Waves · Polarisation
Polarisation Lab
I = I₀ cos²θ
f = 0.60 Hzv = 2.6 m s⁻¹λ = v/f = 4.33 m
Try first: drag Polariser 1 to 90° — rope goes flat.
Mode
Controls
Amplitude after the slit
A out / A₀
0.476 m
A = A₀ cos(α − θ₁) = 0.550 × cos(30°) = 0.476 m
Set |α − θ₁| = 90° → rope downstream goes flat.
Head-on view — oscillation plane at each stage
Arrow = oscillation plane, length ∝ amplitude — projected onto each axis.
Detector trace
Detector displacement in the θ₁ plane.
Live readouts
t / s
0.000
s at detector / m
0.000
What this confirms — cards from "Wave Motion — Everything to Memorise"
- Plane polarisation. After the slit the rope oscillates in ONE plane (θ₁) perpendicular to travel — Mode 1, amplitude A₀cos(α − θ₁), flat rope at Δθ = 90°.
- Why sound cannot be polarised. Mode 2 IS the proof: oscillation lies along the travel direction, so the slot has no perpendicular component to restrict — any slot angle, zero effect.
- Unpolarised → polariser: HALF. Mode 3: the after-P1 meter is frozen at exactly I₀/2 while you drag θ₁ — the average of cos² over all random planes is ½.
- Malus discipline. θ in I = I₀cos²θ is between CONSECUTIVE transmission axes — the meters card computes Δθ = θ₂ − θ₁ explicitly before squaring the cosine.
- Crossed polars + middle filter paradox. Preset: crossed pair alone transmits zero, yet inserting a middle filter at 45° lets I₀/2 · cos²45° · cos²45° = I₀/8 through — adding a filter INCREASES the light.
- Rotating-analyser graph. The live Malus curve is cos² with period 180°: maxima at Δθ = 0°/180°/360°, zeros at 90°/270° — never a sine, never period 360°.
- Test for plane polarisation. Rotate the analyser: intensity swinging max → zero → max means plane polarised light; a steady meter (like after the unpolarised source) means unpolarised.