Seeing pink elephants

Instructions

Use this image or download the PowerPoint slides (see below).

Ask students to stare at the cross on the elephant’s body for 30 seconds and then to look at a blank sheet of paper/white wall (if required, use a countdown for the last 10 seconds to maintain their concentration).

Ask students to describe what they see which should be a “pink” elephant.

Explanation

The eye has three types of cone cells, each of which responds best to different wavelengths of light:

1. L (560nm – red)
2. M (530nm – green)
3. S (430nm – blue)

Light incident on these cone cells is converted to electrical (nerve) potentials by visual pigments at the back of the retina. Mixtures of stimulation of each of these three cell types correspond to all of the colours that we perceive.

When you stare at the green elephant for some time the M cones in those parts of the retina that are stimulated will start to exhaust their pigment. Changing the view to a white background will now stimulate the whole retina across a broad range of wavelengths.

However, until the pigments have been fully regenerated, the retinal area that was previously viewing the elephant sends weaker signals from the M cones in response to the white when compared with the L and S cones. The brain will therefore interpret this input as an elephant shape composed of light that has been stimulating the L and S cones, so we see a magenta elephant.

This after-image disappears when all of the pigments in the receptors “recharge”.

When explaining this to younger students it may be useful to simplify the language and say that we have three types of colour detectors (red, green and blue) and that staring at the elephant makes the “green” detectors run out (of pigment) so that when we change the view we only get red + blue = magenta rather than the full complement of red + blue + green = white.