polarisation
Summary
Polarization means restricting a transverse wave's oscillation, for light the electric field, to one plane. A polarizer cuts unpolarized light to half its intensity. Polarized light meeting a second polarizer at angle θ follows I = I₀cos²θ (Malus's law).
Detail
Meaning: Light is a transverse electromagnetic wave, so its electric field oscillates perpendicular to the direction of travel. In unpolarized light that oscillation direction is random. A polarizer transmits only the field component along its transmission axis. Light can also become polarized by reflection (fully at Brewster's angle) or by scattering. Longitudinal waves, such as sound in air, cannot be polarized. In organic chemistry, a chiral compound can rotate plane-polarized light.
Equation: I = I₀cos²θ, where I is the transmitted intensity (W/m²), I₀ is the intensity of the polarized light reaching the polarizer (W/m²), and θ is the angle between the light's polarization direction and the polarizer's axis (rad or degrees). For unpolarized light entering a polarizer, I = I₀/2.
On the MCAT: Passages use a polarizer and a second polarizer (analyzer) that is rotated. Predict the intensity: maximum when the axes are parallel, zero when crossed at 90°. A third polarizer at 45° between two crossed ones lets light through. Polarizing sunglasses cut glare because light reflected from flat surfaces is partially polarized.
Common confusion: The factor of one half applies only to unpolarized input, not to every polarizer. Polarizing does not change frequency or wavelength. It lowers the amplitude (by cosθ) and so the intensity (by cos²θ).
Sources
- Physics (Cutnell and Johnson)
- Fundamentals of Physics (Halliday, Resnick, and Walker)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.