Light – polarisation

Polarised sunglasses look like regular sunglasses, but they are able to filter out the glare from automobile windscreens and the surface of water. The glasses you wear when watching a three-dimensional (3D) film work because the lens of each eye is polarised in different directions.

What is polarisation and how does it work?

Light travels as a wave

Light travels from one place to another as a wave. In some ways, it is like a wave on a lake. Light travels out in all directions from a source, much like a water wave travels away from a rock dropped into water. In a wave on a lake, the water moves up and down while the wave travels across the surface perpendicular to the motion of the water. A light wave is similar in that something is moving up and down in one direction while the light travels in a direction perpendicular to that motion.

Polarised light waves

To understand polarisation, picture a wave on a rope. If you make a series of up and down motions while holding onto a rope, a series of waves will move from your hand along the rope, transferring the motion down the rope. The rope moves up and down while the wave (and its energy) travels along the rope perpendicular to the motion of your hand. You could also move your hand side to side and create a wave that, rather than moving up and down, moves from side to side. In both of these examples, waves behave the same way – the rope moves in one direction while the wave moves down the rope. The difference is the orientation of the waves.

Consider the person in these images. In the first part of the image, he is moving the ropes up and down, creating vertical waves. In the second image, he is moving the ropes from side to side, creating horizontal waves. However, light waves come out of a light source waving in all directions – horizontal, vertical and all angles in between – as shown in the third part of the image.

Methods of polarisation

One method of polarisation is by the use of a special polarising filter or lens. To understand how a polarising lens works, let’s go back to our rope example. If you make vertical waves on a rope that go through a vertical slit, shown as in figure A in the image below, the waves would travel through the slit easily because the orientation of the slit is the same as the orientation of the wave.

If the slit is turned so that it is perpendicular to the wave, as shown in figure B, the wave is greatly reduced. If the wave is at some other angle relative to the wave, as shown in figure C, only a portion of the wave will make it through the slit.

Light waves are represented in figure D. The pattern on the left represents a light wave moving in many different orientations (directions). When it has passed through the polarising filter, one specific orientation passes through easily – represented by the red line – and the other orientations are greatly reduced.

Examples of polarised light

You can find examples of polarised light in many places. When light is reflected off a surface at a steep angle, such as sunlight off the surface of water or an automobile windscreen, it becomes polarised. The glare from such reflections is greatly reduced by polarised lenses in sunglasses.

The glasses worn by the audience in modern 3D films are polarised so that each eye has a lens that is oriented 90° from the other (like figures A and B). Thus each eye sees slightly different images, which the brain interprets as 3D. The article Depth perception has more information on how we perceive 3D.

If you have one polarising lens (such as a pair of polarised sunglasses), you can test whether another pair of lenses are polarised by looking through two lenses and rotating one. At some position in the rotation, the polarisation of the two lenses will be perpendicular, and at that point, almost no light will pass through.

The screens of many smartphones, tablets and some computers contain polarising filters so that the light that comes out of the screen is polarised. If you look at a cell phone while wearing polarised glasses, you will notice that, as you turn your cell phone from vertical to horizontal, at some angle, the screen will appear to turn black. The polarised light from the phone is filtered out by the polarising filter of the glasses.