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The_Radiation_Specialist
2005-Dec-30, 08:24 PM
I was just wondering, given that our eyes can only, obviously, see the visible visible part of the spectrum. Is this narrow part of the spectrum "special" in any other way? Or is it because throughout evolution of life our eyes got very tiny exposure to x-ray or gamma rays due to earths atmosphere blocking and this made our eyes only able to see the light we can see? If this is so then why cant we see radio waves or microwaves?
Sorry if my question shows a lack of understanding and knowledge :)

Tim Thompson
2005-Dec-30, 08:36 PM
Perhaps not accidently, the visible ("eyeball") wavelengths are where the sun emits most of its radiative energy. Our eyes are way too small to see microwaves, or anything longer (any wavelength larger than about the diameter of the pupil of the eye will go unseen). Higher energies, like X- or gamma-rays will probably destroy the retina before being seen by it.

The_Radiation_Specialist
2005-Dec-30, 10:05 PM
Our atmosphere also lets in some radio radio waves from outer space. Why is it that throughout evolution we havent been able to detect the radio waves. Does it have any affect on us?

ToSeek
2005-Dec-30, 10:42 PM
Our atmosphere also lets in some radio radio waves from outer space. Why is it that throughout evolution we havent been able to detect the radio waves. Does it have any affect on us?

In order to pick up any sort of electromagnetic radiation, you need a receiver at least the size of the waves. "Eyes" that could pick up radio waves would be very large.

archman
2005-Dec-30, 11:08 PM
In order to pick up any sort of electromagnetic radiation, you need a receiver at least the size of the waves. "Eyes" that could pick up radio waves would be very large.
Finally, a rationale for Godzilla!

Sam5
2005-Dec-30, 11:43 PM
Our atmosphere also lets in some radio radio waves from outer space. Why is it that throughout evolution we havent been able to detect the radio waves. Does it have any affect on us?

Our rods and cones are about the length or size of visible light wavelengths. They, in effect, act somewhat like “tuned antennae” for those wavelengths. Radio wavelengths are too long for our eyes.

Fortunate
2005-Dec-31, 12:35 AM
On a marginally revlevant note - some insects and dayactive birds see some frequencies that we don't - mainly in the ultraviolet, I believe.

Sam5
2005-Dec-31, 12:54 AM
There is something unusual about water and light. Water transmits visible light (EM waves) very well but it blocks a lot of other wave frequencies.

Here is an attenuation plot:

http://www.karger.com/gazette/64/fernald/fig_1.html

It represents how much water blocks which frequencies. Note that water doesn’t block or attenuate visible light very much at all, but it does block other frequencies quite a lot.

If light didn’t pass easily through water, all water would be black and all rain and fog would be black.

Nereid
2005-Dec-31, 01:21 AM
There's an interesting capability that eyes have - the ability to form images. If you relax that, and ask only for detection, then some interesting possibilities open up. For example, 'prey detection' by the heat such warm-blooded critters emit (some snakes have this capability).

As for UV and NIR, some other animals can 'see' in these wavelengths; it's perhaps not so interesting (in terms of the OP) as it is a small extension of what we Homo saps can do.

grant hutchison
2005-Dec-31, 01:30 AM
If light didn’t pass easily through water, all water would be black and all rain and fog would be black.And we wouldn't be able to see, since our eyes are mainly water. The transmission properties of water are another reason we use the wavelengths we do, and not others.

Grant Hutchison

gvaldeg
2005-Dec-31, 01:51 AM
I believe most of these posts are on the right track but...I think that the average rod and cone spacing is about 1/10th wave while their diameter is much smaller than that. If this weren't the case, we wouldn't have much resolution in the visable spectrum.

grant hutchison
2005-Dec-31, 02:49 AM
I believe most of these posts are on the right track but...I think that the average rod and cone spacing is about 1/10th wave while their diameter is much smaller than that.That seems very tight. Maximum density is less than 200,000/mm² (see, for instance, the graph here (http://hyperphysics.phy-astr.gsu.edu/hbase/vision/rodcone.html)). That gives each receptor at least 5µm² to itself. So they're spaced a minimum of a couple of microns apart, which would fit with the estimated cone diameter in the fovea of 1.5 microns (see, for instance, here (http://webvision.med.utah.edu/photo1.html)). Wavelengths of visible light are 0.4-0.7 microns, so the scale of retinal receptors is actually rather larger than the wavelength of visible light - which is necessary if the receptors are to interact with the light.

Grant Hutchison

Ricimer
2005-Dec-31, 09:57 AM
also throw in a few other tidbits:

organic compounds don't react well to high energy photons (they tend to break apart) thus explaining why x-rays and even hard UV isn't detected.

Go much below red, into the IR, and our own body heat would begin to blind us, so it would take some interesting measures for a warm-blooded animal to see very far into the IR (snakes use their tongues, which are extended out and cooled for increased sensitivity to IR).

Ken G
2005-Dec-31, 10:05 AM
There's also the evolutionary perspective. What possible advantage could we obtain from seeing radio waves from space? Radio sources on Earth are too weak to be of survival advantage. If there was an advantage, we'd probably all be sporting antennae like our cars.

01101001
2005-Dec-31, 10:09 AM
(snakes use their tongues, which are extended out and cooled for increased sensitivity to IR).
Pit vipers use their... pits (http://www.exergen.com/industrl/irtc/technotes/technote_019.htm).

mickal555
2005-Dec-31, 05:45 PM
What colour are our eyes most sensitive too?
Is it green?

Fortunate
2005-Dec-31, 06:12 PM
What colour are our eyes most sensitive too?
Is it green?

Does the color of the iris factor into how we see things?

mickal555
2005-Dec-31, 06:28 PM
Interesting Question...
I don't think so though, as the light goes through the pupil, which is where the iris isn't...

I was just woundering about green because I heard that our eyes are most sensitive to green light- and then I thought well the sun peaks in green light....

mickal555
2005-Dec-31, 06:30 PM
What interests me is why some colours appear to be bright, take yellow for example it is harder to read on a white background then a red with the same satuaration, hue and luminosity.....

:K

papageno
2005-Dec-31, 08:46 PM
There's also the evolutionary perspective. What possible advantage could we obtain from seeing radio waves from space? Radio sources on Earth are too weak to be of survival advantage. If there was an advantage, we'd probably all be sporting antennae like our cars. Not to mention that the resolution with radiowaves would not be very useful.
With a wavelength of meters, you could hardly sense the presence of other animals, let alone identify them.
With even longer wavelengths you might sense the presence of a mountain, but not much else.

TrAI
2006-Jan-01, 01:18 AM
What colour are our eyes most sensitive too?
Is it green?

Hmmm... Well, it depends on how you look at it. The blue sensitive cones are the most sensitive, though there is few of them compaired to the other types of cones, so the overal sensitivity maxes in the green part of the spectrum(around 550nm) in light adapted conditions. In the dark however, the rods will be what counts, and they are more sensitive in the blue part of the spectrum, so dark adapted sensitivity is in the lower blue spectrum(around 500nm).

As for why yellow looks so bright, well, my guess is that it looks bright due to the closeness of the peaks and overlapping of the green and yellow sensitive cones within this area. red will look weaker due to it being in a lower sensitivity area of the yellow sensitive cones. A very bright red light may seem yellowish however.

UV is absorbed by the cornea, so though the retina would have some sensitivity to the lowest parts of the UV spectrum, it will not be noticed in normal circumstances. Sensitivity may extend a little into the near IR too. But of course, sensitivity and the with of the visible spectrum varies from person to person.

Kaptain K
2006-Jan-01, 02:21 AM
But of course, sensitivity and the width of the visible spectrum varies from person to person.
Not just the sensitivity and width, but also the range. When I was in college, my room mate and I were team mates in physics lab. In the spectroscopy lab, we noticed that I was seeing lines much farther into the "red" end of the spectrum than he was. Conversely, he could see lines in the "violet" end of the spectrum that were completely invisible to me.

He could also see galaxies through the 4 inch finder that I could barely see through the 12 inch main scope!!!

ZaphodBeeblebrox
2006-Jan-01, 03:40 AM
Hmmm... Well, it depends on how you look at it. The blue sensitive cones are the most sensitive, though there is few of them compaired to the other types of cones, so the overal sensitivity maxes in the green part of the spectrum(around 550nm) in light adapted conditions. In the dark however, the rods will be what counts, and they are more sensitive in the blue part of the spectrum, so dark adapted sensitivity is in the lower blue spectrum(around 500nm).

As for why yellow looks so bright, well, my guess is that it looks bright due to the closeness of the peaks and overlapping of the green and yellow sensitive cones within this area. red will look weaker due to it being in a lower sensitivity area of the yellow sensitive cones. A very bright red light may seem yellowish however.

UV is absorbed by the cornea, so though the retina would have some sensitivity to the lowest parts of the UV spectrum, it will not be noticed in normal circumstances. Sensitivity may extend a little into the near IR too. But of course, sensitivity and the with of the visible spectrum varies from person to person.
Not Only that, But I Swear, Someone, On This Board ...

Mentioned How, to People, Who've Had their Corneas Removed, Low UV, Can Be Seen, As a Shade of Brown ...

Anyone Else, Remember this?

:think:

grant hutchison
2006-Jan-01, 03:57 PM
Mentioned How, to People, Who've Had their Corneas Removed, Low UV, Can Be Seen, As a Shade of Brown ...People who have their lenses removed are able to see UV, but it would be odd if it appeared brown: UV is detected by the blue receptors, and brown is a mixture of green and red. So reasoning from first principles, the sensation of UV should be a rather pure shade of violet. But I've never actually read a description written by someone who can see UV, though.

Grant Hutchison

mickal555
2006-Jan-01, 04:34 PM
We need more colours, It would be so strange to see a colour we'd never seen before...

Imagine if you had never seen the colour green for example and then suddenly you were shown a green card...

The_Radiation_Specialist
2006-Jan-01, 05:22 PM
We need more colours, It would be so strange to see a colour we'd never seen before...
That would be freaky!

Lance
2006-Jan-01, 05:31 PM
We need more colours, It would be so strange to see a colour we'd never seen before...

Imagine if you had never seen the colour green for example and then suddenly you were shown a green card...
I once knew someone who could not see yellow until after cataract surgery. She only knew it as muddy-brown.

TrAI
2006-Jan-01, 05:55 PM
Not Only that, But I Swear, Someone, On This Board ...

Mentioned How, to People, Who've Had their Corneas Removed, Low UV, Can Be Seen, As a Shade of Brown ...

Anyone Else, Remember this?

:think:

Might have been me, though I, sadly, did not think to save the link to the source of information.

It might have been wrong though, This guy (http://starklab.slu.edu/humanUV.htm) describes it as a desaturated blue... Hard to say, Blue does logicaly seem to be more likely, though the chemicals involved in color vision may very well have more than one window of sensitivity, its not uncommon for materials to have that, so all cones may be sensitive to more than one band. IIRC the study where they found it to be brownish they did the experiments on people during surgery, and so it may have been affected by other things, like drugs or the equipment used to introduce UV. Since the person from the site linked above seem to have personal experience on a day to day basis, he may be a more accurate source. However, since color perception is individual, its hard to say how any one person would percive something.

It may be that humans do not see UV for a very good reason, as it is likely to cause more injury to the retina than lower energy ranges, sooner or later it would cause irreprable damage. Of course the cornea does get damaged by UV, but perhaps it is more able to sustain this than the retina would. If this thought is correct, it would be likely that UV vision would develop more in short lived species, where long term damage is of less consern.

TrAI
2006-Jan-01, 06:11 PM
We need more colours, It would be so strange to see a colour we'd never seen before...

Imagine if you had never seen the colour green for example and then suddenly you were shown a green card...

Hmmm... My opinion is that color is just a way the visual system represents the data from the eyes to the consiousness, so even with a wider visible spectrum, the brain would not have more actual colors, just a wider band of frequencies to map them to. So, from this view, it would not be possible for the brain to generate any "new" color.

umop ap!sdn
2006-Jan-02, 02:37 AM
It just so happens that it's fairly easy for carbon based molecues that absorb in the visible and the surrounding regions of the spectrum to form naturally. :D

The reason why yellow looks so light to us humans is because only red and green cones contribute to the sensation of lightness in the human eye. Yellow stimulates them both almost as much as white does, therefore it looks very light. Interestingly, while human green cones are most sensitive to about 535nm (a wavelength that is very definitely perceived as green), red cones are most sensitive around 560nm, which also looks greenish. 550nm is right around the maximum of the combined sensitivities of these photoreceptors.

Opsins, which are the light sensitive compounds in the eye's photoreceptors, do indeed absorb in more than one wavelength range; in addition to the primary "alpha" band, they have a weak "beta" band at shorter wavelengths. If I recall correctly, the beta band for our longest wavelength opsin peaks somewhere around 380nm, which is just about the same as the cut off wavelength of the lens. A few mammals such as white tailed deer (longest wavelength opsin somewhere around 535nm) and mice (505nm) have lenses that transmit more of the UV range, presumably because the shorter wavelength opsins can withstand shorter ultraviolet wavelengths than the human red cone opsin can.

gvaldeg
2006-Jan-02, 05:09 PM
That seems very tight. Maximum density is less than 200,000/mm² (see, for instance, the graph here (http://hyperphysics.phy-astr.gsu.edu/hbase/vision/rodcone.html)). That gives each receptor at least 5µm² to itself. So they're spaced a minimum of a couple of microns apart, which would fit with the estimated cone diameter in the fovea of 1.5 microns (see, for instance, here (http://webvision.med.utah.edu/photo1.html)). Wavelengths of visible light are 0.4-0.7 microns, so the scale of retinal receptors is actually rather larger than the wavelength of visible light - which is necessary if the receptors are to interact with the light.

Grant Hutchison
I found these dimensions on a Stanford site:

Number of cones in each retina: 5 x 10^6

Number of rods in each retina: 10^8

Diameter of the fovea: 1.5 mm (5.2 deg); rod-free fovea: 0.5 mm (1.7 deg); foveola (rod-free, capillary-free fovea): 0.3 mm (1 deg); size of the optic nerve head: 1.5 mm x 2.1 mm (5 deg (w) x 7 deg (h)) location of the optic nerve head: 15 deg nasal

Peak cone density: 1.6 x 10^5 cones/mm^2;

Foveal cone size: 1-4 mu (diameter) x 50-80 mu (length); extrafoveal cone size: 4-10 mu (diameter) x 40 mu (length)

Size of rods near fovea: 1 mu (diameter) x 60 mu (length)

S cone spacing (foveal): 10 arc min

L and M cone spacing (foveal): 0.5 arc min

These are dramatically smaller than a micron and about 1/10 wave for green light.

gvaldeg
2006-Jan-02, 05:52 PM
If Stanford is using "mu" for micron instead of millimicron, that may explain at least part of the confusion. A lot of times people use lower case "u" instead of the greek letter "mu" which is the common abbreviation for micron in which case "mu" is millimicron. In any event, I was looking at mu as millimicron which is the same as nanometer.

grant hutchison
2006-Jan-02, 05:54 PM
Foveal cone size: 1-4 mu (diameter) x 50-80 mu (length); extrafoveal cone size: 4-10 mu (diameter) x 40 mu (length)

Size of rods near fovea: 1 mu (diameter) x 60 mu (length)
...

These are dramatically smaller than a micron and about 1/10 wave for green light.One "mu" = a micron: so all these cells are a micron or more in diameter, while green light is around half a micron in wavelength. I'm afraid I don't see where you're getting "dramatically smaller" or "1/10 wave" from these figures.

Grant Hutchison

umop ap!sdn
2006-Jan-02, 06:07 PM
Given that microspectrophotometry has been performed on individual foveal cells (it's how we know what kinds of photoreceptors humans and other species posess) and that the individual photoreceptors themselves have been directly imaged, their size must be greater than a wavelength of light anyway. :D

grant hutchison
2006-Jan-02, 06:46 PM
If Stanford is using "mu" for micron instead of millimicron, that may explain at least part of the confusion. A lot of times people use lower case "u" instead of the greek letter "mu" which is the common abbreviation for micron in which case "mu" is millimicron. In any event, I was looking at mu as millimicron which is the same as nanometer.Our posts obviously crossed - I hadn't seen this when I made my previous post. I'm sure Stanford is using "mu" = "micron". One check on this is to see that their peak cone density figure is the same order of magnitude as the figure I used to calculate that the mean receptor spacing was of the order of a micron. Another is to do an order-of-magnitude sum using the size of the eye (~1cm) and the receptor spacing (~1µm), to see that the angular resolution of such an eye would be of order arctan(1µm/1cm) = ~1/3 arcmin, consistent with the figures quoted by Stanford.
Finally, there is the consideration that receptors smaller than a tenth of the wavelength of green light would be Rayleigh scatterers, rather than absorbers: they wouldn't detect the light, and so we couldn't use them to see.

Grant Hutchison