Assign first the positions of the eye parts, via a diagram on the board, which matches the one outside, but in smaller scale. These children wear a tag around their neck or a Post-It labeling the part of the eye they are representing. Assign the photons by passing to each child a sheet of colored construction paper, judging the numbers of each color you'll need, according to the size of your class. There should be at least one student representing each color of the rainbow. Assign the white and black photons, four of each, by passing out the white and black paper.
Reds are slow, blues are fast and the in between are moderate.
Demonstrate by shaking your arms above your head while holding a piece of paper. Instruct the pupils role-playing the eye parts as to their jobs, as stated below. Line up the students in the following order, so when they go outside they are ready to take their places and move when they hear their part being called by you. Photons' kids will need to walk in pairs through the Iris opening when their color is called in the narrated story. As a pair reaches the retina, another pair enters the eye.