How does the eye see rapid individual movements as in a cinema as a continuous movement?
The eye has the ability to fuse individual “flicker” movements into one homogeneous whole. When a blinking light is presented slowly to the eye, it sees it as “on” and “off” separately. As the speed is increased there is a continuous flicker, but the light stays “on”. As the frequency of the blinking is increased the flicker suddenly vanishes. This fusion frequency is important. The usual level in bright light is 40 cycles per second for the fovea (the most sensitive part of the retina) to 20 cycles per second in the periphery of the retina. However, it decreases in the dark, especially if one is tired or has taken alcohol, and a flickering light can cause a severe headache and nausea. The annoying flicker of a fluorescent tube is similar in origin. Normally the flicker is at 60 cycles per second. But if the tube becomes old or defective the flicker rate drops to 35 cycles per second and the flicker then becomes very noticeable and irritating. Why do we need two eyes? Two eyes provide
