PHY022 · Optics · Class 12 + College
Polarization
Objective: To verify Malus's law for polarised light.
🧰 Apparatus
- Light source
- Two polaroid sheets (polariser and analyser) on rotating mounts
- Light meter or LDR
📐 Formulas and SI units
I = I₀ cos²θ
after the first polariser: I₀ = ½ I_unpolarised
💡 Theory
A polariser lets through only the vibrations along its axis, so unpolarised light loses half its intensity. A second polariser (analyser) at angle θ passes I = I₀ cos²θ of the polarised light: full at 0°, none at 90°.
🪜 Procedure
- Place the polariser in front of the lamp and the meter behind the analyser.
- Set the analyser parallel (θ = 0) and note the reading I₀.
- Rotate the analyser in 15° steps and record the intensity.
- Plot I against cos²θ.
- Check that the graph is a straight line through the origin.
📋 Observations Simulated values
✅ Result and conclusion
At θ = 60° the transmitted intensity is a quarter of I₀; the I–cos²θ graph is a straight line.
🔬 In a real lab
Real polaroids leak a little light even when crossed and absorb some when parallel, so the minimum is not exactly zero.
⚠️ Safety
Lamps get hot; do not look directly into bright sources.
❓ Quick check
1. Crossed polarisers (θ = 90°) transmit…
2. Unpolarised light through one polariser keeps…
3. Polarisation shows that light is…
🏋️ Practice
Easyθ = 45°. I/I₀?
Answer: 0.5
MediumI/I₀ = 0.75. θ?
Answer: 30°
ChallengeName a use of polaroids.
Answer: Sunglasses, LCD screens, 3D films