The text sites Elizabeth Wood's book which discusses light waves as they come in contact with a surface. Wood uses the analogy of an axel with two wheels rolling along concrete and suddenly contacts grass on an angle. As she explains, the first wheel rolls onto the grass and ___________while the other wheel continues to travel at a _____________velocity on the concrete. Once both wheels are on the grass they will travel at the new __________velocity. This analogy explains a waves refraction through surfaces.