KS2 Make a small viewing hole at one end of a shoebox and a slit (about 10cm x 0.5cm) across the lid near the opposite end. Put brightly coloured objects, eg Lego bricks, in the box at this end, replace the lid and look through the viewer in a brightly lit room. Gradually cover the slit to reduce light. Investigate the best colours to wear for safety outdoors on dark evenings.
KS3 Investigate changes in the size of the pupil by exposing each eye in turn to darkness and bright light. Students will observe that the size changes by the action of the muscles of the coloured iris. Link this to the ways the eye protects itself and adapts to different light conditions.
Most textbooks feature simple experiments with two eyes and stereoscopic vision, but you can show which is the more "dominant" eye by holding a pencil at arm's length and, with both eyes open, line the pencil up with a distant object. Then open and close each eye in turn. With one eye, the pencil will seem to jump sideways. This is the eye that was used to line up the pencil in the first place.
KS4 Use Ishihara colour vision test cards to investigate the occurrence of colour blindness in a class. The condition is more common in boys. Explain, using the mechanism of sex-linked inheritance. Information about the history of colour vision tests can be found at www.college-optometrists.orgcollegemuseumcolourishihara.htm
KS5 Look at sections of the retina with a microscope. Research the role of photosensitive pigments in colour vision and what happens to them in conditions such as snow-blindness. Find out about research into artificial rods and cones and their use in bionic eyes; see www.firstscience.comsitearticlesbionic.asp