The polarisation of light: more than just an intensity

What is polarisation of light and what do sunglasses have to do with it?

The past couple of weeks, I have been working on the calibration of a polarimeter. As the name suggests, the instrument measures the polarisation of light. So what exactly is polarisation?

We describe light in terms of three variables: its brightness, its colour and its polarisation. The brightness is a measure of the amplitude of light. We also call this intensity. Intensity is what we measure with cameras. The camera expresses the intensity in a unit of counts which we then convert into another unit: for example, flux. The colour of light is determined by its wavelength¹. This wavelength cannot be directly measured. What I mean by this is that we can not infer the wavelength of light by only using a detector but instead (in the case of wavelength measurements) we look at diffraction patterns. Finally we have the polarisation, which is the direction of vibration of the light. This direction is always perpendicular to the direction of propagation. Thus, measuring polarisation is measuring this direction of the vibrations. Or more accurately, measuring the percentage of waves that have this same vibration direction.

But how can you measure this direction of vibration? I just said that detectors in cameras can only measure intensities. To explain this concept, I will give an example of Polaroid sunglasses and 3D glasses below.

Polaroid sunglasses vs. 3D glasses • The modulation of light.

Sunlight consists of bundles of light rays that all have different directions of vibration. The absence of a preferred direction causes this light to be referred to as unpolarised light. Light beams from light bulbs are unpolarised as well. In contrary, laser light is often polarised. This means that the direction of vibration of laser light has a preferred vibration direction. This preferential direction can result in linear, circular and elliptically polarized light. Usually, the light from lasers is linear polarised. In nature, linear polarised light is also the most common type of polarised light.

But wait a minute... We just said that sunlight is unpolarised? How does (sun)light get polarised? When light from the sun enters the atmosphere, its waves still have a random direction of vibration. In the atmosphere the light can be scattered (reflected) by colliding with molecules in the air, water and small dust particles (aerosols). This scattering is also called Rayleight scattering. Rayleight scattering causes parts of the light to become polarised. When the sun's rays hit flat surfaces, like water, snow and ice on the earth, part of the light becomes horizontally (linearly) polarised. We see this with our eyes as a blinding glare. Polaroid sunglasses have a filter for this specific horizontal linear polarisation. This makes the sunglasses improve vision and prevent eye strain on sunny days.

But then what do 3D spectacles have to do with polarisation? 3D spectacles use polarising lenses to create the illusion of a 3D image. Each of the glasses contains a different polarising filter. With each filter, only light that is polarised in one direction will pass through. Light that is polarised in the opposite direction is blocked. This causes both eyes to see a slightly different image. This causes us to see the image with our two eyes from two apparent positions. As these positions differ, the illusion of a 3D image is produced.

Keep an eye on the blog! Soon I will tell more about how I plan to measure polarisation! ¹ See my post from January 19 for a more detailed explanation regarding sunlight and its wavelengths.