass block.

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Apparatus

Item	Purpose
Rectangular glass block	The medium through which light is refracted
Ray box (with single slit)	Produces a narrow beam of light
Power pack / batteries	Powers the ray box
Protractor	Measures angles of incidence and refraction
Ruler	Draws straight lines and normals
Sharp pencil	Marks ray positions accurately
Plain white paper	Placed under the glass block to trace rays
Optional: 4 pins	Alternative to ray box for tracing rays

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Method

Setting Up

1. Place a sheet of plain white paper on the bench.
2. Place the glass block in the centre of the paper.
3. Draw around the glass block carefully with a sharp pencil so you know its exact position.
4. Mark the midpoint of one of the long sides — this will be the point of incidence.
5. Draw a normal (dashed line) at 90° to the surface at the midpoint.

Performing the Experiment

6. Set the ray box to produce a single, narrow beam of light.
7. Direct the ray towards the midpoint at an angle of incidence of 10°.
8. On the paper, mark the position of the incident ray (entering the block) with two small crosses.
9. Mark the position of the emergent ray (leaving the other side of the block) with two small crosses.
10. Remove the glass block.
11. Use a ruler to draw the incident ray, the refracted ray inside the block (joining the entry and exit points), and the emergent ray.
12. Draw the normal at the entry surface.
13. Use a protractor to measure the angle of incidence (θ₁) and the angle of refraction (θ₂) — both measured from the normal.
14. Record the values in a results table.
15. Repeat for angles of incidence of 20°, 30°, 40°, 50°, 60°, 70° and 80°.

Exam Tip: Always measure angles from the normal, not from the surface of the glass block. This is the most common mistake in this practical.

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Results Table

Angle of Incidence (θ₁) / °	Angle of Refraction (θ₂) / °
10
20
30
40
50
60
70
80

Example Results (for typical glass, n ≈ 1.5)

Angle of Incidence (θ₁) / °	Angle of Refraction (θ₂) / °
10	7
20	13
30	19
40	25
50	31
60	35
70	39
80	42

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Drawing the Ray Diagram

graph LR
    A["Incident ray"] -->|"θ₁"| B["Entry point on glass block"]
    B -->|"Refracted ray inside glass (θ₂)"| C["Exit point on glass block"]
    C --> D["Emergent ray"]
    N1["Normal at entry (dashed)"] -.-> B
    N2["Normal at exit (dashed)"] -.-> C

Key Features of the Diagram

• The incident ray travels from the ray box to the glass block surface.
• At the entry surface: the light bends towards the normal (entering a denser medium).
• The refracted ray travels through the glass block in a straight line.
• At the exit surface: the light bends away from the normal (entering a less dense medium).
• The emergent ray is parallel to the incident ray but laterally displaced (shifted sideways).

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Analysis and Conclusions

What You Should Observe

1. The angle of refraction is always less than the angle of incidence when light enters the glass block (light bends towards the normal).
2. As the angle of incidence increases, the angle of refraction also increases — but not at the same rate.
3. The relationship between θ₁ and θ₂ is not linear — the graph of θ₂ against θ₁ is a curve.
4. The emergent ray is always parallel to the incident ray, confirming the light bends back by the same amount when it exits.

Drawing a Graph

Plot a graph of angle of refraction (y-axis) against angle of incidence (x-axis).

• The graph should show a curve that rises steeply at first and then levels off.
• This non-linear relationship is consistent with Snell's law.

Linking to Snell's Law (Qualitative)

At GCSE level you do not need to calculate the refractive index, but you should understand:

• The refractive index (n) of glass tells you how much it slows down and bends the light.
• A higher refractive index means more bending.
• The ratio of sin(θ₁) to sin(θ₂) is approximately constant — this is Snell's law.

Exam Tip: If asked to describe the relationship, say: "As the angle of incidence increases, the angle of refraction increases, but the angle of refraction is always less than the angle of incidence when light enters a denser medium."

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Sources of Error and Improvements

Source of Error	How to Improve
Inaccurate angle measurement with protractor	Use a sharp pencil and ensure the protractor is correctly centred on the point of incidence; use a protractor with 1° divisions
Glass block moved during experiment	Draw around the block at the start and ensure it is not moved; use Blu-Tack to hold it in place
Thick or diverging light ray	Use a narrow slit on the ray box to produce a thin, well-defined beam
Parallax error when marking ray positions	Mark positions with crosses directly below the ray; keep your eye directly above the mark
Difficulty seeing the emergent ray	Darken the room to make the ray more visible

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Risk Assessment

Hazard	Risk	Precaution
Bright light from ray box	Eye damage from looking directly at the beam	Never look directly into the ray box; direct the beam away from eyes
Hot ray box lamp	Burns	Allow time for the ray box to cool before handling; switch off when not in use
Glass block	May break if dropped, causing cuts	Handle with care; report any breakages to the teacher
Electrical equipment	Electric shock	Check for damaged wires; keep water away from the power supply

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Repeats and Reliability

• Repeat the measurement for each angle of incidence at least three times and calculate a mean angle of refraction.
• This helps identify anomalous results and improves reliability.
• Check that the emergent ray is always parallel to the incident ray — if not, the glass block may have moved.

Exam Tip: In a 6-mark practical question, always mention: (1) the key measurements, (2) how you ensure accuracy, (3) how you improve reliability (repeats and means), and (4) how you would display and analyse the results.

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Worked Example — Exam-Style Question

A student investigates refraction using a glass block. For an angle of incidence of 45°, she measures an angle of refraction of 28°. Another student obtains 29° for the same angle of incidence.

(a) Suggest why the results differ.

The difference could be due to measurement uncertainty when using the protractor, slight differences in the position of the normal, or the glass block being in a slightly different position.

(b) Suggest how the student could improve the reliability of her results.

She should repeat the measurement at least three times for the same angle of incidence and calculate the mean angle of refraction. She should also ensure the normal is drawn accurately using a set square.

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Summary