Humans eyes have three types of photoreceptors, known as cone cells, which are sensitive to the colors blue, green and red. Rods, a different type of photoreceptors, detect small amounts of light and this allows us to see in the dark. Animals process light differently, some creatures only have two types of photoreceptors, which cause them to be partially colorblind, some have four which will enable them to see ultraviolet light, others can detect polarized light which means light waves oscillating in the same plane.
Animals process light differently—some creatures have only two types of photoreceptors, which renders them partially colorblind, some have four, which enables them to see ultraviolet light, and others can detect polarized light, meaning light waves that are oscillating in the same plane.
Although a cat has a poorly developed color vision, seeing the world largely in black, white and grey, it can see far better in the dark, thanks to a crystalline layer in the retina which enables it to absorb 50% more light than human eyes. By day, the cat's irises contract into slits to keep excessive light out.
Sensitivity to ultraviolet light, invisible to humans, enables bees to spot special honey-guide markings on many flower petals which point the way to nectar and pollen. The same sensitivity allows bees to 'see' the sun, even on a cloudy day, so that they can find their way back to the hive. Bees cannot see red, perceiving it as blue.
Most spiders have eight simple eyes known as ocelli, arranged around the top of the head so that they can see in all directions at once. In species such as the jumping spider, which stalks its prey rather than simply waiting for it, two of the eyes at the front are better developed than the rest. This allows the spider to gauge distances accurately for its final pounce.
Many foraging birds, such as chickens and shorebirds, have eyes set on the side of their heads so that each eye sees a different scene. The resulting wide field of vision allows them to spot danger from almost any direction but limits their abilities to judge distances. Shorebirds, such as sandpipers, compensate for the lack of stereoscopic vision by bobbing their heads up and down and sideways to view an object from several angles against its background.
Like other insects, butterflies have compound eyes, made up of numerous separate eyes, up to 28,000 in a dragonfly, but as few as nine in some ant species. Each mini-eye is equipped with its own minute lens so that insects see an object as a mosaic in overlapping points of light. Like a badly tuned television picture. Compound eyes are unable to focus sharply, but they are good at spotting movement.
@frieda