VISION PROBLEMS: If particular parts of the brain are damaged, people may lose some visual perception. They might not be able to recognize faces, for instance (prosopagnosia), or not be able to name colors (color anomia). And sometimes they can lose their stereoscopic vision (visual spatial agnosia) or lose the ability to see objects that are in motion (movement agnosia).
WHAT IS 20/20 VISION? When eye doctors talk about "20/20 vision," they mean what an average person should be able to see when standing 20 feet away from a standard eye chart. This is "normal" vision. If you have 20/40 vision, when you stand 20 feet from the chart, you can only see what the average person sees standing 40 feet from it. If your vision is 20/10, you can see better than the average person: you can see at 20 feet what most people can see from 10 feet.
HOW WE SEE: Scientists refer to light as electromagnetic radiation, because light waves are the product of electric and magnetic fields traveling together through space. We are able to see the world around us because light reflects off objects and radiates outward in all directions. Our eyes detect this light, which the brain then forms into recognizable images. The human eye works in much the same way as a camera captures images on film; its "film" is the retina, a thin layer of neural tissue lining the back of the eye, made of photoreceptor cells that receive light, and other cells that interpret this information and send the signal to the brain via the optic nerve. The retina contains two kinds of photoreceptor cells: cone cells and rod cells. Cone cells are sensitive to bright light and can perceive colors. Rod cells work best in low light and can perceive black-and-white images.