Can the human eye detect just a single photon?

When the photon encounters the rhodopsin molecule in the retina, the rhodopsin absorbs the energy of the photon and a cis-double-bond is temporarily converted into a single bond which then rotates 180°. The double bond reforms with the molecule now in the trans configuration - the shape is changed from curved to straight. The new shape no longer fits its binding site and an unstable linkage, subsequent shape changes, and ultimate breakdown lead to movements that are detected by the nerve cell, which depolarizes its membrane and initiates a signal.

The retina has 10 layers of neural processing in force prior to a signal exiting the optic nerve. A single photon signal from a receptor is not enough to forward ahead to the brain. If the retina is totally unbleached (fully dark adapted after about 30 minutes of total darkness) then the threshold for seeing light is about 6 photons arriving very close to the same place and time in the retina... (spatio-temporal summation) that is enough to make it through processing that results in a signal you will see as a brief point of light.

 

I would think that with 6 photons, and under those ideal conditions, you would still be wondering to yourself "did I see something there"? lol.

Though, I'm not very familiar with human physiology to be of more help.

 

No. In fact, there hardly is a notion of a "single photon".

That depends on the energy of the photon, which can range from zero to somewhere about infinity.

 

Actually, you are on the right track... the doubt about whether something was seen is part of the measurement. In all kinds of discrimination tests like this, the threshold is defined as just when the observer is correct only half the time.

 

OK, I'm surprised no one cited the Physics FAQ yet. This was written back in 1996!

http://math.ucr.edu/home/baez/physics/Quantum/see_a_photon.html

Zz.

 

There is a related phenomenon with cosmic rays. Apparently single sub-atomic particles can be perceived, although the mechanism is not clear.
http://en.wikipedia.org/wiki/Cosmic_ray_visual_phenomena

I'm not sure how this translates to single high-energy photons.

 

I'm not even sure how this is on-topic in this thread!

Zz.

 

This is a very popular view of what a photon must be. Unfortunately it just doesn't make sense. One 'clincher' argument is that a single photon of a Long Wave Radio signal would occupy more than 1km of space and yet it represents less than a millionth millionth of the energy of a single photon of visible light. Don't go looking for a 'size' for a photon in conventional terms.

People claim that single photons of visible light can be perceived after hours of time spent in total darkness, when the eye is fully dark-adapted. I think this is based on using a (measured) low level of light and the rate of 'random' flashes seen correlates with the illumination level. That implies that the individual photons are, in fact perceived.

It is certainly possible to detect individual photons of light using a photo-multiplier, so it's is not that unlikely.