Light normally enters your eyes through the cornea before it reaches the retina. In eyes with normal vision, the light hits the same place on both retinas, which creates a single image in the brain. People with double vision have light reaching different parts of each retina, which produces two separate images. Prism lenses bend light and redirect it to reach precisely the same spot on each retina. Then the brain steps in and combines the two images into one crystal-clear image.