1. You have two polarisers at your disposal. View your surroundings through the first one, turning it with the knob.
2. Set the second polariser in such a way that you are able to view your surroundings through both of them. Again, try to turn the first one and note the darkening image.
3. Set the polarisers in a position, which does not let any light pass through; then, hold an object made of plastic between the polarisers, e.g., a set square. The polariser-set square-polariser system is now letting light through.
How does it work
Light is an electromagnetic wave. If its oscillations take place in one plane, we talk about linearly polarised light. Light may become polarised after passing through an object or reflecting off it. The polarisers presented here only let light in the polarised state through. If they are both oriented so that they are letting through the light which is polarised in the same plane, we see a bright image. When the planes are perpendicular – the image is darkened. Some materials are characterised by their ability to twist the polarisation plane; therefore, a set square placed between two polarisers that block any light allows some light to pass through. The degree to which the plane is twisted is different depending on the colour, hence the beautiful iridescence of the set square.
Polarised light is used to screen three-dimensional films in cinemas and TV with passive technology (LG, JVC, Vision). The polarisation is noticeable to bees, birds, octopuses, squids, and cuttlefish.