How the eye works
The ability to “see” an object begins when light reflects off if it and enters the eye. As the light enters the eye, it is unfocused. The first step in seeing is to focus the light rays onto the retina, which is the light sensitive layer found inside the eye. Once the light is focused, it stimulates cells to send millions of electrochemical impulses along the optic nerve to the brain, enabling us to see the object.
When light enters the eye, it is first bent (refracted), by the front surface of the eyeball (the cornea). The cornea is what provides most of the eye’s optical power (or light-bending ability).
Once the light passes through the cornea, the focus is more finely adjusted by the crystalline lens. The ciliary muscles inside the eye change the shape of the crystalline lens by bending or flattening it to focus the light on the retina. This process of bending or flattening is called accommodation; this is needed to bring objects into focus. The entire process of the bending of light to produce a focused image is called refraction.
Even when the light is focused on the retina, the process of seeing is not complete. The image is inverted, or upside down. Light from the various “pieces” of the object being observed stimulate nerve endings — photoreceptors or cells sensitive to light — in the retina.
Two types of receptors (rods and cones) exist in the retina. Rods are found in the peripheral retina and enable us to see in low light conditions and to detect peripheral movement. They are primarily responsible for night vision and orientation. Cones are found in the central retina and provide detailed vision for such tasks as reading or distinguishing distant objects. They also are necessary for color detection. These photoreceptors convert light to electrochemical impulses that are transmitted via the nerves to the brain.
Millions of impulses travel along the fibers of the optic nerve at the back of the eye, eventually arriving at the visual cortex of the brain, located at the back of the head. Here, the electrochemical impulses are unscrambled and interpreted. The image is re-inverted so that we see the object the right way up. This “sensory” part of seeing is much more complex than the refractive part — and therefore is much more difficult to influence accurately
Most vision problems occur due to an error in how our eyes refract light. In nearsightedness (myopia), the light rays form an image in front of the retina. In farsightedness (hypermetropia), the rays focus behind the retina. In astigmatism, the curvature of the cornea is irregular, causing light rays to focus to more than one place so that a single clear image cannot be formed on the retina, resulting in blurred vision. As we age, we find reading or performing close-up activities more difficult. This condition is called presbyopia, and results from the crystalline lens being less flexible, and therefore less able to bend light.
