Locust Street School in Jersey Shore, Pa., for their question:

HOW DOES A COLOR AFTER IMAGE WORK?

The after image phenomenon is caused by fatigue of certain light sensitive receptors in the eye. It is a fact that every color has a complementary one which may be evoked as an after image under proper conditions, for our seeing organs are constructed so that the complementary relationships are natural and inevitable.

Look at a yellow block of color for 30 seconds under strong light. Shift your gaze to a blank sheet of paper and you should see a blue after image. If you gaze at a blue block of color, a yellow after image will appear. Then stare at a red swatch of color for about 30 seconds and your after image should be the complementary green. Looking at a green spot should return the complementary red.