The audience stares directly at a flag for 20 seconds, and immediately afterwards looks at a white surface. On the white surface appears, after a short while, the flag of Great Britain.
How does it work?
The first flag is the same as the second, but with inverted colours. As long as you stare at the image without moving your eyes or the image, the cones in the retina in the eye, which are responsible for colour vision in bright light, will be charged. In the area that yellow light falls, the protein responsible for seeing yellow, Phototopsin, is consumed. When looking at the white surface, all the colours of the spectrum hit the eye, except the yellow light which cannot be seen without the protein. This means the eye ends up seeing blue. After a few seconds the Phototopsin proteins are again stimulated.
