Light Polarization
Unpolarized light oscillates in every direction. A polarizing filter transmits only the component aligned with its axis. Add filters, rotate them, and watch Malus’s law govern the transmitted intensity. Discover the famous paradox: two crossed filters block all light, but inserting a third at 45° between them lets light through again.
I = I₀ cos²θ • Malus’s Law (1809)
About this lab
Light is a transverse electromagnetic wave — the electric field oscillates perpendicular to the direction of travel. Unpolarized light (from the sun, a lamp) has its E-field oscillating in all directions equally.
A polarizing filter transmits only the component of the E-field parallel to its transmission axis. After passing through the first filter, the light is linearly polarized.
Malus’s law describes what happens when polarized light hits a second filter: the transmitted intensity is I = I₀ cos²(θ), where θ is the angle between the polarization direction and the filter’s axis.
- At θ = 0°, all light passes through (cos²0 = 1).
- At θ = 90°, no light passes through (cos²90 = 0).
- The paradox: two crossed filters (0° and 90°) block everything. But inserting a 45° filter between them lets 1/8 of the original intensity through, because cos²(45°) × cos²(45°) = 1/4, applied to the 1/2 from the first filter.
This lab also shows circular and elliptical polarization, where the E-field vector traces a circle or ellipse as the wave propagates. The first linear filter still converts these to linear polarization with 50% transmission.