Lab 05 — Optics

The Physics of Polarized Sunglasses

The film in polarized sunglasses is stretched polyvinyl alcohol (PVA) doped with iodine. The iodine lines up into chains that act like a grid of wires: the E-field component parallel to the chains is absorbed, only the perpendicular component passes. Once the light is polarized, passing it through another polarizer tilted by θ dims it by Malus’s law, I = I₀cos²θ. Rotate the second filter (the sunglasses) and watch the intensity respond.

I = I_0 \cos^2\theta \qquad 0^\circ \to 45^\circ \to 90^\circ:\ \dfrac{1}{2}\cdot\cos^2 45^\circ \cdot\cos^2 45^\circ = \dfrac{1}{8} = 12.5\%

Cross the sunglasses at 90°, then slip a 45° filter in between

Sunglasses angleθ = 90°

Middle angleθ = 45°

TransmissionI / I₀ = 0.0%

Two filters crossed at 90° block everything — yet inserting a 45° filter between them brings 12.5% of the light back. A polarizer doesn’t sieve light; it re-projects the field onto its axis — a visible classical analogue of quantum measurement. Glare off water and roads is horizontally polarized, which is why sunglasses have a vertical transmission axis.