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The hidden sense in your eyes

By David, 23 July 2020 Activity

Image of two vertical blue balls and two horizontal yellow balls on a computer screen.

You might see some faint, fuzzy coloured shapes!

Your eyes can see colour and brightness, but there’s one more thing they can detect. With the right technique, many people can tell if light is polarised – no glasses needed!

You will need

  • Computer with an LCD screen

What to do

  1. Open a window on the computer that displays only white. An easy way to do this is to open a text editor such as Notepad or TextEdit.
  2. White screen on a computer monitor.Make the window as big as possible.
  3. Relax your eyes and try to stare through the white window.
  4. A girls face with an arrow above indicating a left-right tilt.Slowly tilt your head from one side to the other, keeping your gaze on the window. After a while you might see some faint, fuzzy coloured shapes! Be patient, they can take a while to spot.
  5. Pale, fuzzy blue and yellow shapes on white background.If you don’t know what you’re looking for, it might look a bit like this:

What is polarisation?

Our eyes contain light-sensitive cells called cones to detect colour, and rods that sense if light is bright or dim. But light also has a property that humans normally can’t see, called polarisation.

Light is a wave, which means it waves back and forth as it travels. Some particles of light are waving up and down, some left and right, and some even travel in a spiral!

Your computer screen has filters in it to polarise the light, so it’s travelling in the same direction. Normally, humans don’t notice polarised light. But this activity reveals your hidden polarised light sense!

What’s happening?

Pale, fuzzy blue and yellow shapes on a white background.

Haidinger’s brush

You might see colours in this activity, but they aren’t coming from the screen. This strange effect, known as Haidinger’s brush, happens inside your eyeballs.

Light enters your eyes through your pupils. Then, the light gets projected on the back of your eyeball, where it’s detected by light-sensitive cells. But just before those cells, there’s a layer of blood vessels, nerves and other things that the light goes through.

One of those things is a pigment known as xanthophyll (ZAN-tho-fill). As polarised light goes through the xanthophyll, it forms the pattern you saw. It’s only faint, which makes it hard to see, especially if the pattern doesn’t move.

By turning your head and keeping the screen still, the brush effect moves around on the back of your eye, making it easier to spot!

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Image credits:
Hero image and step 2: ©iStock.com/turk_stock_photographer, David Shaw remix
Step 4: ©iStock.com/Khosrork, David Shaw remix
All other images: CSIRO/David Shaw

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