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Colour reversal illusions

Stare at one colour for a long time, and see inverse colours appearing.
Science content (2016 curriculum): 
Biology: Sensing, Organ Systems (4, 5, 6)
Lessons activity is in: 

Activity 1: Stare at the red and green cross for 20 seconds, then look at some blank white paper. Try not to blink after you move your gaze.

Activity 2: Stare at the black spot in the centre of the red circle for 20 seconds. Then slowly move the image away from your eyes, still staring at the black spot. You will see the inverse colour (light blue, or cyan) appearing as a halo around red circle.

The cells sensitive to a particular colour get tired when you stare at one colour for along time and stop sending signals to the brain. When you put white into the same field of view (either by moving your gaze, or by moving the image away from you), all the colour cells are now stimulated. The tired cells send a weak signal and the other cells send a strong signal, so you see the inverse colour from the original colour stared at.
(Staring at green makes green cells tires. Red and blue cells give a strong signal, perceived as magenta; inverse of red is blue and green giving cyan. Inverse of blue is red and green giving yellow).

Activity 3: Try making your own shapes from one or two bold colours. Use the method in activity 1 or 2 to play with colour perception.

Attached documents: 

This activity is hard for younger students, especially those that have a short attention span. Only a few students in science club grades K-2 were able to see the illusion. The dot is the easier of the two, as they may at least see the halo appearing if they don't see the image on white.
Try doing just the dot, then moving your head back to see the halo.

Grades taught: 
Gr K
Gr 1
Gr 2
Teaching site: 
General Gordon Elementary
General Gordon Elementary Science Club