GENIUS FORUMS

I'm looking to have a dialogue with someone on this subject.

It was because I tried understanding blackbody radiation via this site that I suddenly felt I was lacking in some very fundamental knowledge about the nature of light, color, vision, temperature, god knows what else.

Perhaps one of you might feel warranted to help me understand the following excerpt from the blackbody site linked above:

An object is a "blackbody" if the radiation it emits into space originates completely from its temperature. This means the radiation produced by the object comes from light waves

First of all, what sort of examples do we have of objects whose radiation is a mix between temperature and...._____?

Radiation originating completely from temperature? Well, what causes the activity that causes temperature?

And what is the difference between radiation and lightwaves?

Later in the article, they say:

The important idea is that a blackbody pumps radiation into space in a very special way. Anything which has heat and is dense enough will emit as a blackbody. That means you, the chair you’re sitting on and the Earth on which the chair rests are all blackbodies.

Ok, so I'm confused here. Earlier they stated that an object is a "blackbody" if the radiation it emits into space originates completely from its temperature.

However, in this later excerpt, they are saying: any old old thing, a chair, a tree, a rock is a blackbody that emits radiation (apparently an imperceptibly dim radiation).

Is there ever an instance when light is not emited completely because of temperature?

I've never given much thought to why objects emit light(radiation)

Oh yeah, and is there really any significant difference between light and radiation?

Anyway, I'd appreciate it if anyone could point out any deluded assumptions that I might be exhibiting in my wonderings above.

The first important thing to note is that visible light, ultraviolet radiation, infrared (heat) waves, microwaves, gamma rays, x-rays, and radio waves are all electromagnetic waves, part of the electromagnetic (EM) spectrum. The "type" of wave varies according to its wavelength and frequency. See this page for an introduction.

First of all, what sort of examples do we have of objects whose radiation is a mix between temperature and...._____?

Lightbulbs. Incandescent bulbs, for example, give off far more energy in the "heat" (infrared) part of the EM spectrum than in the "visible light" part of the EM spectrum.

The sun, for another example, gives off all sorts of different EM waves: lots of visible light and infrared (heat) radiation, and also lots of UV and harmful stuff like gamma rays.

And what is the difference between radiation and lightwaves?

As noted above, lightwaves are just a type of electromagnetic radiation.

From what I can gather, the article is saying that if none of the radiation we receive from some object X is reflected radiation coming from some other object Y, then object X is a blackbody: any radiation that we receive from it will depend only on the heat generated by that object. Objects of different temperatures release different amounts of different types of radiation across the electromagnetic spectrum, and so we can tell the temperatures of stars by looking at what sorts of EM radiation they release, and in what proportion.

I have never studied blackbodies before, so somebody can correct that last paragraph if it is wrong.

Philosophaster wrote:The first important thing to note is that visible light, ultraviolet radiation, infrared (heat) waves, microwaves, gamma rays, x-rays, and radio waves are all
electromagnetic waves, part of the electromagnetic (EM) spectrum. The "type" of wave varies according to its wavelength and frequency. See this page for an introduction.

Thanks for the link. It helped put things into perspective. Somewhat anyway.

Question: I wonder how arbitrary our relationship to electromagnetic waves are? For instance, perhaps it is reasonable to imagine a species of intelligent life whose visual system was so radically different from ours that the spectrum percieved would be as low as 20nm and as high as 300nm. Gamma rays and X-Rays would be percievable to them, whereas the spectrum we see would be invisible to them.

Basically what I'm getting at is since we know that bumble bees can see ultra violet, would it be totally unreasonable to suggest that their could be a species that could percieve X-Rays, gamma waves, etc? Likewise, we know that rattlesnakes can percieve infrared, so just imagine a creature out there whose visual system was so weird that infrared was instead of too low and and too long as it is for us, is too high of a freq. and too short of a wavelength for them to percieve. And therefore, micro waves and radio waves would be visible to them.

I also cant help but wonder if their is any limit on how [high/low a frequency - long/short a wavelength] can be.

Philo wrote:

Cory wrote:First of all, what sort of examples do we have of objects whose radiation is a mix between temperature and...._____?

Lightbulbs. Incandescent bulbs, for example, give off far more energy in the "heat" (infrared) part of the EM spectrum than in the "visible light" part of the EM spectrum.

I'm not sure I understand. Are you implying that when radiation is completely the result of temperature, then it gives off energy only in the 'visible light' part of the spectrum, and not so much or not at all in the 'heat' part of the spectrum?

My assumption was that a black body generates pure, colorless light - - - this is in contrast to objects that we see in color. We see color because some of the light has been absorbed and the rest reflected.

Philo wrote:

Cory wrote:And what is the difference between radiation and lightwaves?

As noted above, lightwaves are just a type of electromagnetic radiation.

Ok, so the commonality is that all radiation is comprised of photons. The difference is that photons have varying wave length/frequency.

Question: when we see colors, is that only because the light is not completely the result of temperature, but is also the result of being reflected off of a particular quality of matter?

'Nother Question: when an atom is radioactive(unstable nucleus), is this always accompanied by relatively low wavelength (below 400nm?) Or is there such thing as radioactivity where the atoms are radiating wavelengths above (700nm?). If not, then that's kind of weird considering wave lengths higher than 700nm are radiowaves. Are radiowaves the opposite of the sort of radioactivity that we see emanating from the three major families of radioactive elements: 1) thorium 2) actinium and 3) uranium?

From what I can gather, the article is saying that if none of the radiation we receive from some object X is reflected radiation coming from some other object Y, then object X is a blackbody

I'm assuming that all radiation is photons(which vary greatly in frequency and wavelength) and all photons are produced by reactions occuring at the atomic level. If that assumption is correct, then it would seem that anything that produces photons (as opposed to merely reflecting photons) is a black body. Therefore, what I'm confused about is the statement earlier which said that blackbody radiation is radiation that is completely the result of temperature - - it seems to me that temperature is caused, or even synonymous with 'activity'.

I'm also suprised they don't say 'sufficent temperature'. For photons to be produced, do you not need high temperatures?

I can't help but think that the website on Blackbody radiation that I linked on my first post is comprised of some very bad writing, which probably reflects poor understanding. But then again, maybe I'm just missing something. Afterall, I feel a bit confused about what light is and how its produced.

Any radiation that we receive from it will depend only on the heat generated by that object. Objects of different temperatures release different amounts of different types of radiation across the electromagnetic spectrum, and so we can tell the temperatures of stars by looking at what sorts of EM radiation they release, and in what proportion.

So they are blackbodies because the light that emanates from them as bodies is completely from their own will (to speak metaphorically). Whereas the moon is not a blackbody because the light that often emanates from it is a mere reflection of the light emanating from the will of a blackbody (the sun).

But then again, in a sense is everything not a blackbody? For instance, with infrared goggles, things that don't normally emanate light, appear to actually emanate light on a level that we can't percieve. Therefore, perhaps all density must emanate photons at some level. Some at lower and higher frequencies than others.

?

Cory Duchesne wrote:Basically what I'm getting at is since we know that bumble bees can see ultra violet, would it be totally unreasonable to suggest that their could be a species that could percieve X-Rays, gamma waves, etc? Likewise, we know that rattlesnakes can percieve infrared, so just imagine a creature out there whose visual system was so weird that infrared was instead of too low and and too long as it is for us, is too high of a freq. and too short of a wavelength for them to percieve. And therefore, micro waves and radio waves would be visible to them.

I also cant help but wonder if their is any limit on how [high/low a frequency - long/short a wavelength] can be.

Take the following with a big grain of salt, because I'm hardly an expert in these matters, and I'm kinda half guessing really and it's >3am.

I believe the wavelength of the electromagnetic radiation might generally have some relationship with the possible resolveable resolution of the vision system, and so once you get down to some of the radio wavelengths a single "pixel" may at best be only capable of representing something on the order of say 1 meter(figure plucked from the ether). In other words, the resolution might be horrendous at normal distances.

I also think you generally need relatively very large collectors for each pixel. I vaguely remember the military or someone else attempting to build a device that would act like a camera at these longer wavelengths and each single pixel to be sensed needed a largish dish or antenna. I guess that might be because antenna length is, as I remember, often located within small multiples of the wavelength it is designed to receive(1/4? I can't remember, maybe it varies). So I'm guessing you might also need a massive "eye" to get any decent number of pixels in that range of the spectrum.

I think this may all be kind of the inverse of why for example higher capacity optical media like blu-ray and HDDVD need to move to higher frequency lasers in order to read back smaller details from the disc surface, compared to CDs. Also why the photo-llithograpic(? is that the word) process for use in fabricating silicon chips might have an upper limit.

The higher frequency stuff like gamma rays can also be destructive to organisms AFAIK so that would probably one serous problem on the other end.

Cory Duchesne wrote:I'm not sure I understand. Are you implying that when radiation is completely the result of temperature, then it gives off energy only in the 'visible light' part of the spectrum, and not so much or not at all in the 'heat' part of the spectrum?

No.

My assumption was that a black body generates pure, colorless light - - - this is in contrast to objects that we see in color.

I don't think this assumption is correct, at least not according to the page linked in your first post. It says that a black body is an object such that all the radiation we receive from it originates solely from heat generated within the object, not from reflected radiation or transparence (radiation passing through it).

Question: when we see colors, is that only because the light is not completely the result of temperature, but is also the result of being reflected off of a particular quality of matter?

All light (and hence all perception of color) ultimately results from the temperature of some object or another. The colors we see on earth result from the temperature of the sun if we are outside or from the temperature of electric light bulbs at night and in enclosed rooms.

'Nother Question: when an atom is radioactive(unstable nucleus), is this always accompanied by relatively low wavelength (below 400nm?) Or is there such thing as radioactivity where the atoms are radiating wavelengths above (700nm?). If not, then that's kind of weird considering wave lengths higher than 700nm are radiowaves. Are radiowaves the opposite of the sort of radioactivity that we see emanating from the three major families of radioactive elements: 1) thorium 2) actinium and 3) uranium?

Not all radioactivity consists of EM radiation. Some of it comes in the forms of alpha particles and beta particles. Radiowaves are generated by electric currents moving through antennas.

...then it would seem that anything that produces photons (as opposed to merely reflecting photons) is a black body.

Anything which produces photons and neither transmits nor reflects photons from outside sources.

I'm also suprised they don't say 'sufficent temperature'. For photons to be produced, do you not need high temperatures?

Not at all, although you do need high temperatures to produce the photons that make up visible light.

But then again, in a sense is everything not a blackbody?

No, because lots of things transmit and reflect EM radiation, which means that they are not black bodies.

An object is a "blackbody" if the radiation it emits into space originates completely from its temperature. This means the radiation produced by the object comes from light waves

First of all, what sort of examples do we have of objects whose radiation is a mix between temperature and...._____?

They are only considering the light emitted from the object. You would not count any light reflected by the object.

Actual bodies are not blackbodies. A blackbody is a theoretical object, which a real object may more or less act like.

I think a counterexample would be a laser. A laser emits light at a specific, well-defined frequency. However, that frequency is not determined by the temperature.

The important idea is that a blackbody pumps radiation into space in a very special way. Anything which has heat and is dense enough will emit as a blackbody. That means you, the chair you’re sitting on and the Earth on which the chair rests are all blackbodies.

Ok, so I'm confused here. Earlier they stated that an object is a "blackbody" if the radiation it emits into space originates completely from its temperature

That was misleading. An object like a chair could be considered to be like a blackbody, if it were in a dark room. Without reflected light, it looks black, because it is not emitting enough light on its own to be visible. If you heat it up enough, you will see it.

I also cant help but wonder if their is any limit on how [high/low a frequency - long/short a wavelength] can be.

Wikipedia shows a range of about 81 octaves.

My assumption was that a black body generates pure, colorless light - - - this is in contrast to objects that we see in color. We see color because some of the light has been absorbed and the rest reflected.

No, the color of a blackbody depends on the temperature. As the temperature decreases, it becomes very dim; at room temperature, it is not emitting enough photons to be visible.

When you heat something up, usually it will glow a dull red, then orange, then yellow, as the temperature increases. (Think of the burner on an electric stove.)

Question: when we see colors, is that only because the light is not completely the result of temperature, but is also the result of being reflected off of a particular quality of matter?

The light you see depends on the wavelengths of the light hitting your eye. It doesn't care where the waves come from. Generally the light you see is reflected light, and depends on the physical properties of the object doing the reflecting - what frequencies it tends to absorb or reflect.

I can't help but think that the website on Blackbody radiation that I linked on my first post is comprised of some very bad writing, which probably reflects poor understanding. But then again, maybe I'm just missing something. Afterall, I feel a bit confused about what light is and how its produced.

Yes, the website was a little confused. However, light is a very deep topic.

Cory Duchesne wrote:
Question: I wonder how arbitrary our relationship to electromagnetic waves are? For instance, perhaps it is reasonable to imagine a species of intelligent life whose visual system was so radically different from ours that the spectrum percieved would be as low as 20nm and as high as 300nm. Gamma rays and X-Rays would be percievable to them, whereas the spectrum we see would be invisible to them.

Water is largely opaque to electromagnetic radiation. If you were to plot water's transmissivity vs. wavelength, you would see a spike at the narrow band corresponding to the visible light spectrum. That's why water is clear to humans. Life began in water.

Philosophaster,

Cory: My assumption was that a black body generates pure, colorless light - - - this is in contrast to objects that we see in color.

P: I don't think this assumption is correct, at least not according to the page linked in your first post. It says that a black body is an object such that all the radiation we receive from it originates solely from heat generated within the object, not from reflected radiation or transparence (radiation passing through it).

Actually, when I said colorless, I meant white. So I think we are in agreement then.

C: Question: when we see colors, is that only because the light is not completely the result of temperature, but is also the result of being reflected off of a particular quality of matter?

P: All light (and hence all perception of color) ultimately results from the temperature of some object or another. The colors we see on earth result from the temperature of the sun if we are outside or from the temperature of electric light bulbs at night and in enclosed rooms.

Well, I agree that light primarily results from temperature, but many colors we see are due more to refraction. For instance, when light hits leaves, due to the topography and material of the leaves, certain wavelengths are absorbed, while others get reflected, and thus we see the leaves as green. So that would be an example of color resulting more from reflection than pure temperature.

But as I think you said ealier, scientists can tell how old stars are by the color of stars. Reportedly stars vary in color, despite we are limited to seeing them as mostly bluish white. I'm not too clear on why this is.

C: 'Nother Question: when an atom is radioactive(unstable nucleus), is this always accompanied by relatively low wavelength (below 400nm?) Or is there such thing as radioactivity where the atoms are radiating wavelengths above (700nm?). If not, then that's kind of weird considering wave lengths higher than 700nm are radiowaves. Are radiowaves the opposite of the sort of radioactivity that we see emanating from the three major families of radioactive elements: 1) thorium 2) actinium and 3) uranium?

P: Not all radioactivity consists of EM radiation.

Ok, but the radioactivity that does consist of EM radiation must belong to the short-wave part of the spectrum, as X-rays and gamma rays were discovered during the time when they were studying radioactivity most extenstively (Rutherford and co.). I guess the terms radio-activity (to describe gamma, x-rays, utlra-violet) in contrast to the radio-waves on the opposite end of the spectrum is just an awkward coincidence.

P: Some of it comes in the forms of alpha particles andbeta particles.

Actually, yes - - beta and alpha rays were first discovered emanating from uranium, and were originally assumed to be a form of radiation.

but beta rays were discovered to be streams electrons - - whereas the alpha rays were discovered as postively charged particles of helium.

P: Radiowaves are generated by electric currents moving through antennas.

Alright, so not much to do with the radioactivity of unstable atoms then huh?

cory: then it would seem that anything that produces photons (as opposed to merely reflecting photons) is a black body.

Philo: Anything which produces photons and neither transmits nor reflects photons from outside sources.

What confuses me is that they are saying black-holes are blackbodies as well. Confusing because I can't imagine blackholes producing photons.

C: I'm also suprised they don't say 'sufficent temperature'. For photons to be produced, do you not need high temperatures?

P: Not at all, although you do need high temperatures to produce the photons that make up visible light.

I didnt know that ultraviolet, gamma, radiowaves, and x-rays were classified as photons.

So I guess there different 'species' of photons, different wave lenghts of photons? And photons are synonymous with 'electromagnetic waves'.

C: But then again, in a sense is everything not a blackbody?

P: No, because lots of things transmit and reflect EM radiation, which means that they are not black bodies.

Well, that's what I would have thought. But the website on blackbodies that I linked on my first post said:


Anything which has heat and is dense enough will emit as a blackbody. That means you, the chair you’re sitting on and the Earth on which the chair rests are all blackbodies.

?

Hey Dave Hodges,

Cory: An object is a "blackbody" if the radiation it emits into space originates completely from its temperature. This means the radiation produced by the object comes from light waves

First of all, what sort of examples do we have of objects whose radiation is a mix between temperature and...._____?

Dave: They are only considering the light emitted from the object. You would not count any light reflected by the object.

Actual bodies are not blackbodies. A blackbody is a theoretical object, which a real object may more or less act like.

Well what are some objects from the real world that come close to being a blackbody in your opinion?

Cory: My assumption was that a black body generates pure, colorless light --- this is in contrast to objects that we see in color. We see color because some of the light has been absorbed and the rest reflected.

Dave: No, the color of a blackbody depends on the temperature. As the temperature decreases, it becomes very dim; at room temperature, it is not emitting enough photons to be visible. When you heat something up, usually it will glow a dull red, then orange, then yellow, as the temperature increases. (Think of the burner on an electric stove.)

So let me get this straight - when light is generated completely by temperature (e.g. on an electric stove) it will never be green, blue or purple - because the red wave length must be the first to become established, and once that is established, adding green will only turn it yellow, and then adding blue can only turn it white. Blue, green and purple become eliminated as possibilities - that is, when we are generating heat soley by temperature.

Is this always the case? For instance, what about a blue flame? I suppose a blueflame is blue only because it is fueled by a substance that absorbs green and red wavelength?


That being said, I'm a bit puzzeled about why the hubble telescope reveals such colorful pictures - - green and blue seem prevalent. My guess is that the topography and nature of the debris and materials comprising the galactic panorama absorbs the wave lengths of the more fundamental levels of light, e.g., red - - allowing for green to be displayed. And sometimes both green and red are absorbed allowing for blue, or only green is absorbed, thus allowing for purple. Am I right?

Cory: Question: when we see colors, is that only because the light is not completely the result of temperature, but is also the result of being reflected off of a particular quality of matter?

Dhodges: The light you see depends on the wavelengths of the light hitting your eye. It doesn't care where the waves come from. Generally the light you see is reflected light, and depends on the physical properties of the object doing the reflecting - what frequencies it tends to absorb or reflect.

Good, that's what I thought.

Cory Duchesne wrote:Well what are some objects from the real world that come close to being a blackbody in your opinion?

Sorry, I don't have a good example. The usual example is a hole that is an entrance to a larger cavity.

So let me get this straight - when light is generated completely by temperature (e.g. on an electric stove) it will never be green, blue or purple - because the red wave length must be the first to become established, and once that is established, adding green will only turn it yellow, and then adding blue can only turn it white. Blue, green and purple become eliminated as possibilities - that is, when we are generating heat soley by temperature.

This question is kind of addressed by the URL you posted in the first post in this thread.

Temperature is a statistical property. It is an average motion of the particles making up a body. So some are moving very fast, and emit higher energy photons (shorter wavelengths), and some have less energy and so emit longer wavelenths. There is a distribution of energy, and thus you have, generally, a range of frequencies of light, not a single frequency.

Usually, when the body is hot enough to emit blue light, it will also be emitting a lot of red and yellow light. What we consider colorless, white light is a mix of colors (they can be separated by a prism).

Is this always the case? For instance, what about a blue flame? I suppose a blueflame is blue only because it is fueled by a substance that absorbs green and red wavelength?

You can get blue and green flames. I am not positive, but I think this is solely related to the temperature of the flame. Various substances burn at different temperatures, and often you will see different colors in a flame, with blue at the bottom, bright yellow in the middle, fading off to red at the tips where the flame is coolest.

I've seen green flames particularly in those artificial logs, which leads me to believe there is some particular chemical in them that burns at a temperature that produces green light.

That being said, I'm a bit puzzeled about why the hubble telescope reveals such colorful pictures - - green and blue seem prevalent.

Hubble pictures are not always shown as you would optically see them - they are often shown in "false color" to highlight certain properties. For instance, a picture may actually be taken in infrared, and then different frequencies are shown in different colors, to highlight particular kinds of features.

DHodges wrote:

Cory Duchesne wrote: Is this always the case? For instance, what about a blue flame? I suppose a blueflame is blue only because it is fueled by a substance that absorbs green and red wavelength?

You can get blue and green flames. I am not positive, but I think this is solely related to the temperature of the flame.

I can't imagine that you would be able to get a green flame from a source of light that is purely temperature based. This is because by the time you have generated the kind of energy levels that produce green frequencies, you have already established the frequencies of red. Therefore, by adding green to red, you fail to get green, but instead get orange, yellow.

By generating the color of yellow (the result of establishing both red and green) you are on the the verge of reaching the frequency of energy responsible for blue. As you reach Blue, the yellow gradually becomes white.

According to this applet, a blue light actually comes after you establish white. Which is not what I would have thought. If this is true, then I guess the red wave length must somehow cease to be generated when you reach a certain height of energy, leaving you only with green and blue, which results in sky blue. As long as red is present, any sort of blue should be impossible.

Dhodges wrote: I've seen green flames particularly in those artificial logs, which leads me to believe there is some particular chemical in them that burns at a temperature that produces green light.

Yes, I don't think it's possible to generate a green flame via temperature alone.

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Switching gears a bit, the reason I was interested in Blackbody radiation was because I was under the impression from this book, that quantum physics was a solution to the problem physicists were having understanding the blackbody.

here is a paraphrazed regurgitation of what I learned from the book:

Quantum physics, to a large degree, was concieved in response to a problem that was encountered in attempts at understanding the blackbody. The problem was called the “the ultraviolet catastrophe” It was Max Planck who first began work on the ultraviolet catastrophe, a problem which led him to the idea that energy is not continuous, contrary to prior observations and established science.

I'm not totally sure what they mean by energy not being continuous. I tend to visualize a frequency that grows very gradually.

It seems that what they say 'a break in continuity' they mean that when the frequency gradually grows to a certain point, reaching a certain threshold, the frequency changes very radically. It jumps.

Planck became interested in the problem of accounting for the light and heat sent out by the illumination standard called a ‘black body.’ The black-body was considered the ideal case, and the logic was, if they could understand and predict the ideal case of radiation, then they could go onto to understand the radiation processes in all cases.

Clear enough so far.

Solving the black body problem was to understand how matter produces radiation. It was assumed that the blackbody in its most pure form was some sort of electrically charged particle, the motion of which – speeded up by heat – produced the radiation. The goal was to assign particular movement of particles to particular kinds of radiation. And to understand how motion of such and such kind yielded radiation of such and such energy.

Again, clear enough.

The trouble came when physicists took the next step; in order to arrive at the distribution of energy in the spectrum, one first had to assign energies to the particles responsible for the radiation - but if energy is continuous, as was assumed, there can be no restrictions on it. Restrictions would introduce jumps, abrupt change.

This is where things get a bit hard to follow.

Therefore the motions of electrical particles could not be restricted; the motions had to be indefinitely small at the long wave end of the spectrum. But from this it follows inescapably that the radiation energy at the short-wave end will end up indefinitely large.

I don't know what they mean by 'indefinitely small'. I would think that the particles generating low wave frequencies would simply be what they are.

Therefore, restrictions are neccesary. If there are no restrictions, then there will be a continuous rise to infinity as motion increases and emitted wave length declines.

Again, I would think that a rise to infinity would only occur if you had an infinitely powerful source of energy creating movement in the particles. A mildly powerful energy will produce mildly powerful radiation, moderate, moderate, and strong, strong. I don't understand where this worry about infinitity fits in.

Physics predicted ultraviolet catastrophe – in other words, a glowing body does not radiate an infinite amount of energy.

Well, that's because it would require an infinitely powerful energy source. The radiation produced I would think would correspond to the ammount of energy generating the radiaiton. The energy generating the radiation certainly has limits, and therefore, I would think that the radiation would correspond to those limits.

Experiment shows most of the energy appearing as waves of medium length. The solution is obvious: to avoid the ultraviolet catastrophe and reach an accurate description of black body radiation one must limit the possible energy in such a way that it is not infinite and does not go chiefly to the shortest waves.

Is it just me or does this make no sense?

The assumption of continuous energy made it difficult for Planck and others to see the solution. Black-body measurements were the first evidence that the assumption of unbroken energy was wrong. Energy jumps were apparent.

<shrug>

Cory Duchesne wrote:I'm not totally sure what they mean by energy not being continuous. I tend to visualize a frequency that grows very gradually.

It seems that what they say 'a break in continuity' they mean that when the frequency gradually grows to a certain point, reaching a certain threshold, the frequency changes very radically. It jumps.

What they are talking about is energy being quantized (not the frequency - forget about frequency for the moment).

Light has some smallest amount that it can be (a photon). Energy can not be radiated continuously, but must be in these bundles. There is some smallest amount of energy - a quantum - that can be transferred. For electromagnetic energy that quantum bundle is called a photon.

You know, I think it’s very important to be clear on fundamentals (if such a thing is possible!). For example, energy is not, strictly speaking, a property of any given object. You have kinetic energy (the energy of motion) and potential energy (the potential effect/work of an object dependent upon its location in any given system of things).

By that alone the whole idea of “continuous” (or, dare I say, inherent!) energy is meaningless, is it not?

Leyla Shen wrote:You know, I think it’s very important to be clear on fundamentals (if such a thing is possible!). For example, energy is not, strictly speaking, a property of any given object. You have kinetic energy (the energy of motion) and potential energy (the potential effect/work of an object dependent upon its location in any given system of things).

By that alone the whole idea of “continuous” (or, dare I say, inherent!) energy is meaningless, is it not?

I'm not sure if what they mean by 'continuous' necessarily implies inherently existing.

What I gather is that, by continuous, they mean that the cause corresponds to the effect in a continuous, logical matter.

For instance: Picture a windmill. 1km winds turn it 1rev. 2km turns it 2, 3-3, 4-4. I think that's what they mean by continuous.

The term discontinuous is used to describe seemingly irrational jumps in energy.

For instance, picture a windmill where 1km winds turn the mill 1rev, 2km turns it 2, 3-3, 4-4, but then suddenly 5km turns it 10rev.

So you have a break in continuity, an energy jump, a quantum leap.

To continue the thought experiment further, just to drive the point home, we have 5km turning 10rev, 6km turns it 11rev, 7-12, 8-13, 9-14, but then suddenly 10 km turns it 30rev.

Etc, etc.

DHodges wrote:

Cory Duchesne wrote:I'm not totally sure what they mean by energy not being continuous. I tend to visualize a frequency that grows very gradually.

It seems that what they say 'a break in continuity' they mean that when the frequency gradually grows to a certain point, reaching a certain threshold, the frequency changes very radically. It jumps.

What they are talking about is energy being quantized (not the frequency - forget about frequency for the moment).

I don't understand how I can forget the frequency, because isnt the frequency(the wave length) the photons?

Light has some smallest amount that it can be (a photon).

Do you mean, the smallest that light can be is a photon?

Likewise, the smallest that water can be is a H20 molecule?

Energy cannot be radiated continuously, but must be in these bundles.

Are you equating energy to photons? And what do you mean by these bundles.

There is some smallest amount of energy - a quantum - that can be transferred.

For me to understand you I think your descriptions are going to have to build off of the point where the pioneers of Quantum physics first became puzzled:

Their goal was to assign particular movement of particles to particular kinds of radiation. To me, that is just like saying that they were trying to assign particular levels of wind, to particular revolutions per minute on a windmill.

Their goal was to understand how motion of such and such kind yielded radiation of such and such energy.

Analogously: (how x windpower, generated x revolutions on the mill)

For electromagnetic energy that quantum bundle is called a photon.

I don't get it.

I haven't looked at the referenced article, but from what you have posted I think it's quite poorly written and I'm inclined to ignore it altogether. I wonder if you'd mind starting afresh with me in an attempt to isolate the problem?

Since your initial interest was in blackbodies, have a look at this link and tell me what you make of it:

http://guymcarthur.com/resources/java/applets/planck/

Also, this one--noting that wave frequency is tied to colour (certain colours appear at certain frequencies):

http://www.colorado.edu/physics/2000/qu ... ctric.html

Cory Duchesne wrote:I don't understand how I can forget the frequency, because isnt the frequency(the wave length) the photons?

It will be easier to understand this if you forget about frequency for a minute. Yes, a photon has a frequency, but let's assume we are talking about light just at one fixed frequency for the moment.

Do you mean, the smallest that light can be is a photon?

Likewise, the smallest that water can be is a H20 molecule?

Yes!

Are you equating energy to photons? And what do you mean by these bundles.

By a "bundle" (in this case), I mean a photon. A photon is the particle by which the electromagnetic force is transferred. A photon is a bundle of electromagnetic energy. You can't have less electromagnetic energy transferred than one photon, just like you can't have half a molecule of water. It's the smallest size bundle of energy you can have.

Leyla Shen wrote:You know, I think it’s very important to be clear on fundamentals (if such a thing is possible!). For example, energy is not, strictly speaking, a property of any given object. You have kinetic energy (the energy of motion) and potential energy (the potential effect/work of an object dependent upon its location in any given system of things).

Well, you make a good point. Kinetic (and/or potential) energy is not strictly the property of a particular object, but is relative to some other object or frame of reference. So the energy a particle has is relative to some observer or interaction, not "inherent" or absolute.

The energy of a photon is related to its frequency. Light always travels at the same speed for all observers. Two observers moving relative to each other, and relative to a source of similar photons, would observe similar photons as having different wavelengths. This is known as Redshift or Blueshift, depending on the direction of motion. (This is why I was trying to get Cory to forget about frequency for a moment; that's all true, but a distraction from the more basic point we were discussing.)

The energy of a photon, its frequency, its wavelength, all refer to the same thing.

By that alone the whole idea of “continuous” (or, dare I say, inherent!) energy is meaningless, is it not?

The point is just that electromagnetic energy is transferred in discrete packets (bundles, quanta, photons) - it is not continuous in the sense of being infinitely divisible.

Think of the mathematical concept of a continuous line, such as the real numbers. Any distance on the real number line can be divided in half, as many times as you like.

Compare that with the integers. There is a discrete jump between each integer; they are not continuous. There is no integer halfway between one and two.

In a beam of light, the number of photons will be an integer.

DHodges wrote:

Cory Duchesne wrote:I don't understand how I can forget the frequency, because isnt the frequency(the wave length) the photons?

It will be easier to understand this if you forget about frequency for a minute. Yes, a photon has a frequency, but let's assume we are talking about light just at one fixed frequency for the moment.

Do you mean, the smallest that light can be is a photon?

Likewise, the smallest that water can be is a H20 molecule?

Yes!

Are you equating energy to photons? And what do you mean by these bundles.

By a "bundle" (in this case), I mean a photon. A photon is the particle by which the electromagnetic force is transferred. A photon is a bundle of electromagnetic energy. You can't have less electromagnetic energy transferred than one photon, just like you can't have half a molecule of water. It's the smallest size bundle of energy you can have.

Ok, i followed what you said there. Simple enough. But where are you going with it?

Just to clarify my position: I'm trying to understand what exactly the observation was that physicists found so troubling. I like how Rutherford discovered the Nucleus of the atom, (gold foil experiment) easy to understand, nice and logical, very empirical.

But what exactly was the observation which caused all the trouble?

My understanding so far is that the trouble started when Bohr realized there was something wrong with Rutherford’s atomic model based on the discovery of the nucleus, the model which depicted the atom as a miniature solar system.

According to the model, electrons moved around the nucleus like planets around the sun. But a moving electron was impossible because according to the law of electricity a moving electron(charge) must produce electromagnetic radiation, light. All atoms and therefore all things would glow in the dark. Since Bohr could clearly see that not all things glow in the dark, he detected a problem.

This /\ and This \/ might be worth going into.

Also, since a moving electron must produce radiation, in doing so, it would lose energy and spiral into the nucleus. There should thus be no such thing as an atom, only a nucleus.

Rutherford’s model which represented the atom’s structure simultaneously denied the possibility of that very structure.

It was this problem that Bohr would help solve, founding today’s atomic physics. But to understand Bohr’s logic behind his atomic physics, we need to understand the logic behind Quantum theory which was in part developed by Planck, Heisenberg, Einstein and others.

Leyla Shen wrote:I haven't looked at the referenced article, but from what you have posted

I think it's quite poorly written and I'm inclined to ignore it altogether.

If you are refering to the very first link I gave on my first post, then yes, I'm also inclined to think its poorly written and worth ignoring.

I wonder if you'd mind starting afresh with me in an attempt to isolate the problem?

Like I was just saying to Dave, I'm interested in not so much understanding Quantum physics, as I am in understanding what exactly the empirical observations were that caused all the babbling hoo-ha and mystical intuitive Chandelier-swinging that was reportedly involved in trying to pin down a Quantum mechanics to begin with. From what I've read from the book 'the men who made the new physics', the blackbody as an illumination standard played a vital role in the develop of QM. The blackbody was an illumination standard to help physicists understand how such and such energy produces such and such radiation. By studying or thinking about the logic of a blackbody, they, Somewhere along the line, decided that a logical, continuous, congruent common-sense conception of physics would not suffice.

Leyla wrote: Since your initial interest was in blackbodies, have a look at this link and tell me what you make of it:

http://guymcarthur.com/resources/java/applets/planck/

Alright, lets see:

Guy McArthur wrote:
Any object in the universe has a temperature and thus emits EM radiation at all wavelengths due to thermal motion (a probability distribution of kinetic energy) of charged particles.

Yes, therefore any object in the universe is, to differing degrees, a black body. The more imperfect a blackbody is, the more light it reflects. The moon for instance emits EM radiation, but it is not visible, and the moon reflects light quite apparently. Physics at the eve of QM, were trying to attain absolute certainty, absolute predictive power, by establishing the perfect blackbody, the perfect illumination standard. Due to some troubling observations (or logical implications?), they realized that absolute certainty was impossible in the empirical realm, and had to settle for not only probability models, but they also had to develop strange models that involve breaks in continuity, and jumps. And I'm not very clear on what the observations or logic was that demanded these strange models.

The intensity of the radiation as a function of wavelength is given by Planck's function.

This is a bit over my head. I would have been pleased if it said something simple like: the intensity of the radiation is proportional to the intensity of the kinetic energy. (much like
the revolutions per min on a windmill is prop. to the intensity of the wind)

Leyla wrote: Also, this one--noting that wave frequency is tied to colour (certain colours appear at certain frequencies):http://www.colorado.edu/physics/2000/qu ... ctric.html

I like the way the info. was presented in that one, but I always find after reading something about electricity and light, I end up with more questions than I started with.

colorado.edu/physics web site wrote: It's been determined experimentally that when light shines on a metal surface, the surface emits electrons. For example, you can start a current in a circuit just by shining a light on a metal plate. Why do you think this happens? Well...we were saying earlier that light is made up of electromagnetic waves, and that the waves carry energy. So if a wave of light hit an electron in one of the atoms in the metal, it might transfer enough energy to knock the electron out of its atom

There is something I find very unsatisfactory about merely visualizing a 'current'(of electrons) running through a circuit because electrons are being knocked out of atoms by a beam of light. For instance, questions arise in my mind like: What was the frequency of the light? Do different frequencies produce different currents? Can we adjust the intensity of a particular frequency of photon in the same way we can adjust the power of the water coming out of a fire-hose and thus is there a corresponding variation in current power? A current power that corresponds to how intense a particular frequency is applied?

colorado.edu/physics web site wrote: Historically, light has sometimes been viewed as a particle rather than a wave; Newton, for example, thought of light this way. The particle view was pretty much discredited with Young's double slit experiment, which made things look as though light had to be a wave. But in the early 20th century, some physicists--Einstein, for one--began to examine the particle view of light again. Einstein noted that careful experiments involving the photoelectric effect could show whether light consists of particles or waves. The photoelectric effect would still occur no matter which view is correct - Either way, the light would carry energy, so it would be able to knock electrons around - However, the details of the photoelectric effect come out differently depending on whether light consists of particles or waves.

?

Absurd! Did Einstein do two different experiments, one with light as particle, one with

wave? How.

[quote=""colorado.edu/physics web site"]
If it's waves, the energy contained in one of those waves should depend only on its amplitude--that is, on the intensity of the light. [/quote]

Ok, that sounds sensible enough. A bright glowing green light logically contains more energy than a dimmer light of the same color.

Other factors, like the frequency, should make no difference. So, for example, red light and ultraviolet light of the same intensity should knock out the same number of electrons, and the maximum kinetic energy of both sets of electrons should also be the same.

Very sensible.

Decrease the intensity, and you should get fewer electrons, flying out more slowly; if the light is too faint, you shouldn't get any electrons at all, no matter what frequency you're
using.

That's right, only an idiot with think otherwise.


Ok Leyla - so that second website was particularly helpful. What we are looking for is the precise point when things get queer. I havent clearly pinpointed it yet, but that site has helped me at least feel like I've got a grip on the fundamentals. What i'm looking for is just around the corner I think.

Tomorrow I will see if I can deal more effectively with the radiation/blackbody work done by Max Planck - because right now its time for some sleep.

Cory:

Like I was just saying to Dave, I'm interested in not so much understanding Quantum physics, as I am in understanding what exactly the empirical observations were that caused all the babbling hoo-ha and mystical intuitive Chandelier-swinging that was reportedly involved in trying to pin down a Quantum mechanics to begin with.

[laughs] Yes, I completely understand.

I have to help my daughter with her homework now. I'll try to beat you back here (to finish reading your post properly and reply)!

Sorry, Cory. I’ve been distracted by guns, microwave tanks, mothers, children and mothers’ day all day. I’m going to have to continue this (and most all else for the time being) piecemeal.

Accordingly: it was a near-perfect site! :)

However, the details of the photoelectric effect come out differently depending on whether light consists of particles or waves.

Cory: Absurd! Did Einstein do two different experiments, one with light as particle, one with wave? How.

All they are referring to here is this:

If all else is equal, blue light has more energy than red light with yellow light somewhere in between. But this means that if light is a wave, a dim blue light would have the same amount of energy as a very bright red light. And if this is the case, then why won't a bright red light produce a current in a piece of metal as well as a dim blue light? Einstein realised that the only way to explain the photoelectric effect was to say that instead of being a wave, as was generally accepted, light was actually made up of lots of small packets of energy called photons that behaved like particles.

http://www.einsteinyear.org/facts/photoelectric_effect/

Here is the experiment itself, FYI:

http://www.einsteinyear.org/facts/photoelectric_effect/

More on electricity, etc., later.

Leyla Shen wrote:Sorry, Cory. I’ve been distracted by guns, microwave tanks, mothers, children and mothers’ day all day. I’m going to have to continue this (and most all else for the time being) piecemeal.

Accordingly: it was a near-perfect site! :)

Yes, you're right, it's a good one.


You posted also:

http://www.einsteinyear.org/facts/photoelectric_effect/

The following was taken from the link above:
(I enjoy questioning things piece by piece and then reading further later, to see how reasonble or unreasonable my discrepancies and questions were, for instance:)

einsteinyear website wrote: The photoelectric effect is the name given to the observation that when light is shone onto a piece of metal, a small current flows through the metal. The light is giving its energy to electrons in the atoms of the metal and allowing them to move around, producing the current.

Clear enough.

However, not all colours of light affect metals in this way. No matter how bright a red light you have, it will not produce a current in a metal, but even a very dim blue light will result in a current flowing.

Ok, fine.

The problem was that these results can't be explained if light is thought of as a wave.

This I don't understand. Why did they think of light as a wave to begin with? And when they say wave, don't they mean frequency? I suppose the word frequency is a word used to describe the nature of a wave.

Waves can have any amount of energy you want - big waves have a lot of energy, small waves have very little.

When they say waves here, are they refering to the frequency?

Big waves=low frequency. Small waves=high frequency.

?

And if light is a wave, then the brightness of the light affects the amount of energy - the brighter the light, the bigger the wave, the more energy it has.

Hold on here. I would have thought different. How about: the brighter the light, the greater quantity(volume) of waves, and thus the more energy. If we say we are increasing the wave-size of a blue light, we are in effect increasing its frequency, and therefore changing it's color. Are we not?

The different colors of light are defined by the amount of energy they have.

I thought they were defined by their frequency. Hopefully what they mean is: Higher frequency equals higher energy.

blue light has more energy than red light with yellow light somewhere in between. But this means that if light is a wave, a dim blue light would have the same amount of energy as a very bright red light.

What do they mean 'if' light is a wave? The fact of the matter, regardless of whether it's a particle or a wave, is that red light doesnt have what it takes to push around electrons.

Maybe the red wave-lengths have some sort of magnetic charge that is different from the magnetic charge of blue, leaving the red-wave lengths impotent.

And if this is the case, then why won't a bright red light produce a current in a piece of metal as well as a dim blue light?

It might be because the red light has a differing magnetic charge(or some kind of quality) that renders it impotent.

Einstein realised that the only way to explain the photoelectric effect was to say that instead of being a wave, as was generally accepted, light was actually made up of lots of small packets of energy called photons that behaved like particles. Einstein wasn't the first person to use the idea of photons, but he was the first to make it the starting point of an explanation rather than a convenient fiddle to explain away odd results.

I don't see how introducing photons solves anything. Not yet anyway. I'll read more soon.

Cory Duchesne wrote:This I don't understand. Why did they think of light as a wave to begin with? And when they say wave, don't they mean frequency? I suppose the word frequency is a word used to describe the nature of a wave.

Waves have two separate properties: frequency and amplitude. You seem to be confusing them.

DHodges wrote:

Cory Duchesne wrote:This I don't understand. Why did they think of light as a wave to begin with? And when they say wave, don't they mean frequency? I suppose the word frequency is a word used to describe the nature of a wave.

Waves have two separate properties: frequency and amplitude. You seem to be confusing them.

Damn. Of course, you're right. I knew that. I just had a bit of block.

So I'll correct myself:

Cory wrote:

Waves can have any amount of energy you want - big waves have a lot of energy, small waves have very little.

When they say waves here, are they refering to the frequency?

Big waves=low frequency. Small waves=high frequency.

Alright, so:

Big waves = high amplitude

Small waves = low ampltude

Blue, Red, whatever frequency, can have a varying amplitude, from strong to weak.

Cory wrote:

And if light is a wave, then the brightness of the light affects the amount of energy - the brighter the light, the bigger the wave, the more energy it has.

Hold on here. I would have thought different. How about: the brighter the light, the greater quantity(volume) of waves, and thus the more energy. If we say we are increasing the wave-size of a blue light, we are in effect increasing its frequency, and therefore changing it's color. Are we not?

I see my mistake, although I think my mistake raises an interesting question about the 'quantity of light'.

Is it just me or is it tempting to think of light as comprised of thread-like waves, and that greater quantity/high concentration of these threads equals greater brightness?

For instance, light being emitted from a small hole can only produce a limited ammount of brightness. By dimming that brightness you are only decreasing the quantity/concentration of whatever light is at the basic level. By increasing the brightness you are increasing the quantity/concentration.

(I think the problem is that frequency/quantity is easier for me to visualize objectively than amplitude/quality)