On the back surface of the eyeball is the retina and it is covered with light sensitive cells. These cells are activated by light energy so that when light falls on one, a chemical reaction takes place which then sends a particular electrical signal along the cell’s axon to the brain (refer back to page 5). The chemical balance in the cell is constantly being re–set. Staring at an image (for more than 30 seconds) tends to deplete the light sensitive chemicals in certain cells which can mean there is a small delay before the balance is re–established. If we stare at a blank area during this time we will see a negative of the image we stared at initially.
125 million of the cells on the retina are called rods and their role is to detect the difference between light and dark. Only 6 million cells are responsible for detecting colours. These are called cones and there are different types for different colours. After – images appear in opposite colours called complements. The complement of blue is orange, red’s is blue–green, and yellow's is blue–purple.
The final thing you need to know about is an illusion called ‘persistence of vision’. This refers to when an image is changing faster than our visual system can process information. The brain blends these images together, either interpreting them as a single composite image (as in the third part of this activity) or as motion (as in TV and movies where 24 separate images are shown every second).
Stare steadily at the image for at least 30 seconds (45 is better). It's OK to blink as long as you don’t look at anything other than the image. After the time is up, move your gaze immediately to a blank, white piece of paper. Do you see an afterimage on the paper? Does it seem to glow?
Stare again for at least 30 seconds, but this time at an orange. When you transfer your gaze to the white paper the image won't be orange at all!
Cut a 10 cm square of cardboard with a 5 cm circular hole in the middle. Push this onto a tennis ball, then use your fingers to spin the new device. If it's spinning fast enough, the square looks like a circle!