Two polarizers

What you can do here
  1. You have two polarisers. Look around you through the first one turning it with the knob.
  2. Set the second poralizer so that you can look around through both of them. Turn the first one again and note that the image gets darker.
  3. Set the poralizers in a position which does not allow any light through; next, put an object made of plastic between the polarizers, e.g., a square ruler. The polariser-square-polarizer system will now let the light through.

How it works
Light is an electromagnetic wave. If it oscillates on one plane, the light is linearly polarized. Natural unpolarised light (with different random wave vibration planes) may be polarized after it goes through a polarizer which only lets through a component with a specific vibration plane. If both polarizers are oriented so that they let through the light polarised in the same plane, you get a bright image. When the planes are perpendicular, the image gets darker. Some materials can twist the polarization plane and therefore, a square ruler put between two polarizers that block light allows some light to pass through. The degree to which the plane is twisted differs depending on the colour, hence the beautiful iridescence of the square.

Interesting facts
Polarized light is used to screen three-dimensional films in cinemas and TV with passive technology (LG, JVC, Vision). The polarization is noticeable to bees, birds, octopi, squids, and cuttlefish.