Large Hardon Collider

[Khamsin]

Without us, all your hard work would be for naught! We'd know everything about the universe, but no one would know how to turn it into anything useful!

[Eliseos]

Exactly, you'd have no one to design your space elevator and perpetual motion machine.

[Kuya]

I have a physics and visual perception question. As i understand it, light is electromagnetic waves, although colour is not clear to me. I heard it explained as that different wavelengths represent different colours, for example, shorter wave lengths corresponds to blue and longer wavelengths corresponds to red. What is not clear to me however is, how the process of colour works: do objects not have colour? As it was explained to me is made it seem that objects do not have colour but rather they vary in the kinds of wavelengths they reflect. They absorb some and reflect others and those reflected waves constitute the colour. Is this correct?

Also, what is it that makes certain matter reflect some electromagnetic waves but absorb others?

Next, a question related to biology, is colour inherent in those wavelengths or is it the visual sensory organs of organisms that give these wavelengths their colour? I ask this because it appears that a specific wavelength does not necessarily mean that an organism will see the colour that is often associated with that wavelength, and not because the organism's sensory cells are defective. It seems that ilumination, past experience, contrast, and interaction between different electromagnetic wavelengths form the end result which is colour, but, and this part might be hard to explain: does this mean that colour only exists due to the interaction between the factors i mentioned or does it exist independently and the factors i mentioned only modify it?

[Chun Liroy]

That has to be the most interesting color question I've heard. I wish I knew the answer, but as expect the answer to the first question affects (effects? I can never get those 2 right) the answer to the second one.

[Eliseos]

Basically, our eyes have developed sensitivity to the "color spectrum" (and some infrared/UV) which is just one small part of the spectrum of electromagnetic waves. I'll try to explain the quantum mechanical effects (nothing too in depth or equation involving). A short answer is no, objects do not inherently have color, they just reflect certain wavelengths of light better. I probably won't explain this very well but here goes. Also, I will be using light and electromagnetic radiation interchangeably, so when I say one I mean the other and vice versa.

So let's say light hits some object, we'll say a cloud of hydrogen in space. The incident light hits a hydrogen atom in this cloud (we'll just consider only one to make things easier), and this light imparts energy onto the atom. The atom gets excited for a brief (ie on the order of nano-seconds) time, and re-emits this light. Since energy conservation takes place, there will be some energy loss in the wavelengths that get re-emitted (since the light from a star emits electro-magnetic radiation over a vast range of frequencies). The spectrum will be missing most of the spectral lines due to energy conservation (if energy conservation didn't occur, you could potentially get an unlimited amount of energy from this process). The colors that reach your eyes are what color you see.

A good example are neon signs. When energy (electricity) travels through the neon gas, the molecules become excited and constantly absorb and emit light. Here's a picture from the wiki of neon emission spectra (if you've been wondering what spectral lines are, this may answer that question).

http://upload.wikimedia.org/wikipedi...n_emission.png

The white lines on the far right and left are UV/infrared wavelengths. As you can see, most of the color emitted is red/orange in wavelength, and this is why neon signs have this color. The same phenomena occurs in sodium street lamps, giving them that familiar yellow glow and being the bane of astronomers like Miz trying to see a clear night sky.

The following is me applying what happens at the atom level to macroscopic items, so I may not be correct in what I say here (or what I said above, if I said something incorrect someone please point it out). When light is incident on say a book, the dye used to print it and the type of material the page is made out of is what gets re-emitted back to your eyes. Different paints use different dyes, which (I assume) are slightly different materials, so the only colors that are emitted from this book are the colors that reach your eyes. So it is kind of the opposite of what one would normally think, the book only emits the colors it didn't absorb.

I don't know if this answered your question, I think what you wrote is basically correct.

As for the biology question, I would assume it's an evolutionary trait. The receptors in our eyes have adapted to seeing the color wavelength, just like bees eyes have adapted to the ultraviolet spectrum. It would be advantageous for a species to be able to differentiate color. Think of a hunter in the woods, if you can spot a camoflaged animal because you can see its colors, as opposed to just black and white, you have a much higher chance of survival. I am going to stop speculating here because I know next to nothing about biology.

[Tristam]

Speculating on the biology aspect, as I understand it, when you see a color all that is happening is your brain is saying "here is light of wavelength x". Certain cells in the eye absorb different wavelengths I think, and send the absorbed energy as an electrochemical potential instead of an electromagnetic signal. edit: In retrospect, it is alot more complicated than this. I can't wrap my head around it.

Now, since the ability to perceive a color is dependent on the absorption of energy, does that set some sort of limit on what wavelengths of the spectrum are able to be processed by biology? As I understand it, the probability that a given molecule will absorb, reflect, or transmit light depends on the bending, stretching, etc. motions of the bonds between atoms (and the motions of the atoms themselves?). Beyond this, for me, it gets strange.

Why some cells absorb some wavelengths and not others is a great question, and I'm sure there's a cool evolutionary story that describes the gradual adoption of the eye to perceive a narrow spectrum of wavelengths in such a cool way. Wish I knew it, but I would imagine it would take some scholarship.

[Lornkeit]

This might be totally unnecessary, but I thought it was a cool coincidence that I scanned this picture to make/laminate the image for classrooms just yesterday.

http://img.photobucket.com/albums/v1...001cropped.jpg

[Eliseos]

Speculating on the biology aspect, as I understand it, when you see a color all that is happening is your brain is saying "here is light of wavelength x". Certain cells in the eye absorb different wavelengths I think, and send the absorbed energy as an electrochemical potential instead of an electromagnetic signal.

Now, since the ability to perceive a color is dependent on the absorption of energy, does that set some sort of limit on what wavelengths of the spectrum are able to be processed by biology? As I understand it, the probability that a given molecule will absorb, reflect, or transmit light depends on the bending, stretching, etc. motions of the bonds between atoms (and the motions of the atoms themselves?). Beyond this, for me, it gets strange.

Why some cells absorb some wavelengths and not others is a great question, and I'm sure there's a cool evolutionary story that describes the gradual adoption of the eye to perceive a narrow spectrum of wavelengths in such a cool way. Wish I knew it, but I would imagine it would take some scholarship.

For a physics add-in to this, there is theoretically an upper-limit to the smallest wavelength a being (strictly speaking something that appears on this earth, not alien doodz) would be able to detect and have it's wavelength detectors (eyes) survive. Smaller wavelengths mean the photon has a higher energy, meaning it can ionize the molecules in said wavelength detectors and permanently damage them. Most of our eyes are made up of water, and Hydrogen ionizes at 13.6 eV, which is somewhere around 88.9 nm wavelength, which is in the ultraviolet region. Yeah yeah, water isn't pure hydrogen, it's just an approximation of why we haven't evolved an ability to see wavelengths smaller than that.

And I think what you said about the energy from a photon creating an electric potential in the brain is fairly accurate, from what I remember of my physics discussions on light.

EDIT: I also have this poster http://unihedron.com/projects/spectrum/ hanging up in my room. Besides it being interesting (to me at least) to look at, it was a nice thing to have around to check calculations in some of my classes. One of the more interesting facts on it in the color section says

The visible spectrum covers approximately one octave.

[Cyn]

It's always strange to me that people don't really stop to wonder why black gets so much warmer than white in the sun.

[Chun Liroy]

There's no need to wonder... it's fairly simple to explain.

[Eliseos]

What is more interesting to me is why the sky is blue. I remember my teachers in grade school saying it was blue because of all the water on the earth reflecting back blue light into the sky. Not at all true, in fact, water is blue because of the sky, since water on it's own is obviously clear.

EDIT: I know why it's blue but it is still neat to think about.

[Tristam]

For a physics add-in to this, there is theoretically an upper-limit to the smallest wavelength a being (strictly speaking something that appears on this earth, not alien doodz) would be able to detect and have it's wavelength detectors (eyes) survive. Smaller wavelengths mean the photon has a higher energy, meaning it can ionize the molecules in said wavelength detectors and permanently damage them. Most of our eyes are made up of water, and Hydrogen ionizes at 13.6 eV, which is somewhere around 88.9 nm wavelength, which is in the ultraviolet region. Yeah yeah, water isn't pure hydrogen, it's just an approximation of why we haven't evolved an ability to see wavelengths smaller than that.

And I think what you said about the energy from a photon creating an electric potential in the brain is fairly accurate, from what I remember of my physics discussions on light.

EDIT: I also have this poster http://unihedron.com/projects/spectrum/ hanging up in my room. Besides it being interesting (to me at least) to look at, it was a nice thing to have around to check calculations in some of my classes. One of the more interesting facts on it in the color section says

Cool Cool, thanks, I figured there were both evolutionary and actual physical reasons why we see certain wavelengths and not others. And yeah I think my simple explanation is valid but only to a certain extent. I mean, after it's an electrochemical potential it's no longer a wavelength, and there aren't different rod cells for each wavelength of light that the brain can distinguish right? So there most be some very subtle processing depending on how each type of cell integrates signals, and ow the brain interprets that signal.

But that's biology for ya, it's always more complicated than it seems at first glance.

[Kuya]

No one really answered this question:

does this mean that colour only exists due to the interaction between the factors i mentioned or does it exist independently and the factors i mentioned only modify it?

But what i am gathering is that colour exists independent of the phototopic mapping of organisms. As in, the factors i mentioned modify a colour rather than form the components of the experience we call colour (perceptually speaking).

edit: do you guys agree with this?

[Woozie]

Thanks for the explanations and links, Tristam and SilentRoy.

The only one in astrophysics is Miz. Most of us are picking field where there is a tiny chance of finding a job, instead of none.

So true.

Well I just use that term as a catch all for anything dealing with space because I don't really know the niches of the field all that well XD

Well technically, the term still wouldn't apply. My specific interest would be particle physics if the job market wasn't a problem. The only person (to my knowledge) who's sticking to anything specifically space related is Miz.

You'll never get rid of us.

Without us, all your hard work would be for naught! We'd know everything about the universe, but no one would know how to turn it into anything useful!

The same guy I talked to about biology made a good point about engineers.

I don't care about applications to my research, I learn it just for fun. However, the funding I get from my research are going to be from people who do care about applications. So, technically, we rely on engineers for us to be able to continue our own research.

I still say we get rid them though.

I have a physics and visual perception question. As i understand it, light is electromagnetic waves, although colour is not clear to me. I heard it explained as that different wavelengths represent different colours, for example, shorter wave lengths corresponds to blue and longer wavelengths corresponds to red. What is not clear to me however is, how the process of colour works: do objects not have colour? As it was explained to me is made it seem that objects do not have colour but rather they vary in the kinds of wavelengths they reflect. They absorb some and reflect others and those reflected waves constitute the colour. Is this correct?

Also, what is it that makes certain matter reflect some electromagnetic waves but absorb others?

Next, a question related to biology, is colour inherent in those wavelengths or is it the visual sensory organs of organisms that give these wavelengths their colour? I ask this because it appears that a specific wavelength does not necessarily mean that an organism will see the colour that is often associated with that wavelength, and not because the organism's sensory cells are defective. It seems that ilumination, past experience, contrast, and interaction between different electromagnetic wavelengths form the end result which is colour, but, and this part might be hard to explain: does this mean that colour only exists due to the interaction between the factors i mentioned or does it exist independently and the factors i mentioned only modify it?

I'm on my sleeping pills while typing this and I'm not going to proof read. So there's bound to be lots of mistakes, and some of the stuff I say may not even make sense.

I'm going to use frequency in my explanation instead of wavelengths. When it comes to color, it makes no difference since every specific wavelength corresponds to some specific frequency. So you could say color depends on wavelength, or equivalently that it depends on frequency. Since frequency is easier to work with mathematically, it's what I'm used to using. As you probably already know, lower frequency means higher wavelength and vice versa.

Our perception of color is pretty much something our brain makes up to distinguish wavelengths, kind of like how we can distinguish pitch. SciAm had an article once about how they could make people see strange or unusual color perceptions by tricking the brain to interpret signals differently somehow.

You are correct in that objects don't have color, they simply absorb and reflect different wavelengths. The reason there are differences between what's absorbed and reflected is related to the quantum nature of light.

First note that the energy of a photon is directly related to it's frequency (the constant of proportionality is planks constant). So red light, which is the lower frequency of the visible spectrum, is simply light composed of photons with lower energy. Blue is light composed of photons with higher energy. So the color we perceive is simply a matter of how energetic the photons in the light are.

For simplicity, suppose we have a hydrogen atom. The electron on the atom will have a certain amount of energy at any given point in time. Quantum theory puts a limit on how low the energy can possibly be. So in other words, you can't make the energy of the electron zero (or arbitrarily close to zero). You're eventually going to reach what's called the ground state, where the electron's energy cannot drop any further under any circumstance.

The electron can become more energetic though. But not by arbitrary amounts. The electron is going to have a "next highest" state. In other words, the electron will either have the ground state energy, or the energy of the next highest state (which we call the first excited state). The electron cannot have any energy between these two states or under the first state. It's either in the first or the second.

So what if I attempt to give the electron an energy between the two states? Suppose, for example, I bombard the atom with photons who's energy is exactly half the amount required to jump from the ground state to the first excited state. Will the electron go halfway up?

Well, as I said, quantum theory wont allow this. So these photons will simply bounce off of the atom instead of being absorbed. We call this "scattering". So whatever energy these photons are, if we divide it by planks constant, we get the frequency (the color) of that light. Since this light is being scattered, we see the object as being that color.

What would happen if we shined a light at exactly the frequency we need to jump from ground to the first excited state (or some other excited state)? The photon would be absorbed. So the object would appear black.

If instead of shining light of one energy on it, we were to shine white light on it, the color we perceive would be the combination of all of the frequencies that are scattered.

For objects other than the hydrogen atom, it works basically the same way. Certain energies are allowed, and certain energies aren't. So certain photons are scattered, and certain photons are not.

Actually, I guess my post has been somewhat misleading so far. There are a bunch of different types of scattering that occur for different reasons. I can't get into those without a bunch of math, so I wont. Just note that the color of an object depends on what frequencies are scattered more easily for any reason. Most of the scattering isn't really the process I described above.

But I did describe the process above for a good reason. Scattering only determines the color of objects that don't emit their own light. In the example above, the frequencies that the hydrogen absorbed are the "absorption spectra" (see Eli's post). When the electron gets excited, it wants to go back to it's ground state. In order to do so, it has to emit a photon. The photon energies it can emit are the same as those it can absorb.

As Eli explained, this is what gives neon its color. The neon is excited by electricity, falls back to a less excited state, and emits light of the frequencies corresponding to changes in energy level. We see the frequencies associated with these energies. In Eli's picture, we see that the emission spectra is simply a bunch of stripes. In most everyday materials, the emission spectrum will be broad bands instead of stripes, or the entire spectrum will be the emission spectrum. In other words, most objects can emit light at any wavelength. However, they do not emit all frequencies equally. Some frequencies will be emitted a lot more than others. The amount of each wavelength emitted depends on the temperature of the object.

If you were to look at a piece of metal, it doesn't appear to emit any light. It actually does, but it emits a very small amount, and at wavelengths that we can't see. The human body emits it in the infrared frequencies, which is why we can track people down with infrared vision technologies. If our eyes could naturally see infrared, everybody would look as if they were gave off their own light (because they do, but we just can't see the infrared frequencies). If we heat up a piece of metal, it eventually starts to emit much more radiation at the red frequencies, so the light appears red. This is essentially where the color of objects who give off their own light come from.

By the way, you misspelled color. For some reason you put a "u" in it, but everyone knows color has no u.

[Kuya]

Cool Cool, thanks, I figured there were both evolutionary and actual physical reasons why we see certain wavelengths and not others. And yeah I think my simple explanation is valid but only to a certain extent. I mean, after it's an electrochemical potential it's no longer a wavelength, and there aren't different rod cells for each wavelength of light that the brain can distinguish right? So there most be some very subtle processing depending on how each type of cell integrates signals, and ow the brain interprets that signal.

But that's biology for ya, it's always more complicated than it seems at first glance.

As i understand it, there are three photopigments that absorb visible light, so as it was mentioned earlier, we can't see some lights because energy is too high, and i believe (correct me on this) we can't see others because we do not have the photopigments that absorb slight lower or higher wavelengths.

[Chun Liroy]

I didn't know puerto rico is located in great Britain.

[Cyn]

Our eyes can only discern a small fragment of the electromagnetic spectrum as light. What we think of as "color" is our way of interpreting that small spectrum. Light can be manipulated in several ways because it can not only be absorbed and reflected, but also pass through different objects.

[Woozie]

Cool Cool, thanks, I figured there were both evolutionary and actual physical reasons why we see certain wavelengths and not others. And yeah I think my simple explanation is valid but only to a certain extent. I mean, after it's an electrochemical potential it's no longer a wavelength, and there aren't different rod cells for each wavelength of light that the brain can distinguish right? So there most be some very subtle processing depending on how each type of cell integrates signals, and ow the brain interprets that signal.

But that's biology for ya, it's always more complicated than it seems at first glance.

I don't know much about how we process color, but one interesting fact is that if you were to take a green light and a non-green light at the same intensity, humans would perceive the green one as being much brighter. Our brains adapted to growing up in green environments by getting better at seeing green things specifically (as opposed to an overall increase in vision).

What is more interesting to me is why the sky is blue. I remember my teachers in grade school saying it was blue because of all the water on the earth reflecting back blue light into the sky. Not at all true, in fact, water is blue because of the sky, since water on it's own is obviously clear.

EDIT: I know why it's blue but it is still neat to think about.

My teachers told me that too, now that I think about it. When I go back in time to punch the teachers who taught me the structure of an atom, I'm going to give these teachers a visit too.

Edit (I don't want to double post): An interesting thing I read earlier today is that Pauli didn't come up with the exclusion principle. He summarized someone else' work and got published. When he originally got published, he was eager to give credit to the appropriate person. But when he got his Nobel Prize, he decided to take all of the credit for himself. I'm kind of glad he got the credit, because otherwise it would have been called the "Stoner Principle", and professors everywhere would have to deal with lame marijuana jokes from their undergrads.

[Eliseos]

If you guys are interested in more light-y things, check out some of MIT's physics lectures on iTunes U. http://deimos3.apple.com/WebObjects/...51.01299615455 *should* link to the ones I have. Lectures 30 and 31 are pretty light on the math, it's mostly just him explaining stuff and drawing pictures to illustrate his points. I absolutely love Walter Lewin, and I used his lectures when I was taking the equivalent class at my university. Don't be intimidated by the letters MIT, Walter does a very good job explaining everything. The best part about these videos is that they are all free.

[Chun Liroy]

What is more interesting to me is why the sky is blue. I remember my teachers in grade school saying it was blue because of all the water on the earth reflecting back blue light into the sky. Not at all true, in fact, water is blue because of the sky, since water on it's own is obviously clear.

EDIT: I know why it's blue but it is still neat to think about.

Well the only reason I said that about the black=hot white=cooler is because everyone I know understands why so there's no reason to actually "wonder" about it, while the reasons as to why the sky is blue is much more interesting, to me at least. And wow that was one long sentence.