Answer: Afterimage
Step-by-step explanation:
Afterimage is the effect that occurs in form of optical illusion to the eyes.An image glimpses can be seen even when exposure towards it has ended.These glimpses are illusion that are presented through visual system where retina creates an impression .
According to the question,professor is describing about afterimage concept through making children stare at screen square image for a while and then showing them white screen which makes them see an illusion of red colored square image that is not actually present.Illusion is present where image shape is same but color is changed due to reception mechanism.