Eye is The Human Sensory Organ
Sensory System of Class 11
The eye receives light reflected from objects (visual field) and converts it into electrical signals which is translated by brain into visual understanding.
Light is emitted and absorbed in discrete quanta of photons. It moves as waves of electromagnetic radiation, its wavelengths detected by human eye is in the range of 380-760 nm and most sensitive for 500 nm
(i.e., green colour)
Free-living animals need sensitivity to light to locate their food, enemies and the shelter. Photoreceptors have developed for this purpose which also has undergone gradual advancement with evolution.
The Planarian (flat worms) have “eye cup” as photosensory spot to detect the intensity and direction of light.
In annelids (polychaetes) and onwards (higher animals) eye has developed as separate organ.
In insects and crustaceans there is compound eye, each unit of which is called ommatidium. They are adapted for apposition (mosaic) vision in bright light and superposition image in dim light (darkness). It
also forms coloured images. Honey bee can see UV radiation.
Among all invertebrates only cephalopods (molluscs) have eye with true lens similar to the vertebrates.