Andrea Sella's Haidinger's Brush Trainer

Be the envy of your friends! Tell them the angle of polarisation of the light in the blue sky, with no equipment at all!

The purpose of this site is to help Andrea, and maybe you too, to learn to do this.

When you see a flat plane of polarised light such as the blue sky, with the naked eye, there is an extremely faint coloured pattern visible, due to the characteristics of your retinas. This pattern is called Haidinger's Brush. Here's how you can learn to develop a "sixth sense" and see it yourself.

First, learn what it looks like.

It looks like this (maybe rotated), and extremely dim, in the centre of your field of vision. It's about as big as the tip of your thumb at arm's length. Move the slider; try to see the pattern when it's as dim as possible. Actually, due to Method 2 below, it is visible on your monitor even when the slider is all the way to the left.
Haidinger's Brush

Second, learn to see it in an artificial situation.

Method 1, with a polarising filter.

Method 2, using an LCD display.

Third, learn to see it in the sky.

Learn more about Haidinger's Brush on Wikipedia.