Green Laser Pointer

TeaTime wrote:
... and it has been shown that the dark-accustomed and healthy human eye can
actually detect single photons, with sensitivity peaking in the green
wavelengths. Remarkable.

References, please...

"lal_truckee" wrote in message
...
TeaTime wrote:
... and it has been shown that the dark-accustomed and healthy human eye
can actually detect single photons, with sensitivity peaking in the green
wavelengths. Remarkable.

References, please...

Just one of many:

http://hyperphysics.phy-astr.gsu.edu...n/rodcone.html

(see 'Rod Details')

"lal_truckee" wrote in message
...

TeaTime wrote:
... and it has been shown that the dark-accustomed and healthy human eye
can
actually detect single photons, with sensitivity peaking in the green
wavelengths. Remarkable.

References, please...

I think it's pretty standard stuff on most intro Nuclear Physics books. For
example, the spinthariscope, invented by Sir William Crookes in 1903, had the
capability of generating individual photons, which were detectable by the eye.

http://en.wikipedia.org/wiki/Spinthariscope

The alpha particles from Ra that fall on the zinc sulfide screen, generate a
single photon and the eye is able to pereceive the tiny flash through the
little eyepiece.
--
Ioannis
-------
Every man has his price. Mine is $3.95.

"Paul Schlyter" wrote in message
...
It's more like 1/2000 of that of green light.

Wien's displacement law says that the peak wavelength of the Planckian
blackbody radiation is inversely proportional to the blackbody
temperature.

The Sun has its peak near green light, and the surface temperature
of the Sun is some 6000 K

6000 / 2.73 = approx. 2200

The peak wavelength of the 2.73 K background radiation is some 1.2 mm

Yes, inverted wavelength arithmetic on my part. I was mentally dividing 1.2
by 5600 instead of 560nm by 1200000nm. Still pretty feeble by anyone's
reckoning (bit like me really) )

In message , TeaTime
writes

"Sam Wormley" wrote in message
news:IgU3h.227152$FQ1.59356@attbi_s71...
Mij Adyaw wrote:

Is that really true? I am not sure that any light from the pointer will
make it out into space. I just isn't powerful enough.



Unless the light gets absorbed it just keeps going no matter how weak!


Quite so. I should have thought the fact we can use telescopes to see light
that left distant stars thousands of millions of years ago would speak for
the longevity of light photons.

Isn't their longevity zero? :-) From the viewpoint of a photon time does
not pass.

The oldest photons I've consciously seen (in my frame) are 2 thousand
million years old (from 3C273). Not bad for a small reflector in
suburban London, though I suspect people her can do better.

"Jonathan Silverlight" wrote
in message ...
In message Isn't their
longevity zero? :-) From the viewpoint of a photon time does not pass.
Isn't their longevity zero? :-) From the viewpoint of a photon time does
not pass.

That's a philosophical poimt, Jonathan! Isn't longevity, like beauty, in
the eye of the beholder?

lal_truckee wrote in
:

TeaTime wrote:
... and it has been shown that the dark-accustomed and healthy human
eye can actually detect single photons, with sensitivity peaking in the
green wavelengths. Remarkable.

References, please...

That is pretty standard stuff. Fully dark adapted rod cells can respond to
a single photon. On the average though, about seven to ten photons have to
hit before the rod cell fires. However the visual processing system doesn't
normally register a single rod cell firing. Several adjacent rods usually
have to fire before the visual system responds. For this reason night
vision is of lower resolution than day vision. The overall quantum
efficiency of the dark adapted human visual system is probably around one
percent.

Kalzmon



http://en.wikipedia.org/wiki/Rod_cells

Fleetie skrev:
Is that really true? I am not sure that any light from the pointer will make it out into space. I just isn't powerful enough.
....
Some photons will be absorbed, I guess, and others will be Rayleigh-scattered
out of the intended direction of the beam as they propagate through the
atmosphere.

I would imagine the scattering of the photons in the atmosthere
is the whole clue behind the GLP, isn't it? Somehow, the photons
(or at least the energy, depending on the scattering mechanisms)
that leave the laser needs to be dispersed back to reach the
eyes of the observers.

Rune

On 11 Nov 2006 05:44:50 -0800, "Rune Allnor"
wrote:

I would imagine the scattering of the photons in the atmosthere
is the whole clue behind the GLP, isn't it? Somehow, the photons
(or at least the energy, depending on the scattering mechanisms)
that leave the laser needs to be dispersed back to reach the
eyes of the observers.

Of course, what you see is scattered light from the laser. But that
doesn't change the fact that only a small fraction of the light is
scattered when a GLP is pointed straight up. Most of the beam's energy
will escape the atmosphere, and could be detected very easily in space
from a low Earth orbit- probably even visually.

_________________________________________________

Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com

In message , Chris L
Peterson writes
On 11 Nov 2006 05:44:50 -0800, "Rune Allnor"
wrote:

Of course, what you see is scattered light from the laser. But that
doesn't change the fact that only a small fraction of the light is
scattered when a GLP is pointed straight up. Most of the beam's energy
will escape the atmosphere, and could be detected very easily in space
from a low Earth orbit- probably even visually.

A 2 watt argon laser was imaged by Surveyor 7 on the Moon in 1968
http://www.w7ftt.net/laser2.html
That's a fairly impressive bit of kit, but I wonder what a dark-adapted
human eye could see, especially with modest optical aid.