Peter Walwyn, of Paraparaumu College, asks :-

Polaroid sunglasses polarise light so that when when looking through two sunglasses at right angles to each other no light gets through. However when a third sunglass lens is added between them light can get through. Why?

John Campbell, a physicist at the University of Canterbury, responded.

Isn't that a little doozy of a conundrum. It always surprises people.

Another example of a polariser is the standard TV aerial. They respond only to electric fields oscillating parallel to the cross-rods of the aerial. In optical polarisers, oriented, long molecules in the lens behave in a similar way.

The front polariser polarises the light say vertically. A second one (the back one) set to polarise horizontally does block the light because there is no component of the light vector (electric field) which is in the direction which it transmits.

Now consider when the middle one is inserted and rotated till the light passed is brightest. The polarisation of it is at 45 degrees to the vertical polarized light from the front one. So the middle one has light incident on it at 45 deg to its axis. It transmits the component parallel to its axis. So now the back polariser has light incident on it whose polarisation is at 45 degrees to its axis ie it has a horizontal and a vertical component so the back polariser blocks the vertical component but passes the component which is horizontal. Hence some light gets through the complete system.

It can be getting a bit faint by then and maybe coloured as cheap polarisers dont work so well in the blue (short wavelengths).

This is the principle of the dark field polarising microscope. Two crossed polarisers give a dark field until something with polarising properties is inserted between. This can be a crystal whose optical properties are different in different crystal directions (alinged atomic arrangements), a transparent material under strain, or even a material, such as stretched polythene, whose long intertwinned molecules partially aligned because of the stretching. You can observe the latter yourself between crossed sunglasses.