Fundamentals

As the magnetic field is always perpendicular to and proportional to the electric field, it may be conventionally ignored when considering polarization, so only the electric field need be described for simplicity. The orientation of the electric field, as it oscillates perpendicularly to the direction of propagation, defines the state of polarization. Note that the electric-field vector, which is shown oriented in a single plane in Figure 2-3A, typically changes rapidly and randomly, resulting in randomly polarized light that is said to be unpolarized. When the movement of the electric field vector is not random, the light is said to be polarized. Pictorially, the polarization state can be represented by the path that the tip of the electric field vector traces with time as viewed along the propagation axis looking toward the source (Lissajous figures). Figure 2-3A depicts linearly polarized light, with the electric field vector being constrained to a single plane; when viewed head-on it traces a single line. In circularly polarized light, the electric field vector rotates, tracing a corkscrew pattern as the light propagates. Viewed head-on, the pattern mapped out by the tip of the electric-field vector is a circle. In elliptically polarized light, which represents a more general case of circular polarization, the electric-field vector rotates and at the same time changes its amplitude as the light propagates, tracing an ellipse instead of a circle when viewed head-on.

Polarized light can be produced in a number of ways. One way to produce complete or partially polarized light is by reflection. Partial polarization, as the name implies, is a mixture of unpolarized and polarized light (linear, circular, or elliptical). Fresnel showed that the polarized component of reflected light tends to be linear, parallel to the interface. Reflected light is completely polarized if the angle of incidence equals the Brewster angle:

where nt and ni are the refractive indices of the transmitted and incident media, respectively. At the Brewster angle, all the reflected light is linearly polarized, but not all the linearly polarized light is reflected. Consequently, the transmitted light is a mixture of linearly and randomly polarized (commonly referred to as unpolarized) light—that is, it is partially polarized.

Applications and clinical relevance

Ophthalmic applications of polarization are numerous, some of which are discussed briefly here. Polarizing sunglasses are sometimes useful for reducing glare from reflected sunlight. Reflected light is somewhat polarized parallel to the reflecting surface, and in most environments, reflecting objects tend to be horizontal. In boating, for example, sunlight reflected from the water surface is partially polarized, usually horizontally, as is light reflected from a road surface or from the hood of a car. Even the sky acts as a partial polarizer by means of the scattering properties of air molecules. Accordingly, sunglasses incorporate vertically oriented linear polarizers (polarizing filters) to block the horizontally polarized component of reflected light.

Several stereopsis tests incorporate the use of linear polarizers. The well-known Stereo Fly test, for example, displays 2 slightly displaced images that linearly polarize light in perpendicular meridians. The person wears glasses containing linear polarizers, also at right angles to each other; each eye sees just 1 of the images, thus a single 3-dimensional (3D) image is perceived by someone with normal 3D or stereoscopic vision.

Polarized light is used in some ophthalmic instruments, such as the scanning laser polarimeter (discussed in Chapter 8). The retinal nerve fiber layer is birefringent, which means that polarized light travels through it at different speeds depending on whether the polarization is along or across the fibers. This retardation is linearly related to the thickness of a birefringent medium. Scanning laser polarimetry employs this effect to determine the thickness of the nerve fiber layer, for glaucoma diagnostic purposes.

Excerpted from BCSC 2020-2021 series : Section 3 - Clinical Optics. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.