Fig. 47. Light travels slower on the plastic side of the step near the lens compared with the speed at which it moves through aqueous. This results in a difference in the total number of wavelengths from those points to the fovea and back. It is this phase delay that creates two points of focus, one for distance and one for near. (Courtesy of Mike Simpson, Alcon Laboratories, Fort Worth, TX, and James A. Davison, MD, Wolfe Eye Clinic, Marshalltown, IA.)