Polarization proves light is a transverse wave. In unpolarized light, the electric field $\vec{E}$ vibrates in all planes perpendicular to propagation. A polaroid transmits only one direction.

Types of polarized light:

1. Plane (linear) polarized — $\vec{E}$ oscillates in one fixed plane
2. Circularly polarized — $\vec{E}$ rotates at constant magnitude
3. Elliptically polarized — $\vec{E}$ rotates with varying magnitude
4. Partially polarized — mixture of polarized + unpolarized

Through a polaroid:

• Unpolarized → polaroid: $I_\text{out} = I_0/2$ (always)
• Plane polarized → polaroid (Malus's law): $I = I_0\cos^2\theta$
• Crossed polaroids ($\theta = 90°$): $I = 0$
• Three polaroids (crossed ends, middle at 45°): $I = I_0/8$