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The human eye is a sensory organ, part of the sensory nervous system, that reacts to visible light and allows us to use visual information for various purposes including seeing things, keeping our balance, and maintaining circadian rhythm.
The eye can be considered as a living optical device. It is approximately spherical in shape, with its outer layers, such as the outermost, white part of the eye and one of its inner layers keeping the eye essentially light tight except on the eye's optic axis. In order, along the optic axis, the optical components consist of a first lens that accomplishes most of the focussing of light from the outside world; then an aperture in a diaphragm that accomplishes the remaining focussing of light into images; then a light-sensitive part of the eye where the images fall and are processed. The retina makes a connection to the brain via the optic nerve. The remaining components of the eye keep it in its required shape, nourish and maintain it, and protect it.
Three types of cells in the retina convert light energy into electrical energy used by the nervous system: rods respond to low intensity light and contribute to perception of low-resolution, black-and-white images; cones respond to high intensity light and contribute to perception of high-resolution, coloured images; and the recently discovered photosensitive ganglion cells respond to a full range of light intensities and contribute to adjusting the amount of light reaching the retina, to regulating and suppressing the hormone melatonin, and to entraining circadian rhythm.