16m opto

[Szali Sandor]

From: "jhidvegi"
>> +me'r, hat ha egy fotont is meg bir latni akkor ez miert akadaly?
>
> Ez azért meredek állítás! Ha így lenne, akkor - szerintem - semmiféle
> távcsőre nem lenne szükség. A borzasztóan távoli csillagokról ugyanolyan
> energiájú fotonok jöhetnek, mint a napból, és esélyünk sincs meglátni
> szabad szemmel. Pedig biztosan nem csak 1-1 darab érkezik a szemben lévő
> 1-1 csapra vagy pálcikára.

+a tavoli csillagokrol nem biztos hogy folyamatosan jonnek a fotonok :)
De tenyleg nem igaz, en a buborekkamras tortenetre emlekztem, mikor meg a
tudosok felorakat ucsorogtek a sotetben hogy hozzaszokjanak, kozben marha
jokat elmelkedtek-beszelgettek. Na ott irkaltak olyanokat hogy egy fotont is
meglattak. Gugliztam ezt itt, valszeg nem lehet egy fotont meglatni de nem
kell sokkal tobb!!!
a vegeredmeny: 9 foton mar lathato!!!
es _bizonyitottan_ : 1 foton is kivalt reakciot a palcikaban.


[Physics FAQ] - [Copyright]
Original by Philip Gibbs 1996.

Can a Human See a Single Photon?
The human eye is very sensitive but can we see a single photon?  The answer
is that the sensors in the retina can respond to a single photon.  However,
neural filters only allow a signal to pass to the brain to trigger a
conscious response when at least about five to nine arrive within less than
100 ms.  If we could consciously see single photons we would experience too
much visual "noise" in very low light, so this filter is a necessary
adaptation, not a weakness.

Some people have said that single photons can be seen and quote the fact
that faint flashes from radioactive materials (for example) can be seen.
This is an incorrect argument.  Such flashes produce a large number of
photons.  It is also not possible to determine sensitivity from the ability
of amateur astronomers to see faint stars with the naked eye.  They are
limited by background light before the true limits are reached.  To test
visual sensitivity a more careful experiment must be performed.

The human retina at the back of the eye has two types of receptors known as
cones and rods.  The cones are responsible for colour vision but are much
less sensitive to low light than the rods.  In bright light the cones are
active and the iris is stopped down.  This is called photopic vision.  When
we enter a dark room the eyes first adapt by opening up the iris to allow
more light in.  Over a period of about 30 minutes there are other chemical
adaptations which make the rods become sensitive to light at about a
10,000th of the level needed for the cones to work.  After this time we see
much better in the dark but we have very little colour vision.  This is
known as scotopic vision.

The active substance in the rods is rhodopsin.  A single photon can be
absorbed by a single molecule which changes shape and chemically triggers a
signal which is transmitted to the optic nerve.  Vitamin A aldehyde also
plays an essential role as a light-absorbing pigment.  A symptom of vitamin
A deficiency is night blindness because of the failure of scotopic vision.

It is possible to test our visual sensitivity by using a very low level
light source in a dark room.  The experiment was first done successfully by
Hecht, Schlaer and Pirenne in 1942.  They concluded that the rods can
respond to single quanta during scotopic vision.

In their experiment they allowed human subjects to have 30 minutes to get
used to the dark.  They positioned a controlled light source 20 degrees to
the left of the point on which the subjects eyes were fixed so that the
light would fall on the region of the retina with the highest concentration
of rods.  The light source was a disk which subtended an angle of 10 minutes
of arc and emitted a faint flash of 1 millisecond to avoid too much spatial
or temporal spreading of the light.  The wavelength used was about 510 nm
(green light).  The subjects were asked to respond "yes" or "no" to say
whether or not they thought they had seen a flash.  The light was gradually
reduced in intensity until the subjects could only guess the answer.

They found that about 90 quanta had to enter the eye for a 60% success rate
in responding.  Since only about 10% of photons which arrive at the eye
actually reach the retina this means that about 9 photons were actually
required at the receptors.  Since the photons would have been spread over
about 350 rods they were able to conclude statistically that the rods must
be responding to single photons even if the subjects were not able to see
them when they arrived too infrequently.

In 1979 Baylor, Lamb and Yau were able to use rods from toads placed into
electrodes to show directly that they respond to single photons.