The human retina is incredibly sensitive to light

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Many neuroscientists argue that the retina is part of the brain, as it develops from the same tissue and is directly linked to it. The eyes take in light through the pupils and lenses at the front, which lands on the retina at the back. The retina is a complex layer of photoreceptors, specialised neurons for detecting light, some of which can be activated by as little as half-a-dozen photons (the individual ‘bits’ of light). This is very impressive sensitivity, like a bank security system being triggered because someone had a thought about robbing the place. The photoreceptors that demonstrate such sensitivity are used primarily for seeing contrasts, light and dark, and are known as rods