The human eye

The sensitivity of the human eye is quite remarkable. When fully adapted to darkness it can detect a signal consisting of as little as 5 or 6 photons. To put this into perspective, a small bulb on a Christmas tree emits about 1020 photons per second. Perhaps it is just as well that we have not evolved so far as to see a single photon, as then, even in the darkest night, stray photons would appear as a series of intermittent tiny flashes with which our brain might find it difficult to cope.

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