2.1 Visual Mechanics

Our eyes gather information by processing light reflected from or emitted by objects. Light rays enter the cornea and are refracted by its curved surface. As a result the light rays are squeezed together. This allows them to enter the pupil. The iris regulates the amount of light entering the pupil. Bright light causes the iris to contract and dim light causes it to dilate.

the human eye

Having passed through the iris, light waves strike the lens. With its two curved surfaces, the lens further refracts light. Muscles in the eye control the focal length, determining the amount of refraction. Eyes that are too long or have too much refracting power cause images to be focused in front of the retina. This condition is known as near sightedness. Farsightedness occurs when the eye is too short or refracts too little causing images to be focused behind the retina. A perfect eye focuses light energy into an image on the retina.

cones and rods in the human eye

The retina is the inside lining of the rear surface of the eye. It contains cells that convert light energy into electrical impulses. These specialized light receptor cells are the cones and rods. Cones are highly concentrated in the central fovea, a small pit near the intersection with the optic nerve. They are used for detailed acuity in bright light and allow us to see colour. In contrast, rods are spread in the periphery of the retina, and are used in reduced light allowing us to see only black and white. The reception of light at the retina is the last passive stage in the vision process.

Having been converted to electrical impulses, light energy now travels through the axons of the cones and rods. The axons join together to compose the optic nerve. It carries the electrical stimulus to the brain stem, where receiver nerve cells connect with the optic nerve cells. Signals from the stimulated receiver cells travel to the occipital lobe at the back of the brain where they connect with other brain cells to produce vision.