Can the wavelength of an individual photon be measured (with a
major uncertainty) from the angle of its reflection or refraction
by an optical diffraction grating? The large uncertainty comes
primarily from the fact that photons emerging from the grating
have a statistical distribution that peaks at a specific angle,
but each individual photon can emerge at any angle.

Is that correct? If it is correct, does it have any utility?

-- Jeff, in Minneapolis

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"I find astronomy very interesting, but I wouldn't if I thought we
were just going to sit here and look." -- "Van Rijn"

"The other planets? Well, they just happen to be there, but the
point of rockets is to explore them!" -- Kai Yeves

How are you detecting the photons? If you somehow measure the energy of the photon, wouldn't that tell you the wavelength?

Can't a human detect a single photon hitting the retina, and recognize it's color?

No.

If you look for the evidence of wave you can find it.
If you look for the evidence of particle you can find it.

No. One photon would not trigger your color recognition.

'you might just get ' What was that ?'...

Single photon detection requires fully adapted night vision, and that implies rod cells only detection, and rods are black and white detectors. The cone cells do the color detection, and they only work well in daylight. That's why we see in black and white only in dim light.

Interesting that the eye contains a photon detection system and a photon wavelength detection system. You could get a lot of information out of that, enough to avoid getting eaten by unfriendly critters both day and night.

For all we know, a single visible-light photon coming out of otherwise total darkness may not be enough to stimulate even a rod cell. A barely visible star puts some thousands of photons per second into our pupils. I don't know how many rods are involved in a sharply focused image.

If a single cone is stimulated, it tells us only that the light that stimulated it is somewhere in the visible range. All three types respond over most of the visible range, with a large overlap even between the blue and red ones. If I am not mistaken, our sensation of color is the result of the brain's comparison of the relative amounts of output from at least two of the cone types.

Experiment has shown that dark adapted, well seeing eyes can detect a single photon.

Researchers have been able to focus a laser on a single cone: The subject "sees" the color the cone is most sensitive to, regardless of the actual frequency of the laser. I'm sure this applies to rods as well.

__________________
Any day you wake up on "the right side of the dirt" is a good day.

T. Anderson

Hornblower, that is generally correct.

IIUC, the way in which rhodopsin (the visual pigment in rod cells) works, it takes the absorption of one photon in the right part of the cell to trigger a nerve impulse. There are two effects here that make it very difficult (or perhaps impossible) to actually perceive a single photon.

One is that not all photons entering a rod cell get absorbed by retinal (which is the vitamin-A derivative that binds to rhodopsin and allows the absorption of a photon to trigger a conformational change and hence trigger the signalling cascade that leads to the nerve impulse).

The second is that one's visual perception contains a certain background noise. Even on the darkest night, or underground, you never perceive unremitting blackness. You see a bit of noise in it. This noise makes it hard to see very very faint light sources. And it makes the absorption of a single photon pass unnoticed.

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The quarrelsome oarsmen were rowing,
The great violinist was bowing;
But how is the sage
To tell, from the page:
Was it pigs or seeds that were sowing?

I would be interested to read a bit more about that. Any chance you have a link?

__________________
The quarrelsome oarsmen were rowing,
The great violinist was bowing;
But how is the sage
To tell, from the page:
Was it pigs or seeds that were sowing?

Off topic but there has recently been some studies of people with inactive cones seeing a simplified colour set with rods! The reason rods have been asociated with black & white is because they turn on at much lower intensities than cones; but they don't become colour active until intensities that would turn on cones anyway. Once the cones turn on they drown out the colour info from the rods. Therefore it has now been assumed that dogs see in colour too!

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"New study reveals that 2030 will occur in 21 years!!"

I have heard that, too. (My physics teacher in high school). There is an anectode Geiger would have used coffe with a little arsenic to make the eyes more sensitive.

I have no idea if this is true. And I also would be interested in a link to someone who made real experiments on the eyes sensitivity.

__________________
Andre

"They did not know it was impossible, so they did it!"
Mark Twain

Are you saying red photons will excite "blue" color cones? The sensitivity of our color cones do overlap in the midrange colors (eg green), so each cone will be excited by these wavelengths, to varying degrees. I would think, though, that this would not be the case for our blue color cone with the typical red laser. Is there a quantum issue that comes into play that would cause a red photon to register with a blue cone? If so, under these very controlled circumstances, the red light would appear blue.

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Lighten up! This is a stellar board! Author: duh.

"The Sun, with all the planets revolving around it, and depending on it, can still ripen a bunch of grapes as though it had nothing else in the universe to do..." Author: Galileo supposedly.

This article states dogs do have cones, but only about 1/10th the number as ours.

There is a lot of processing our brains do regarding color. For instance, the actual sensitivity of our "blue" cone is very poor compared to our other two. However, thanks to our processing ability, blue is boosted to a comparable level with the others.

Rods, I think, can also contribute somewhat to how we see color. I believe this comes into play within the midrange between weak and strong light intensities. This is the mesopic range, between the scotopic and photopic ranges. I am unsure of this, admittedly.

__________________
Lighten up! This is a stellar board! Author: duh.

"The Sun, with all the planets revolving around it, and depending on it, can still ripen a bunch of grapes as though it had nothing else in the universe to do..." Author: Galileo supposedly.

They sure will. I'm sure you have a graph of human color sensitivity
at hand. Preferably one that doesn't normalize the peaks. You can
see that even the red and blue have a great deal of overlap. Only
the relative rate of stimulations in the different cones enables the
signal procesing mechanisms in the eyes to distinguish different
colors, when a sufficiently large number of stimulations occur.

-- Jeff, in Minneapolis

__________________
http://www.FreeMars.org/jeff/

"I find astronomy very interesting, but I wouldn't if I thought we
were just going to sit here and look." -- "Van Rijn"

"The other planets? Well, they just happen to be there, but the
point of rockets is to explore them!" -- Kai Yeves

It's not about two types of cones having overlap. The question is whether "blue" cones have any response to light at, let's say, 650 nanometers wavelength. I don't have a non-normalized graph available, but many normalized graphs I've seen don't show S (blue) cones having any relative sensitivity for light at red wavelengths; the S cone sensitivity seems to fall off at 550 nanometers. But then again, those graphs are pretty inexact and normalized so I can't say either way. Hope someone has better info on this.

You are right. I was replying to my interpretation of what George
said, rather than to what George said. While there is a great deal
of overlap in spectral sensitivity of the blue and red receptors, that
overlap does not extend, for blue receptors, all the way to red light
wavelengths.

However, if the laser Kaptain K introduced to the thread is somehow
able to stimulate a blue receptor using red light, then the person will
see blue, as long as other receptors nearby are not stimulated.

-- Jeff in Minneapolis

__________________
http://www.FreeMars.org/jeff/

"I find astronomy very interesting, but I wouldn't if I thought we
were just going to sit here and look." -- "Van Rijn"

"The other planets? Well, they just happen to be there, but the
point of rockets is to explore them!" -- Kai Yeves

Blue-responsive cones' responsiveness does drop to effectively nothing at frequencies too high to be called "red". The limit is more of a yellowish green. Red-responsive cones do, however, have a response well into "blue frequencies". So a blue laser could stimulate red-receptive cones even though a red laser couldn't stimulate blue-receptive ones.

BTW, although it's not really an answer to a specific question in this thread: reptiles & birds have four kinds of cones, more evenly spaced than ours (as opposed to the way our green and red are crowded together), and no rods. It's one of the reasons why evolutionary biologists think the common ancestor of mammals had spent a long time evolving heavy specialization for a nocturnal lifestyle.

Hi folks,
Very interesting topic. Reading through this thread and the one referenced in the opening post got me wondering:

*- Do we know of any indiscrete wave entities?
*- Aren't all known waves (none electromagnetic, sound for example) made of matter which is in turn made of particles?
*- If so what is the basis for distinguishing the wave/particle feature for light?
*- A 150 kHz sound wave could be produced in a gas whose atoms/molecules vibrate at one or more different frequencies. Could a similar analogy be applied to light?

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"They reasoned that an object situated at the center and related equally to the extremes in every direction can have no impulse to move in any specific direction. In fact, they compared the situation of such an object with that of a man violently but equally hungry and thirsty, standing at the same distance from food and drink and unable to decide in which direction to move." - Aristotle

Since I sorta started this, I guess I should (attempt to) clarify some what. I did not mean that a laser at any frequency the eye is responsive to could be seen as red or blue or green, depending on the single cone it was focussed on. The unspoken given was that it was in the range where the sensitivities overlap.

__________________
Any day you wake up on "the right side of the dirt" is a good day.

T. Anderson

A couple thoughts. On second thought, let me grab my biology book. Brb.

I'm not sure on the overlap of colors that cones can see, but I am somewhat familiar with the general stuff, so don't expect me to be specific. I am working off my book (conveniently pictured in the BA's blog!) so if I get it wrong it's my fault.

All photoreceptors require an two protiens to work. First is a protien that bonds to the light-absorbing pigment retinal. Retinal is a protien that is part of the light sensitive cell. The other protien is something called an opsin which bonds to the retinal, affecting its light gathering abilities.

Rods work a little differently than color cones (to follow), in that they create something called rhodopsin. Rhodopsin, from what I understand (and I may be wrong, the book is somewhat technical here) takes on a funny property when it absorbs light. With too much light the retinal part of the rhodopsin actually seperates from the opsin, which in turn changes the shape of the opsin protien rendering it relatively ineffective. In a dark-ish environment the chemical reactions that bleach (deactivate) the opsin in light cease and the rhodopsin can form. I am not sure what the threshold is, but I would imagine it varies by species, and likely even person to person.

It takes time, 3-10 minutes for basic, functional night vision, up to 3 hours for the full amount. As far as I know, rods are not sensitive to color, but are highly sensitive to contrast. The book mentions that as well, though it was published in...I want to say 2003. Do you have a newer reference George? I'm not going to buy the next edition of a $400 book just to check if rods can see color. I would be interested if someone had a good study on rods and color.

Color vision requires cone cells (they look like cones, you can stop guessing!) and cone cells use photopsin. There are three types--green, red, and blue as mentioned in other posts. Each cone's retinal is sensitive to one of those colors, and each binds to a unique opsin to form photopsin.

And now to quote from the book Taken from "Biology", Campbell and Reece, 2006. Pages 1066 and 1067. And don't slight my reference, this isn't a paper and I don't remember the specifics. Each department had their own "style" for references...oh, don't even get me started. Forget I said it.

The color part is fascinating to me...sounds an awful lot like how a computer creates colors. I'm sure it's not a pure coincedence, though I don't know the extent of any intentional design in that way. Seems like I should but it's early in the morning and I'm done thinking after this--it's the weekend!

There's a ton more, the book has several pages on the eye and vision, even outlining the chemical reactions one step at a time. It's quite fascinating, but a heck'va lot more than I care to type here. And without diagrams most of you wouldn't care to read it all anyway. Even if you did read, it would be hard to follow, so ~p->q and I'm done.

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None to speak of

tetrachromatic women

Not fair! this really makes me see......... yellow

__________________
"New study reveals that 2030 will occur in 21 years!!"

Thaks for the clarification.

In one sense, I suppose, rods can be said to be sensitive to color; but rods alone do not produce enough information to allow a specific color to be rendered. Rods are most sensitive to green with a peak of ~510 nm, and they work within a color range between over 400nm to under 600nm. But, in the scotopic range, the brain has no way to know the spectral energy distribution of the light source as it is only getting one signal, so to speak.

However, in the mesopic range (between the rod-only scotopic and the cone-only photopic ranges), both rods and cones are working together. Since the cones have a net sensitivity peak of 555nm, then there is a bit of a blueshift when light intensities reduce to the level within the mesopic range. This color shift is known as the Purkinje effect. Objects will look more green or blue than they normally would. Or, said another way, objects will look less yellow, orange, or red.

The way I see it, in the mesopic range, it could be argued that we have 4 color receptors since all three cones and rods are still active signal generators for our color processing. [Dr. Lamb coined the term "retinex" to address the combined effect of our eyes with our brain's color rendering process.]

This is not an area I know much about, admittedly, which is why I was curious about Kaptain K's laser circumstance.

__________________
Lighten up! This is a stellar board! Author: duh.

"The Sun, with all the planets revolving around it, and depending on it, can still ripen a bunch of grapes as though it had nothing else in the universe to do..." Author: Galileo supposedly.

These explanations are all very good. You may find a more in-depth and complete explanation on the Wiki entry for retina, with particular emphasis on rods and cones.

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After careful consideration of NASA's Constellation Program, I believe Directv3 is the only viable means of achieving the objectives of NASA's Vision for Space Exploration.

"...only nuclear power can now halt global warming." - James Lovelock, independent scientist, author, researcher, environmentalist, and an open member of Environmentalists for Nuclear Energy

I was digging in my little filing cabinet and recalled your request. You might enjoy this article.

"In a 1996 paper entitled, "The Reengineering of Lighting Photometry," Dr. Sam Berman sets forth a new theory on the workings of the human eye where the function of the rods and cones are not mutually exclusive as previously believed."

[Sorry 'bout the delay.]

__________________
Lighten up! This is a stellar board! Author: duh.

"The Sun, with all the planets revolving around it, and depending on it, can still ripen a bunch of grapes as though it had nothing else in the universe to do..." Author: Galileo supposedly.