In general, all living things respond to the stimulus of light. Almost all multicellular organisms have specialized light receptor cells in which light energy can cause changes in a light-sensitive pigment. In most invertebrates, the light receptors do not function as eyes and as a result, they are unable to form images. However, they are able to perceive the presence of light and can detect any changes in light intensity. As a result, some of these receptors can give no indication of the direction of the light source and hence the animal responds mainly by random movements. However, there are some cases in which light receptors are arranged in such a manner as to indicate direction.
One of the earliest forms of “vision” is known as phototaxis which is a light-controlled motion. This phenomenon has been observed in some photosynthetic bacteria such as Chromatium, which move selectively towards illuminated areas rather than dark places.
In higher life forms, they have more complex eyes that generally have a lens which is capable of concentrating light onto a photosensitive area. This increases the sensitivity of the eye to dim light. It also increases the ability of the eye to perceive direction and movement. The light from each source is focused onto some of the receptor cells at any moment. There are basically two different types of image-forming eyes in animals; compound eyes and camera-type eyes. Many insects and crustaceans have compound eyes which utilize many closely packed lenses. The ommatidia point in various directions and as such will be stimulated by light from different points. Various animals such as mollusk and vertebrates possess a camera-type eye which uses a single lens system to focus light onto a photosensitive surface, known as the retina, which functions similarly to a piece of photographic film. The recognition of the shapes of objects involves the formation of an image on this photosensitive area.
For humans, the term “vision” is a complex process of information regarding the environment of a living organism. The human eye is capable of detecting a variety of colors, forming images of objects miles away, and responding to as little as one photon of light. However, it is actually the brain that sees.
The eye has often been compared to a camera. It would be more appropriate to compare it to a TV camera attached to an automatically tracking tripod; a machine that is self-focusing, adjusts automatically for light intensity, has a self-cleaning lens and feeds into a computer with parallel-processing capabilities so advanced that engineers are only just starting to consider similar strategies for the hardware they design. The gigantic job of taking the light that falls on the two retinas and translating it into a meaningful visual scene is often curiously ignored, as though all we needed in order to see was an image of the external world perfectly focused on the retina. Although obtaining focused images is no mean task, it is modest compared with the work of the nervous system—the retina plus the brain. No human inventions, including computer-assisted cameras, can begin to rival the eye.
The cornea and lens, which are two constituents of the light-focusing system, form an inverted image on the retina. The iris regulates the opening of the lens while the eyelids prevent light from entering and also prevents any possible damage to the surface of the cornea. The ciliary muscle controls the lens so that objects from different distances may be brought sharply into focus. The focusing of light onto the retina can be accomplished by this mechanism and also by the curvature of the cornea (Thomas, Yiannis 2001). The cornea has a refractive index of 1.38; the lens is 1.42 whereas the refractive index of both humors is 1.33.
The largest difference in refractive index occurs between the air and cornea and therefore it is essential for image formation. The delicate and accurate control is achieved by the lens which acts as a fine adjustment (Davson, Hugh 2012).