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Microscope: Ghostly afterimages

By Mike McRae, 26 October 2022

Line drawing of a bird next to the same bird coloured red.

Stare at the red bird’s eye for 30 seconds, then quickly stare at the white bird’s eye. The bird should turn blue-green!
Credit: ©iStock.com/sonicken remix

Double Helix magazine is looking for your questions! Our Microscope column answers the most intriguing science, tech, engineering and maths queries you can throw at us.

Comment down below with your question, or email us at Helix.Editor@csiro.au. The best questions will be published in our magazine! Here’s a sample question to get you thinking.

Aditi asks
When we look at a very bright object and then look away from it, why do we still see the object’s outline?

After light from an object enters our eyeball, it’s focused into an image on the back wall of the eye. Here, a thin layer of tissue called the retina absorbs the light and transforms it into nerve impulses, ready for the brain to interpret.

Retinas are made up of a few different kinds of cells, which respond to different colours and intensities of light. To do their work, chemicals inside each cell change shape whenever light with the right characteristics hits the retina.

The chemicals in the retina’s cells take time to reset to their original shape after being activated. This is especially the case if they’ve been switched on by a long look at a bright image. To make sense of shapes and colours, the brain balances the intensity of colours in pairs – reds with greens, blues with yellows, and light with dark.

Even without those cells being active for a moment, the brain keeps adjusting, adding patches of one colour wherever another colour has tired out a part of the retina.

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