plane polarized light waves

Meaning: Light is a transverse electromagnetic wave. In plane polarized light the electric field E points along one fixed axis, so it oscillates in one plane. Unpolarized light, such as from the sun or a bulb, has E directions spread randomly. A polarizer passes only the component of E along its transmission axis. Reflection can also polarize light: at Brewster's angle the reflected beam is fully polarized parallel to the surface. Only transverse waves can be polarized, so sound in a fluid cannot.

Equation: I = I₀cos²θ, where I is the transmitted intensity (W/m²), I₀ is the intensity of the incident polarized light (W/m²), and θ is the angle between the light's polarization and the polarizer's axis (rad or degrees). For unpolarized light entering a polarizer, I = I₀/2. Brewster's angle: tan θB = n₂/n₁, where n₁ and n₂ are refractive indices (dimensionless).

On the MCAT: Expect two polarizers in a row. Parallel axes transmit everything that got through the first, and crossed axes (90°) transmit none. A third polarizer at 45° between crossed ones lets some light through. Polarized sunglasses cutting glare is another common setup. In organic chemistry, chiral compounds rotate plane polarized light, which a polarimeter measures.

Common confusion: Students apply cos²θ to unpolarized light. That law needs light that is already polarized, and the first polarizer simply halves unpolarized light. The polarization plane is defined by E, not by the magnetic field or the direction of travel.