Polarization facts for kids
Polarization is a special property of some types of waves. Imagine a wave moving forward, like a ripple on water. Besides moving forward, the water also moves up and down. This up-and-down motion is what we call a wave's direction of vibration.
Light waves are like this; they are transverse waves, meaning their vibrations are at right angles to the direction they travel. Sound waves, however, are longitudinal waves. They vibrate back and forth in the same direction they travel. Because of this, sound waves do not have polarization. Only transverse waves can be polarized.
Light and Polarization
When light bounces off shiny, clear surfaces, like water or glass, it often becomes partly or fully polarized. This means its vibrations are mostly aligned in one direction. The only time this doesn't happen is if the light hits the surface straight on.
The idea of polarization was first discovered in 1808 by a mathematician named Etienne Louis Malus. You can see this effect yourself with a polarizing filter, like those in sunglasses. If you look at a reflection off a flat surface, like a road or water, and slowly turn the filter, you'll notice the reflected light gets dimmer or even disappears. This is because polarizing filters block light that vibrates at a 90-degree angle to the filter's special axis. If you put two polarizing filters on top of each other and turn one 90 degrees, very little light can pass through both.
Light also becomes partly polarized when it scatters through the atmosphere. This scattered light makes the sky look bright and blue. Photographers use polarizing filters to make the sky look darker in pictures, which helps clouds stand out more. This effect is easiest to see at sunset, especially when looking at the horizon at a 90-degree angle from the sun.
Polarizing filters are also used in sunglasses because they greatly reduce the bright glare from reflections off horizontal surfaces. This is why they help you see better on sunny days. Sometimes, if you wear polarizing sunglasses, you might see rainbow-like patterns on car windows or other clear plastic items. This happens because of how the light interacts with the special glass or plastic. You might also notice that liquid crystal displays (LCDs), like those on phones or calculators, can look strange or even unreadable through polarizing sunglasses. This is because LCDs use polarization to create images.
How Polarizing Filters Work
Polarization of light becomes very useful after it has been filtered. A filter can separate light that vibrates in one direction from light vibrating in other directions. Most everyday light, like from the sun or a light bulb, has a mix of vibrations. However, lasers are an exception; their light is usually already polarized.
Think of a polarizing filter like a comb. If you try to push a playing card through the comb, it will only fit if you turn it the right way, so it lines up with the comb's teeth. Light that is "turned" a different way will be blocked by the filter.
Liquid crystal displays (LCDs) use this idea to block light and create the letters or numbers you see on screens. Another cool use is in 3D movies and 3D televisions. Special glasses have different polarization filters for each eye. This separates the light meant for your left eye from the light meant for your right eye, creating the 3D effect.
In nature, light that bounces off a surface, like a window or water, often has the same polarization. This is what we call 'glare'. A polarizing filter on a camera can remove this glare, helping you see through the window or into the water. Depending on how you turn the filter, it can also increase the glare if you want to.
The picture on the right was taken through polarizing sunglasses and a car's rear window. Light from the sky reflects off the other car's windshield at an angle, making it mostly polarized horizontally. The rear window is made of tempered glass. The way this glass is treated with heat changes how light passes through it. This change allows some of the horizontally polarized light to become vertically polarized, which can then pass through the sunglasses. This is why you can see the regular pattern from the heat treatment in the window.
Other Electromagnetic Waves
Other electromagnetic waves, like radio waves or microwaves, can also have polarization. However, their polarization can happen in different ways.
Generally, some waves have their electric field vibrating at a right angle to the direction the wave travels. These are called TE (transverse electric) waves. Others have their magnetic field vibrating at a right angle to the wave's direction, and these are called TM (transverse magnetic) waves. These are the most common types of wave polarization. They can also be called vertical or horizontal polarized waves. If both the electric field and magnetic field vibrate at right angles to the wave's direction, the wave is called TEM (transverse electromagnetic).
Linear, circular, and elliptical polarizations are specific types of TEM polarization. These are easier to understand if you imagine looking straight at the wave as it comes towards you.
- Linear polarization: Imagine looking at a wave head-on. The electric field of the wave vibrates along a single straight line. Even if the strength of the vibration changes, as long as it stays on that straight line, it's called linear polarization. Think of shaking a jump rope up and down while looking at it from one end. The moving rope makes a straight line shape.
- Circular polarization: With circular polarization, the electric field traces out the shape of a circle as the wave moves towards you. The strength of the vibration stays the same, but its direction keeps changing in a circle. This circle can be traced clockwise or counterclockwise. This is like swinging a jump rope around in a circle for jumping; it makes a circular shape.
- Elliptical polarization: This is similar to circular polarization, but the strength of the vibration changes as it rotates, forming an ellipse (an oval shape) instead of a perfect circle.
See also
In Spanish: Polarización para niños
