Eye: The organ of sight. Eyes are organs that detect light, and send electrical impulses along the optic nerve to the visual and other areas of the brain.
Overview:
The eye is a very complex organ with multiple components. The eye allows you to see and interpret the shapes, colors, and dimensions of objects in the world by processing the light they reflect or emit and the eye is able to detect bright light or dim light, but it cannot sense an object when light is absent. We use our eyes in almost every activity we perform, whether reading, working, watching television, writing a letter, driving a car, and in countless other ways. Most of the people probably agree that sight is the sense they value more than all the rest.
Our eyes work much similar to a camera. In a camera, light passes through the lens and then focuses on the film. Each of our eyes also has a crystalline lens, which is located just behind the iris. As the image passes through the cornea, our lens focuses on those lights, colors and shapes on the retina. The retina then registers these images and sends them to our brain but sometimes the shape of our eye doesn't bend the light properly, and hence this can lead to common vision problems like nearsightedness and farsightedness. By contracting and relaxing the muscles that make up the ciliary body, the thin vascular tissue which secretes transparent liquid within the eye, we are able to focus our vision on things which are close up, far away and everywhere in between. This function allows us to see objects throughout our range of vision.
Normal eyes:
Eye is one of the most important organ in a human body. In a normal eye, the rays of light, coming from a distant object gets focused by the cornea and the lens of the eye onto the retina and thus it forms a sharp image. The normal eye has no refractive error such as nearsightedness, farsightedness or astigmatism. The normal eye retains good distance vision with aging, but usually begins to need reading glasses by middle ages like 40’s.
Pit eyes:
Pit eyes are also known as stemma which are the eye-spots which may be set into a pit to reduce the angles of light that enters and affects the eyespot, to allow the organism to deduce the angle of incoming light. The directionality can be improved by reducing the size of the aperture which is done by incorporating a reflective layer behind the receptor cells, or by filling the pit with a refractile material. Pit eyes are found in about 85% of phyla, which are the basic forms, probably the precursors to more advanced type of simple eye.
Different types of eyes which come under Pit Eyes are:
Spherical lensed eye: spherical lensed eye is seen in mammals, also in some cases of gastropods, copepods & cephalopods.
Multiple lenses: Multiple lenses are seen in some of the marine animals, Eagles, jumping spiders.
Refractive cornea: Refractive cornea is only useful out of water and these are found in most of the mammals, reptiles & birds whose vitreous fluid has a higher refractive index than the air.
Reflector eyes: Many small organisms such as rotifers, copeopods and platyhelminths use this kind of eyes, but these are too small to produce usable images. Some larger organisms, such as scallops, also use reflector eyes.
Compound eyes:
Arthropod eyes are also called as compound eyes because they are made up of repeating units, the ommatidia, each of which functions as a separate visual receptor and this type of eye is commonly found in arthropods, including many insects and crustacea. A compound eye generally allows only a short range of vision.
The two main kinds of compound eyes are:
Apposition eyes: Apposition eyes are the most common form of eye, and they are presumably the ancestral form of compound eye. These are found in all arthropod groups, some annelids and bivalves also have apposition eyes.
Superposition eyes: The superposition eye is used mostly by nocturnal insects.
