When light reaches a curved surface it bends. This process of bending light is called refraction. The two main refractive surfaces of the eye are the cornea, and the lens, inside the eye. These two curved surfaces, along with other internal structures of the eye, work together to focus light onto the retina in the back of the eye. If light does not focus directly on the retina, the eye will be nearsighted, farsighted, or astigmatic. These problems are called refractive errors.