The simplest "eyes", such as those in microorganisms , do nothing but detect whether the surroundings are light or dark , which is sufficient for the entrainment of circadian rhythms .  From more complex eyes, retinal photosensitive ganglion cells send signals along the retinohypothalamic tract to the suprachiasmatic nuclei to effect circadian adjustment and to the pretectal area to control the pupillary light reflex .
The human eye is an organ which reacts to light and pressure. As a sense organ , the mammalian eye allows vision . Human eyes help provide a three dimensional, moving image, normally coloured in daylight. Rod and cone cells in the retina allow conscious light perception and vision including color differentiation and the perception of depth. The human eye can differentiate between about 10 million colors  and is possibly capable of detecting a single photon . 
Just behind the iris and pupil lies the lens, which helps focus light on the back of your eye . Most of the eye is filled with a clear gel called the vitreous. Light projects through your pupil and lens to the back of the eye. The inside lining of the eye is covered by special light-sensing cells that are collectively called the retina. It converts light into electrical impulses. Behind the eye, your optic nerve carries these impulses to the brain . The macula is a small extra-sensitive area in the retina that gives you central vision.
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