The eye functions like a camera. Light rays enter the eye through the cornea (the clear window in front), pass through the pupil (the opening in the center of the iris), focused through the lens, and finally reaching the retina at the back of the eye. The retina is analogous to the film of a camera. When light rays land on the retina, they form an upside-down image. The retina converts the image into impulses that travel through the optic nerves to the brain, converting them into upright visual images.
Vision is clear only if the cornea and lens correctly bend or "refract" the light rays and focus them at the point on the retina known as the fovea. The cornea's focusing power is matched to the eye's length in the normal eye. Blurry vision may be due to what is called a "refractive error" - the failure of the cornea and lens to focus light properly. Prescription eye glasses, contact lenses, and refractive surgery correct or improve refractive errors by focusing light rays closer to or directly onto the retina.
Some common eye conditions include: