Vision & The Eye
When you understand how the eye works, it is easier to comprehend why people need glasses or contact lenses to see well.
The eyes are complex sensory organs. They are designed to optimize vision under conditions of varying lighting. The basic elements are similar to those of an average photography camera.
The primary function of the eye is to focus light. For the eye to see, light rays must be bent or "refracted" to meet at a single point through the cornea, the clear window at the front of the eye that provides most of the focusing power. The transparent dome cornea is a firm covering and is susceptible to dryness and abrasion injuries.
The iris of the eye is the color portion behind the cornea. Our eye color is a function of the amount of pigment within the iris (brown eyes have the most pigment, while blue eyes have the least). The iris contains muscles that open and close its central opening called the pupil in response to decreases and increases in light exposure (exactly like the camera aperture).
Light then travels through the lens, where it is fine-tuned to focus properly on the retina, the nerve layer that lines the back of the eye and connects to the brain. The retina acts like the film in a camera, and clear vision is achieved only if light from an object is precisely focused onto it. If the light focuses either in front of or behind the retina, the image you see is blurred. A refractive error means that the shape of eye structures does not properly bend the light for focusing.