What polarizations can a plane electromagnetic wave have?

• A. Linear only
• B. Linear, circular, and elliptical
• C. Linear and circular
• D. Circular only

Answer: (B)

Polarization describes how the electric field vector oscillates at a fixed point as the wave passes. For a plane wave, the electric field has two perpendicular components in the transverse plane. The way these components relate in time—their relative phase and their magnitudes—determines the path the tip of E traces.

If the two components are in phase (phase difference 0 or π), the field points along a fixed line and the wave is linearly polarized. If the phase difference is ±90 degrees and the two components have the same amplitude, the tip of E traces a circle, giving circular polarization. If the phase difference is ±90 degrees but the amplitudes are unequal, the path is an ellipse, giving elliptical polarization. Therefore, a plane electromagnetic wave can be linearly, circularly, or elliptically polarized depending on the phase difference and amplitudes of the transverse components.