Light is an electromagnetic wave, in which the electric field vector and the magnetic field vector are perpendicular to each other and both are perpendicular to the direction of wave propagation. The asymmetry of the vibration direction of the electric field vector relative to the propagation direction is referred to as "polarization." Based on the vibration characteristics of the electric field vector, light can be categorized into "unpolarized light," "linearly polarized light," and "circularly polarized light."
· Linearly Polarized Light
When the electric field vector of a light wave vibrates consistently along a fixed direction, the light is known as linearly polarized light. The vibration direction can be any orientation within the plane perpendicular to the direction of propagation (e.g., vertical, horizontal, or diagonal).
Figure 1 Schematic Diagram
Figure 2 Schematic Diagram
Linearly polarized light is widely used in daily life and industry: Liquid Crystal Displays (LCDs) modulate linearly polarized light to produce images; early 3D movie technologies (such as linear-polarized 3D) utilize linearly polarized light with different vibration directions for the left and right eyes to create stereoscopic vision; in material inspection, when linearly polarized light passes through stressed materials, the "photoelastic effect" occurs, allowing analysis of stress distribution based on changes in polarization state.
· Non-Polarized Light
Non-polarized light, also known as natural light, constitutes most of the visible light in everyday life, such as sunlight and incandescent light. In non-polarized light, the vibration directions of the electric field vector are randomly distributed within the plane perpendicular to the direction of propagation, with equal amplitudes in all directions. Non-polarized light can be imagined as a chaotic combination of countless linearly polarized light waves with varying vibration directions, resulting in a composite vibration direction that changes constantly and irregularly.
Figure 3 Schematic Diagram
Figure 4 Schematic Diagram
· Circularly Polarized Light
In circularly polarized light, the electric field vector rotates uniformly within the plane perpendicular to the direction of propagation, tracing a circular path. Depending on the direction of rotation, circularly polarized light is further divided into "left-handed circularly polarized light" and "right-handed circularly polarized light"—when viewed facing the direction of propagation, clockwise rotation of the electric field vector corresponds to right-handed polarization, while counterclockwise rotation corresponds to left-handed polarization.
Circularly polarized light can be produced by superimposing two linearly polarized light waves with mutually perpendicular vibration directions, equal amplitudes, and a phase difference of π/2 (90°).
Figure 5 Schematic Diagram
Circularly polarized light is widely used in modern 3D display technologies. For example, in RealD and other 3D cinema systems, the left and right lenses of the glasses employ left-handed and right-handed circular polarizing filters, respectively, allowing each eye to receive different images and creating a stereoscopic effect through binocular disparity.
· Formation of Linear and Circular Polarization
Polarizing filter transmission is the most common method for generating linear and circular polarization. Polarizers have a specialized optical structure that only allows light with a vibration direction parallel to the "polarization axis" to pass through, while absorbing light with other vibration directions. When unpolarized light passes through a polarizer, it is converted into linearly polarized light. If two polarizers with mutually perpendicular polarization axes are placed in line, under ideal conditions, the light will be completely blocked.
Figure 6 Schematic Diagram
When linearly polarized light is incident perpendicularly on a quarter-wave plate, and its vibration direction forms a 45° angle with the wave plate's "optical axis," the wave plate decomposes the linearly polarized light into ordinary light (o-light) and extraordinary light (e-light). After passing through the wave plate, these two components acquire a phase difference of π/2, and their superposition results in circularly polarized light.
Figure 7 Schematic Diagram
Linearly polarized light can also be generated through other methods, such as:
Reflection and Refraction: When natural light is incident on the interface between two media (e.g., air-glass) at a specific angle (the Brewster angle), the reflected light becomes completely (or nearly completely) linearly polarized, with its vibration direction perpendicular to the plane of incidence, while the refracted light becomes partially polarized.
Birefringent Crystal Effects: Certain crystals (e.g., calcite) exhibit birefringence. When natural light is incident on such crystals, it splits into two linearly polarized beams with mutually perpendicular vibration directions (o-light and e-light). Through appropriate optical arrangements, one of these beams can be isolated to obtain linearly polarized light.