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Physics · 3.2 Light · Paper 6 practical

Refraction. Bend it.

Trace a ray of light through a rectangular glass block using the optical-pin method. Measure the angle of incidence i and angle of refraction r, then determine the refractive index n = sin i / sin r.

0625 Topic 3.2.2 — Refraction 0625 Topic 3.2.3 — Refractive index Paper 6 — ATP
Setup — drag the angle slider (or the incident ray endpoint) to set the angle of incidence, then press Record reading.

Tip: drag pin P₁ or P₂ on the canvas to set the incident ray. Shortcuts Space record · R reset.

Variables

40
1.50
⚠ Inside the block, the ray reaches the second face at an angle beyond the critical angle — total internal reflection would occur. Reduce i.

Live readouts

Angle of incidence i
40.0°
Angle of refraction r
25.4°
sin i
0.643
sin r
0.428
n = sin i / sin r
1.503
Snell's law: n₁ sin i = n₂ sin r. With n₁ = 1.00 (air), n₂ = sin i / sin r.

Trial data — vary i, measure r

Set an angle then press Record reading. Take five values of i from 10° to 70°.

sin i vs sin r — gradient = n

📋 Method (Cambridge ATP procedure)
  1. Place the glass block in the centre of a sheet of A4 paper and trace its outline carefully with a sharp pencil. Remove the block.
  2. Draw the normal (perpendicular line) at a point on the longest boundary edge.
  3. Using a protractor, construct an incident ray at the angle of incidence i = 30°.
  4. Press two optical pins (P₁ and P₂) on the incident ray, at least 5 cm apart, to reduce error in tracing the line.
  5. Replace the block on its outline.
  6. Look through the opposite side of the block at the images of P₁ and P₂. Move your head until the images appear in a single line.
  7. Press pins P₃ and P₄ on the paper so that they line up with the images of P₁ and P₂.
  8. Remove the block and pins. Draw the emergent ray through P₃ and P₄ back to the block boundary.
  9. Join the entry and exit points to mark the refracted ray inside the block.
  10. Measure the angle of refraction r with a protractor.
  11. Repeat for five values of i between 10° and 70°.

Analytical control: plot sin i (y) against sin r (x). The gradient of the straight line through the origin is the refractive index n.

⚠ Sources of error & precautions
  • Pin alignment — use thin, sharp pencil lines and space the pins at least 5 cm apart so a small alignment error makes little change to the ray direction.
  • Block slipping — do not move the block once its outline is traced; if it moves, re-trace it.
  • Protractor reading — measure both i and r from the normal, eye perpendicular to the paper to avoid parallax.
  • Parallel sides — use a block with parallel sides; the emergent ray is then parallel to the incident ray but laterally displaced.
  • Range of i — at very small angles, both i and r are small and a 1° error becomes a large percentage error. Use a sensible spread.
🧪 Apparatus list
  • Rectangular glass block
  • Soft board / pin board
  • A4 plain paper
  • Four optical pins (P₁, P₂, P₃, P₄)
  • Sharp pencil and ruler
  • Protractor (resolution ± 1°)
🎯 Syllabus reference (0625)
  • 3.2.2 Refraction of light — describe an experimental demonstration of the refraction of light using a transparent block; define angle of incidence and angle of refraction; recall the meaning of the term normal.
  • 3.2.3 Refractive index — recall and use the equation n = sin i / sin r.
  • Paper 6 — record results in a table; plot a graph with a straight line through the origin; calculate a gradient using a large triangle on the line of best fit.

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