Human eyes are able to see the flame of a candle at a distance of 50 km

surface of the Earth in your field of vision begins to bend at a distance of 5 km. But the sharpness of human vision allows you to see far beyond the horizon. If there were no curvature, you could see a candle flame 50 kilometers from you.

Range of vision depends on the number of photons emitted by the remote object. 1 000 000 000 000 stars of this galaxy collectively emit enough light to thousands of photons reaches each square. see the earth. This is enough to excite the retina of the human eye.

Since, on the ground, check the sharpness of human vision is not possible, the scientists resorted to mathematical calculations. They found that, in order to see the flashing lights, you need to hit the retina from 5 to 14 photons. A candle flame at a distance of 50 km, taking into account the scattering of light, giving the number, and the brain detects a faint glow.

via factroom.ru