PHY019 · Optics · Classes 10–12
Total internal reflection
Objective: To observe total internal reflection and measure the critical angle for glass–air.
🧰 Apparatus
- Semicircular glass block
- Ray box with a single slit
- Protractor
- White paper
📐 Formulas and SI units
sin θc = n₂ / n₁
for glass in air: θc = sin⁻¹(1/n)
💡 Theory
When light goes from a denser to a rarer medium, the refracted ray bends away from the normal. At the critical angle θc it grazes the surface; beyond θc no light leaves and all of it is reflected: total internal reflection. sin θc = n₂/n₁.
🪜 Procedure
- Place the semicircular block on paper and draw its outline and centre.
- Aim the ray at the curved side towards the centre so it enters undeviated.
- Increase the angle of incidence at the flat face slowly.
- Note the angle where the refracted ray just grazes the surface: θc.
- Increase i further and observe total internal reflection.
- Compare θc with sin⁻¹(1/n).
📋 Observations Simulated values
✅ Result and conclusion
For glass (n = 1.5) in air, θc ≈ 41.8°; above it all light reflects inside the block.
🔬 In a real lab
Near θc a weak refracted beam and a strong reflected beam coexist, so the exact angle is judged to about ±1°.
⚠️ Safety
Ray boxes and lamps get hot: let them cool before touching; use the low-voltage supply only.
❓ Quick check
1. Total internal reflection needs light to travel…
2. Critical angle for n = 2 (in air)?
3. Which uses total internal reflection?
🏋️ Practice
Easyθc for water (n = 1.33)?
Answer: ≈ 48.8°
MediumWhy do diamonds sparkle?
Answer: Very small θc (24.4°), so light reflects many times
ChallengeCan TIR occur from air into glass?
Answer: No