Mind-boggling Union Flag optical illusion shows how colour blindness works

A resurfaced optical illusion clip used to explain how colour blindness works has left viewers in awe.

The BBC Archive shared a 1985 video of an unusual Union Flag in the colours of yellow, black and green.

At the beginning of the clip, the presenter asks viewers to stare at a black dot in the centre of the flag until it disappears.

The screen then subsequently appears being blank, and most people will see the Union Flag in its traditional red, blue and white colours.

The presenter reveals that the illusion can be explained by the simple "three receptor theory" – which is based on the colour-sensitive cones in the retina of the human eye, corresponding roughly to red, green, and blue sensitive detectors.

As you stare at the green area of the image, your green and blue cones get tired and respond less than normally to the white screen when it flashes up.

As the red cones were not in use in this case, the imbalance creates an appearance of a "red image" in the white screen.

Meanwhile the yellow areas of the flag pattern activate and tire out the red and green cones.

When the image flashes "white" it is only the blue cones working normally, leading this to show the typical red, blue and white Union Jack Flag.

This mimics the experience of some "colour blind" people who might lack cones sensitive to one of these three colours.

It also leads to a different version of the colour being seen.

The simple test results fascinated the viewers. One said: "I see violet, light purple when the screen goes white!"

Another wrote: "Cool! In saw the Union Jack in pale tones of red white and blue. The red was more a faded pink and the blue was like a lilac blue… Awesome!!"

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