Physics · 3.2.3 Total internal reflection · Paper 6 practical
Total Internal Reflection. Find c.
Investigate what happens when light travels from glass (denser) to air (less dense). Below the critical angle c, the ray refracts. At i = c, the refracted ray grazes the surface (r = 90°). Beyond c, total internal reflection occurs — used in optical fibres and prismatic periscopes.
0625 Topic 3.2.3 — TIR & critical angle
Applications — optical fibres, prisms
Shortcuts C find critical angle · Space record · R reset.
Variables
30.0
1.50
Live readouts
Angle of incidence i
30.0°
Angle of refraction r
48.6°
Critical angle c
41.8°
Status
Refracting
Formula
sin c = 1/n ⟹ c = sin⁻¹(1/n)
Snell's law: n sin i = 1·sin r. When sin r > 1, no refraction is possible — total internal reflection occurs.
Trial data
Record angles below the critical angle to confirm Snell's law.
📋 Method (Cambridge ATP procedure)
- Place a semicircular glass block flat-side outermost on a sheet of paper; trace the outline.
- Mark a point at the midpoint of the flat edge — light enters through the curved face along a radius, so refraction occurs only at the flat face.
- Draw the normal at that point.
- Shine a ray of light into the curved side, aimed at the midpoint, at an angle of incidence i from the normal (measured inside the glass).
- Mark where the refracted ray emerges on the air side; remove the block and measure the angle of refraction r.
- Increase i step by step until the refracted ray just disappears along the surface — this is the critical angle c.
- For i > c, observe that the ray is completely reflected inside the glass (TIR).
Analytical control: for the critical angle, sin c = 1/n. Therefore n = 1 / sin c.
⚠ Sources of error & precautions
- Edge of disappearance — at i ≈ c the refracted ray is very dim. Find c by approaching from both sides.
- Block alignment — the curved side must face the ray box and the light must travel along a radius so that there is no refraction at the curved face.
- Mark dimness — at high angles, the refracted ray brightens and reflects partially; the critical point is where the refracted ray just vanishes.
- Protractor reading — measure both i and r from the normal, not from the flat surface.
🧪 Apparatus list
- Semicircular glass block
- Ray box with single-slit fitting
- Soft board, A4 paper, pins (optional)
- Sharp pencil and ruler
- Protractor (resolution ± 1°)
🎯 Syllabus reference (0625)
- 3.2.3 Total internal reflection — state that the angle of incidence equals the angle of reflection in TIR; define critical angle; recall and use the equation sin c = 1/n; describe internal reflection and TIR using both diagrams and an experimental demonstration; describe uses of optical fibres (in telecommunications and medicine).