Rays of light enter the eye, and an image of the thing is targeted on the retina. Focusing bends the light rays, a phenomenon known as refraction, so the image falls on the fovea centralis . Refraction occurs when mild rays pass at an oblique angle from a substance of one optical density into a substance of a different optical density. This occurs at the curved floor between the air and the cornea and on the curved surfaces of the lens itself. A lens with a convex surface causes mild rays to converge.