Most refraction experiments are theory-laden and far from everyday experience. Accordingly, many
students fail to apply the law of refraction to phenomena. To guide students from phenomena to
theory, teachers can use a refraction experiment proposed by Kepler, where measurements are based on
shadow images. For a different look at Kepler?s experiment, one can use the principle of
reversibility to get equivalent results, but based on apparent depth. For this reversal, rays of
light are reinterpreted as lines of sight, and vice versa. The principle allows students to relate
refracted rays to shifted images, and applies to other optical phenomena.