In order to see, light rays must be focused on the retina. The resulting nerve impulses must be transmitted to the visual areas of the cerebral cortex in the brain.
Refraction of light rays is the deflection or bending of a ray as it passes through one object and into another of greater or lesser density. The refraction of light within the eye takes place in the following pathway of structures:
- cornea
- aqueous humor
- lens
- vitreous humor
The lens is the only adjustable part of the refraction system.
When looking at distant objects, the ciliary muscle is relaxed and the lens in elongated and thin.
When looking at near objects, the ciliary muscle contracts to form a smaller circle. The elastic lens recoils and bulges in the middle, having a greater refractive power.
When light rays strike the retina, they stimulate chemical reactions in the rods and cones.
In rods, the chemical rhodopsin breaks down to form scotopsin and retinal (a Vitamin A derivative). This chemical reaction generates an electrical impulse. Rhodopsin is then resynthesized in a slower reaction. A deficiency of Vitamin A will decrease the sensitivity of rods, resulting in night blindness.
In cones, the chemical reactions are brought about by different wavelengths of light. It is believed there are three types of cones:
- red-absorbing
- blue-absorbing
- green-absorbing
Each type absorbs wavelengths over about one-third of the visible light spectrum, causing red cones, for example, to absorb light from the red, orange, and yellow wavelengths.
The chemical reactions in cones also generate electrical impulses. Absent or nonfunctional cones are the cause of colorblindness, with the most common form being the inability to distinguish between red and green.
Impulses from the rods and cones are transmitted to ganglion neurons, which converge at the optic disc to become the optic nerve, passing posteriorly through the wall of the eyeball.
The optic nerves from both eyes converge at the optic chiasma, which is just in front of the pituitary gland. Here, the medial fibers of each optic nerve cross to the other side. Crossing permits each visual area to receive impulses from both eyes, important for binocular vision.
Visual areas are located in the occipital lobes of the cerebral cortex of the brain. Although each eye transmits a slightly different picture, the visual areas put them together or integrate them to make a single image. This is called binocular vision. Visual areas also right the image since it appears on the retina upside down. The image on film in a camera is also upside down, but this is not apparent because, when they are viewed, they appear right side up, an accomplishment of the brain.
Errors of Refraction:
- Astigmatism results in an irregular curvature of cornea or lens, where parts of the visual field appear blurred.
- Myopia (nearsightedness) occurs when distant objects appear blurred. It happens when the eyeball is too long or the lens too thick.
- Hyperopia (farsightedness) occurs when near objects appear blurred. It happens when the eyeball is too short or the lens too thin.
