Polarisation is a property of light. Light has many properties. The properties we can see most easily with our eyes are colour and brightness. To understand what determines the brightness, the colour, and other properties of light, you'll first need a description of what light actually is.
What is light?
Light is actually a very complicated phenomenom. How to describe it best, depends on the property of light that you are interested in. To understand colour, brightness and polarisation, it is probably easiest to imagine that a beam of light consists of myriads of tiny waves.
The brightness of a beam of light is linked to the amount of energy that the waves carry. Brightness is usually expressed as the energy that flows through a surface per unit of time, for example as W/m2. Your eyes accomodate to various levels of brightness by adapting the diameter and with that the surface area of your pupils: when it grows dark, your pupils will widen to intercept more light waves. Telescopes also work like that: the larger the diameter of the telescope mirror, the fainter the astronomical objects that can be observed.
The colour of a beam of light depends on the wavelengths of the waves in the beam. The wavelength λ of a wave is the distance covering a complete wave, for example, from one top to the next. The wavelength of a wave that enters our eyes determines the colour that we perceive. For example, we identify waves with a wavelength λ of about 400 nm as blue light, and waves with λ of about 700 nm as red light. The abbreviation nm stands for nanometer.
Because 1 nm equals 0. 000 000 001 m, the 2 cm diameter of a 10 eurocent coin would equal about 50,000 lengths of blue light waves, and almost 30,000 lengths of red light waves.
Human eyes are sensitive to light waves with wavelengths from violet-blue (380 nm), through blue, green, yellow, and orange, all the way to red (700 nm). Light with waves in this wavelength region is therefore called visible light. On the short wavelength end, the visible range is bordered by ultra-violet light, and on the long wavelength end by infrared light, which is basically thermal radiation. Light with wavelengths outside the visible range is often referred to with the more general term radiation instead of as light.
A beam of light or radiation that is composed of waves with the same wavelength is monochromatic. Natural radiation, such as that from the Sun or other stars, is not monochromatic. Instead it is composed of waves with a wide range of wavelengths.
Not all human eyes have the same sensitivity: some people can see light with shorter or longer wavelengths than other people. More familiar is the variation in the ability to distinguish different colours: people than can distinguish less different colours than the average person, are said to be colour blind. There are different types of colour blindness, depending on which colours are missing from a person's vision, but the most common types are those where green and red appear to be very similar.
Very simply said: polarisation refers to the direction of the vibrations of the waves that form a beam of light or radiation. If the waves all vibrate in the same direction, the beam is said to be fully polarised or 100% polarised. If the vibrational directions are completely random, the beam is unpolarised. If a fraction of the waves in a beam has a specific vibrational direction, while the rest vibrates in random directions, the beam is partially polarised. If 20% of the waves vibrates up and down, while 80% has a random vibrational direction, the degree of polarisation of the partially polarised beam is 20%.
As an example, sunlight that reaches the Earth can be considered to be unpolarised (although light from specific regions on the Sun can be polarised), but it gets polarized when it is scattered by gas molecules in the Earth's atmosphere. As a result, the Earth's blue sky is polarized.
The state of polarisation of radiation holds information about the source of the radiation or about the objects or processes that interacted with the radiation. Unfortunately, humans have eyes that are not very sensitive to the state of polarisation of light, but we can design and use instruments that can measure polarisation. Studying the state of polarisation of radiation and light, helps us to uncover information that cannot be obtained through other methods.