Light entering the eye first comes into contact with the tear film, which protects and bathes the corneal surface. The cornea is a transparent structure at the front of the eye that bends incoming light and provides most of the eye’s optical power, or light-bending ability. Beyond the cornea is the iris, a colored muscle that contracts and relaxes depending on light levels to adjust pupil size. Aqueous humor fills the space between the iris and cornea, providing nourishment. Aqueous fluid is also responsible for the pressure within the eye.
Once light is focused through the pupil by the cornea it is finely focused by the crystalline lens, which changes shape allowing focal adjustment of light onto the retina. This adjustment is known as accommodation and begins to diminish in our fourth decade of life. Losing the ability to focus is known as presbyopia.
After light passes through the crystalline lens it enters the vitreous humor before striking the light sensitive layer of cells known as the retina. Embedded within the retina are millions of light sensitive cells, including rods and cones. Rod photoreceptors are used in poor light conditions and are found more in the peripheral retina. Cone photoreceptors are densely packed within the macular region, which is responsible for fine detail, central vision. Cone receptors sense blues, reds and greens separately and are sensitive in bright environments.
Most vision problems are due to an error in how our eyes refract light. Once light is focused on the retina the image or visual stimulus needs to be processed and interpreted, which occurs within the brain.