Linear polarization is a property of light (or any electromagnetic wave) where the electric field oscillates along a single straight line as the wave travels forward. Ordinary light from the sun or a lightbulb vibrates in every direction perpendicular to its path. Linearly polarized light vibrates in just one of those directions, like a rope being shaken strictly up and down rather than whipped around in circles.
How Light Becomes a Single Vibration
Light is an electromagnetic wave, meaning it carries an electric field and a magnetic field that oscillate together as the wave moves through space. In unpolarized light, the electric field points in random directions from moment to moment, all perpendicular to the direction the wave is traveling. There’s no preferred orientation.
In linearly polarized light, that randomness disappears. The electric field swings back and forth along one fixed line, tracing a straight path over time. If you could freeze the wave and look at it end-on, you’d see the field moving along a single axis rather than jumping around. This is sometimes called “plane polarized” light because the electric field stays within a single flat plane as the wave propagates.
Linear polarization is one of three polarization states. The other two are circular (the electric field rotates in a circle as the wave moves) and elliptical (it traces an oval). Linear is the simplest and most commonly encountered.
Ways to Produce Linearly Polarized Light
Polarizing Filters (Dichroism)
The most familiar method is passing light through a polarizing filter. Certain materials, both natural crystals and synthetic films, absorb light vibrating in one direction while transmitting light vibrating in the perpendicular direction. This property is called dichroism. The filter essentially strips away every orientation of the electric field except one, leaving linearly polarized light on the other side.
When unpolarized light hits a perfect polarizing filter, exactly half the intensity gets through. The transmitted half is the component already vibrating along the filter’s transmission axis. If you then place a second polarizer in the beam, the transmitted intensity depends on the angle between the two filters. This relationship is described by Malus’s Law: the intensity equals the incoming intensity multiplied by the square of the cosine of the angle between the two polarizers. At 0 degrees (both aligned), all the light passes. At 90 degrees (crossed), no light passes at all.
Reflection at Brewster’s Angle
Light also becomes polarized when it reflects off a smooth surface at a specific angle. When unpolarized light strikes glass, water, plastic, or pavement, the reflected beam favors one polarization direction over the other. At a particular angle of incidence known as Brewster’s angle, the reflected light is 100% linearly polarized, with its electric field running parallel to the reflecting surface. Brewster’s angle depends on the material’s refractive index: for ordinary glass (refractive index around 1.5), it’s roughly 56 degrees from vertical.
Atmospheric Scattering
Sunlight arriving at Earth’s atmosphere is essentially unpolarized. As it scatters off air molecules, though, it picks up partial polarization. The degree of polarization depends on the scattering angle. Light scattered at 90 degrees to the incoming beam is the most strongly polarized. If you look straight up when the sun is near the horizon, you’re seeing light that scattered through roughly 90 degrees, so it carries significant linear polarization. Many insects, including bees, use this polarization pattern in the sky for navigation.
Polarized Sunglasses and Glare
The most common everyday application of linear polarization is glare-reducing sunglasses. Glare occurs when sunlight bounces off a smooth horizontal surface like water, a car hood, or a wet road. That reflected light is predominantly polarized in the horizontal direction. Polarized sunglasses contain a filter oriented to transmit only vertically vibrating light. Horizontal light waves are blocked, so the glare disappears while other light in the scene, scattered from rougher surfaces in many orientations, still reaches your eyes.
This is why polarized sunglasses dramatically reduce the blinding reflection off a lake but don’t make the whole world darker the way a simple tinted lens would. They’re selectively removing the most polarized component of the light reaching you.
How LCD Screens Use Polarization
Every LCD screen relies on two linear polarizers sandwiching a layer of liquid crystal. The front polarizer is oriented in one direction (say, horizontal) and the rear polarizer is oriented perpendicular to it (vertical). Without anything between them, no light would get through, because the first filter blocks everything the second one would transmit.
The liquid crystal layer between them solves this by twisting the polarization of the light as it passes through. When voltage is applied to a pixel, the liquid crystal molecules change their alignment, controlling how much the polarization rotates. This determines whether light for that pixel passes through the second filter or gets blocked, creating bright and dark areas on screen. Tilt your head 90 degrees while wearing polarized sunglasses in front of an LCD and you may see the screen go dark, because your lenses are now crossed with the screen’s output polarizer.
Comparing Polarization States
- Linear: The electric field oscillates along a single straight line. It can point in any fixed direction (horizontal, vertical, or any angle), but it stays on that line.
- Circular: The electric field rotates in a circle as the wave propagates, maintaining constant magnitude. This happens when two equal perpendicular linear components are 90 degrees out of phase with each other.
- Elliptical: The electric field traces an ellipse, a general case where the two perpendicular components differ in amplitude or aren’t exactly 90 degrees out of phase. Both linear and circular polarization are technically special cases of elliptical.
Measuring and Detecting Polarization
The simplest way to check whether light is linearly polarized is to look at it through a polarizing filter and rotate the filter. If the brightness changes as you rotate, the light has some degree of linear polarization. If it drops to zero at one rotation angle, the light is completely linearly polarized. If the brightness stays constant, the light is either unpolarized or circularly polarized.
Photographers use this principle with circular polarizing filters attached to camera lenses. Rotating the filter changes how much polarized light (sky glare, reflections on water or glass) gets through, letting them deepen blue skies or remove unwanted reflections from a scene. Scientists studying everything from crystal structure to distant stars also analyze polarization to learn about the materials and processes that shaped the light before it reached their instruments.