Physical optics

Lore & Background

Physical optics is defined as the branch of optics that examines interference, diffraction, polarization, and other phenomena where the ray approximation of geometric optics is invalid. This definition excludes quantum noise in optical communication, which falls under coherence theory. The term also names an approximation used in optics, electrical engineering, and applied physics, positioned as an intermediate method between geometric optics and precise full wave electromagnetism. The word 'physical' indicates it is more physical than geometric or ray optics, not that it is an exact physical theory.

Reader's Guide

The physical optics approximation involves using ray optics to estimate the field on a surface, then integrating that field over the surface to calculate the transmitted or scattered field. This approach resembles the Born approximation, treating details as a perturbation. In optics, it is a standard way to estimate diffraction effects, typically by integrating ray-estimated field over a lens, mirror, or aperture. In radio, it models some interference, diffraction, and polarization effects but not the dependence of diffraction on polarization. As a high-frequency approximation, it is often more accurate in optics than for radio. In radar scattering, it involves taking the current found on a tangent plane of similar material at each point on the geometrically illuminated part of a scatterer, with current on shadowed parts set to zero, then integrating to obtain the approximate scattered field. This is useful for large smooth convex bodies and lossy surfaces. The ray-optics field or current is generally inaccurate near edges or shadow boundaries unless supplemented by diffraction and creeping wave calculations.

Did You Know?

• Physical optics studies interference, diffraction, and polarization, but not quantum noise in optical communication, which is studied in coherence theory.
• The physical optics approximation is an intermediate method between geometric optics and full wave electromagnetism.
• In radar scattering, physical optics takes the current on a tangent plane for illuminated parts and zero for shadowed parts.