Can a single photon be seen?

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The human eye is very sensitive, but can a human see a single photon?

The retina has two types of receptors: cones and rods. Cones are responsible for the perception of colors, but they are much less sensitive than rods. In bright light, the cones are active, but the rods are not, but when darkness falls, the eyes adapt and the rods become more active.

The sensitivity of the rods is about 10,000 times that of the cones. Due to this, after adaptation to darkness, we see much better in the dark, but the perception of colors decreases.

A photon hitting a stick receptor changes the energy of molecules at the tips of the receptor. Certain chemical reactions take place and in a very short time the signal from the photon reaches the optic nerve.

The light sensitivity of the human eye has been tested by scientists: Hecht, Schlayer and Pirenne. In their experiment, the subject was placed in a completely dark room for 30 minutes so that his eyes could get used to the darkness. After that, with the help of a very weak light source, they shone in his eyes and asked to answer when he sees the light.

At first, the subjects were able to confirm the presence of light without any problem. Then the power of the light source began to be lowered until the subjects began to make mistakes in more than 50 percent of the cases.

During the experiment, it was found that at least 9 photons must reach the retina every 100 milliseconds in order for the subject to correctly determine the presence of light at least 60 percent of the time.

Later, another group of scientists experimented with stick photoreceptors connected to electrodes and proved that the photoreceptor responds even to a single photon. Why does it take more to see the light?

As it turned out, neural filters in the brain limit our perception in such a way that we begin to distinguish visible light only if at least 50-100 photons per second come from one source.

If we could see and perceive each photon, there would be too much visual «noise» in our field of vision. Neural filters seem to cleanse the visible image for us from too weak and insignificant light sources.