Composition of Light: Questions & Answers

[RobertDSmeets]

Hi everyone,
I have a few questions about the composition of light:

First, what is it? Is white light the result of all color wavelengths present in an area?
Second, if so, then why is there no interference in waves of light (or is there)?
Third, if photons all travel at the same speed, then doesn't this imply that the amplitude of a red wave would be greater than the amplitude of a yellow wave?
Fourth, why is a photon absorbed or reflected? In other words, what happens, in terms of energy, that causes a photon to bounce, be absorbed, or to have partial reflection/absorption?

Thanks.

 

[sophiecentaur]

Hi
Well, you seem to have managed to ask almost all of the questions about light possible.
I think that you need to read more around this topic and get some basic in before you try to construct actual questions.
Here is one of the earliest hits I found from a google search into "the nature of light". Try reading it and follow the links too. Don't skip things that are a bit boring or tedious as they may be vital for your understanding. There are many other links that Google will give you. Hopefully, you will be able to spot the ones which are mainly BS and there are several of those on the web (as with all subjects).
I am assuming that you don't want to buy an A Level textbook but that's really what you need.

 

[RobertDSmeets]

Hi sophiecentaur,
I've read that page as well and have done some basic googling to little avail. If you can recommend an A level text, I'd consider the purchase (and honestly, prefer it so the researchers can get paid!). But, I have not been able to find anything that directly addresses these particular aspects of the functioning of light. Logically, at least the part about amplitude makes sense to explain why redshift occurs in space, being basically that red light is left behind as the beams of light travel with indigo (or more likely sky blue) leading the beam (or something beyond our visible spectrum). But yes, I'm interested in hearing more about this and thank you for any recommendations.

 

[Khashishi]

Jackson, Classical Electrodynamics is a standard text on electromagnetism (and therefore, light).

 

[sophiecentaur]

Hi sophiecentaur,
I've read that page as well and have done some basic googling to little avail. If you can recommend an A level text, I'd consider the purchase (and honestly, prefer it so the researchers can get paid!). But, I have not been able to find anything that directly addresses these particular aspects of the functioning of light. Logically, at least the part about amplitude makes sense to explain why redshift occurs in space, being basically that red light is left behind as the beams of light travel with indigo leading the beam (or something beyond our visible spectrum). Red light essentially is left behind if this is correct. But yes, I'm interested in hearing more about this and thank you for any recommendations.

There are many different A level textbooks and anyone of them is worth using as long as it is recommended by one of the examining boards. I wouldn't recommend a 'course book' that's aimed at a modular course because they can be quite limited. You need a full textbook which is aimed at a standard Physics A Level course - it can be pretty much as old as you like because things like Dark Matter do not come into basic Physics. Plenty of time for that later.
You clearly have some large gaps in your knowledge of light. It is one of the most important facts that the speed of EM waves in space is the same for all frequencies and for all observers under all conditions. I can't think where you got that stuff about slow red photons. I have frequently made the point on PF that learning of Physics (or any other Science - or in fact any subject) is not achievable by simple Question and Answer on Science Forums. You have to start at the beginning and work through and you can then avoid the sort of mis information that you seem to have acquired somewhere. What have you actually read so far?

 

[RobertDSmeets]

My official training in physics is basic first year collegiate physics (Big10). I come from a philosophy background and started this research in relation to aesthetics and color. The part about the standard speed of EM waves is kind of where the question lies. Basically, is what we call "the speed of light" the speed of a photon or the speed of a wave? And, if it's a wave, then which one? A red one? A green one? This is the crux of what I am really trying to understand. As a result, if the speed of photons is what we mean when we say that the speed of light is constant, then it should follow that, even though the speed of the photons are constant, in order for the frequency to change from color to color, the amplitude must also change. This means that a photon would have to travel along a longer path over a shorter distance to make red at the same speed. This is what is confusing to me about what we mean when we say "the speed of EM waves" or "the speed of light." Sorry, but I don't remember the exact text titles and authors of the books.

Khashishi, thank you for the recommendation.

 

[RobertDSmeets]

To clarify, I'm imagining the photon traveling in a wave pattern (like a sine wave). Maybe that is the issue?

 

[Khashishi]

By the way, Jackson's book won't really tell you what a photon is, since it is a classical treatment. Nevertheless, you really should understand classical electrodynamics before you jump into quantum electrodynamics. It's simple enough to picture light as a wave, but don't bother trying to picture a single photon. It's just not something that can be pictured.

 

[phinds]

My official training in physics is basic first year collegiate physics (Big10). I come from a philosophy background and started this research in relation to aesthetics and color. The part about the standard speed of EM waves is kind of where the question lies. Basically, is what we call "the speed of light" the speed of a photon or the speed of a wave? And, if it's a wave, then which one? A red one? A green one? This is the crux of what I am really trying to understand. As a result, if the speed of photons is what we mean when we say that the speed of light is constant, then it should follow that, even though the speed of the photons are constant, in order for the frequency to change from color to color, the amplitude must also change. This means that a photon would have to travel along a longer path over a shorter distance to make red at the same speed. This is what is confusing to me about what we mean when we say "the speed of EM waves" or "the speed of light." Sorry, but I don't remember the exact text titles and authors of the books.

Khashishi, thank you for the recommendation.

You seriously misunderstand light. ALL light travels at c. Different wavelengths register on the human retina as different colors. Light travels as a wave. Photons are the result of an interaction of that wave with an object.

Now read post #5 again and follow his suggestion. This Q&A stuff isn't going to cut it.

 

[Drakkith]

To clarify, I'm imagining the photon traveling in a wave pattern (like a sine wave). Maybe that is the issue?

Yes, that's a widespread misconception. Photons do not travel in up and down or side to side wave-like motions. What makes an EM wave a wave is the alternating electric and magnetic field vectors. These give the direction and amplitude of the electric and magnetic fields at any point and it is these which alternate back and forth, changing the direction and strength of the force that would be applied to a charged particle.

Photons cannot even be said to travel in a classical motion. They are not tiny point-like balls that move in straight lines or sine waves. They are the quantized interaction of the EM wave with matter.

 

[RobertDSmeets]

Thanks for the information. I'll have to see what the University bookstore has, or order something, or go back to school. We'll see.

Just for kicks, the question that I was thinking about that began this was this:

What was the order of colors that came about at the creation of the universe? Darkness into...

Any takers?

 

[RobertDSmeets]

They are the quantized interaction of the EM wave with matter.

Isn't this still being debated though? Isn't that what string theory is about; the idea that matter is made of vibrations (ie. sound and light)? If true, doesn't that also imply that the interaction with matter isn't so much of an interaction but the changing of state of energy from a vibration to matter? That would imply that the EM wave is a different state of energy and that photons are the result of that change in state from vibration to matter.

Going to the bookstore...

 

[sophiecentaur]

What was the order of colors that came about at the creation of the universe?

Colours only existed after Human Beings developed their tristimulus colour vision - a long time after the beginnings of the Universe.

 

[sophiecentaur]

Isn't that what string theory is about;

Don't even go there until you have a proper grasp of Physics that is actually based on experiment. You are managing to show a serious contempt for the several millennia of Scientific Learning by treating it all as word salad.

 

[RobertDSmeets]

Colours only existed after Human Beings developed their tristimulus colour vision - a long time after the beginnings of the Universe.

This is kind of like the tree falling in the forest question. Do colors exist if nobody is there to see them?

Also,
Please accept my apology, but I think Newton would be asking similar questions.
It's not contempt at all. It's an effort to understand things that are nearly(?) incomprehensible and we use words to exchange ideas.

Now, I'm going to the bookstore, for real. Again and sincerely: apologies and thanks for the information.

 

[sophiecentaur]

I think Newton would be asking similar questions.

Newton did all his homework before shooting off his mouth. Remember his statement about standing on the shoulders of giants? He would, in 2018, be asking the sort of questions that showed he was up to date with the current state of learning.

It's not contempt at all.

Maybe not intended but you are constantly undervaluing it and implying that you are just round the corner from understanding it all. Judging by your words, it is a lot harder than you could even dream of. "Understanding" comes from hard graft and not from a few PF posts with questions.
Just get down to the bookstore - or even eBay. And good luck with it.

 

[Drakkith]

Just for kicks, the question that I was thinking about that began this was this:

What was the order of colors that came about at the creation of the universe? Darkness into...

Colors aren't physical, quantifiable concepts. They are subjective concepts that exist purely in the mind of the beholder. My stepfather is red-green colorblind and he absolutely does not see colors like you and I do. Even "normal" people's perception of color is subjective, with appreciable differences existing between many people. So the question of which color came into existence first is a question which cannot be answered.

In addition, the extremely early universe was filled with a very hot plasma and any EM radiation present would have been well into the gamma ray range.

They are the quantized interaction of the EM wave with matter.

Isn't this still being debated though?

Not as far as I know, no. Any theory that successfully replaces quantum electrodynamics would still need to quantize the interaction between the EM wave and matter. This is because any future theory would have to make the same predictions that QED does, which requires a quantized interaction.

Isn't that what string theory is about; the idea that matter is made of vibrations (ie. sound and light)? If true, doesn't that also imply that the interaction with matter isn't so much of an interaction but the changing of state of energy from a vibration to matter? That would imply that the EM wave is a different state of energy and that photons are the result of that change in state from vibration to matter.

String theory states that point particles (elementary particles) are made out of 1-D strings whose different vibrational states determines its properties. But note that it is the strings that can have different states, not energy. Energy cannot have a state, as energy isn't a physical object or a field or something.

 

[RobertDSmeets]

String theory states that point particles (elementary particles) are made out of 1-D strings whose different vibrational states determines its properties. But note that it is the strings that can have different states, not energy. Energy cannot have a state, as energy isn't a physical object or a field or something.

So, if we were using an analogy, it'd be like one end of the string is a liquid and the other end a solid?
It's seems more likely (to me) that they'd be shaped more like a rubberband where one side might be vibrating differently than the other and that the differences might cause the variable states.

As for the original post, I went to the bookstore.
Jackson, chapter 10.1-5 similarly describes what I was talking about down to theta=wavelength/linear distance. What is not explained, and what I was asking, is theta related to the amplitude of the wave being addressed. I liked in 10.2 that Rayleigh had similar questions about why the sky was blue.

To sophiecentaur,
I've also read Standing On the Shoulders Of Giants. I may not have a degree in physics, but I have read Hawking, Riemann, Newton, Liebniz, and many of the greatest thinkers in the past few thousand years. I am aware of the many contributions made over the centuries, the longstanding traditions, and the amount of time it takes for even the smallest steps of progress. However, I can also remind you that science itself is standing on the shoulders of people like Plato and Aristotle and has still not fully contended with Popper, Kant and many other metaphysicists. I think that the issue is that my language sounds crude because my understanding of what a "vibration" or "frequency" (or other scientific term) might be is likely much more shallow in terms of scientific rigor than a practiced physicist, and in some cases, our definitions may vary. Those differences, however, do not negate that what I say might be partially correct (or not), but that our understanding of the subject matter is very different. I'm not "shooting off my mouth." I was asking people who I expected to know more about the subject because I know that I don't know everything about everything. So, again, apologies and thank you for the information.

 

[Drakkith]

So, if we were using an analogy, it'd be like one end of the string is a liquid and the other end a solid?

No, not at all.

It's seems more likely (to me) that they'd be shaped more like a rubberband where one side might be vibrating differently than the other and that the differences might cause the variable states.

Different sides of the string can't vibrate differently. A string from string theory is analogous to a classical string. If you look at a guitar string, for example, each string only supports certain frequencies that it can vibrate at for a prolonged period of time. If you slightly increase or decrease the frequency, that vibration will quickly die out. Plucking a string at one end causes the entire string to vibrate. You cannot have the two ends of the string vibrate differently. Similarly, strings in string theory have certain "frequencies" that they can vibrate at. Others are not supported. We can say that the frequencies at which the string can vibrate are quantized.

As for the original post, I went to the bookstore.
Jackson, chapter 10.1-5 similarly describes what I was talking about down to theta=wavelength/linear distance. What is not explained, and what I was asking, is theta related to the amplitude of the wave being addressed. I liked in 10.2 that Rayleigh had similar questions about why the sky was blue.

I assume you mean this question:

Third, if photons all travel at the same speed, then doesn't this imply that the amplitude of a red wave would be greater than the amplitude of a yellow wave?

As far as know, the amplitude of two EM waves is equal when they have the same energy density. For two waves of different wavelengths, this means that if you measure the wave with a longer wavelength you will observe more photons per second than if you measure the shorter wavelength wave. So no, there is nothing that, in general, means a shorter wavelength wave will have a lower amplitude than a longer wavelength wave.

 

[bhobba]

Jackson, Classical Electrodynamics is a standard text on electromagnetism (and therefore, light).

It is.

But you should build up to it - I would start with Susskind first:
https://www.amazon.com/dp/0465093345/?tag=pfamazon01-20

Then the good old Feynman Lectures which are available online fro free:
http://www.feynmanlectures.caltech.edu/

Although IMHO it's so important as a resource I would buy a copy - of course I have and read it every now and then.

Then Jackson.

Although I prefer one not that well well known, it's a bit different but IMHO in a good way - Schwinger:
https://www.amazon.com/dp/0738200565/?tag=pfamazon01-20

Jackson dismisses as silly things that greatly interest me, such as how to reasonably justify Maxwell's Equations. I think its important to have an 'intuitive' idea why its true - but Jackson is in the camp - its experiment stupid - not pretty arguments.

But for questions about Photons start with Feynman's good old QED - The Strange Story Of Light And Matter.
https://www.amazon.com/dp/0691024170/?tag=pfamazon01-20

Thanks
Bill

 

[bhobba]

\As for the original post, I went to the bookstore.Jackson, chapter 10.1-5 similarly describes what I was talking about down to theta=wavelength/linear distance. What is not explained, and what I was asking, is theta related to the amplitude of the wave being addressed. I liked in 10.2 that Rayleigh had similar questions about why the sky was blue..

Jackson is considered THE textbook, but its at graduate level - you need to build up to it. See my other posts.

Read the Feynman Lectures - it will likely answer any questions you have - do not go for the sledgehammer of Jackson to start - that's like crucifixion - first offence - not the best idea.

Thanks
Bill

 

[sophiecentaur]

@bhobba Those suggests are fine but the OP needs to be sure of the basics of A level (and earlier) Science plus the associated maths. There's no point in trying to run before one can walk. People often try to read the more advanced stuff as it it's Popular Science and then misinterpret things.
PS The OP should learn that λ (the usual symbol for wavelength) is Greek Lambda.

 

[bhobba]

@bhobba Those suggests are fine but the OP needs to be sure of the basics of A level (and earlier) Science plus the associated maths

Of course Jackson and other advanced books mentioned is fine. Its just this is a beginner level thread. The OP has just done first year standard physics - you could possibly get by and if you can its of course a no brainer suggestion. The trouble is as you mention, has he the basics down pat well enough to do it. Even if he does I personally would still do it in stages - Susskind, then the Feynman Lectures, then Jackson. Just my personal way of doing things. I am at the moment reacquainting myself with GR. My favorite when I was really into it was Wald but its the most advanced. I am doing it in stages - Soper - Classical Field Theory, Ohanian - Gravitation and Space-Time, MTW, then Wald. I could have gone straight to Wald - but my view is its not a race - taking a slow, easy gradual approach makes for an easier ride. And along the way the OP may find he doesn't need the more advanced texts to answer his queries.

Actually if the OP wants EM in the context of general field theories Soper is a good choice.

Thanks
Bill

 

[sophiecentaur]

Actually if the OP wants EM in the context of general field theories Soper is a good choice.

I was trying to make realistic suggestions of stuff the would be appropriate. Have you actually read his comments?

 

[bhobba]

I was trying to make realistic suggestions of stuff the would be appropriate. Have you actually read his comments?

Point taken. I certainly didn't make it clear why I was suggesting it. It was in relation to:

I come from a philosophy background and started this research in relation to aesthetics and color. The part about the standard speed of EM waves is kind of where the question lies. Basically, is what we call "the speed of light" the speed of a photon or the speed of a wave? And, if it's a wave, then which one? A red one? A green one?

I was thinking with a philosophy type background he would want to understand the foundational principles it is built on - ie field theory in a general sense.

Thanks for pointing out I should have been clearer - context of suggestions is always important.

Thanks
Bill

 

[RobertDSmeets]

Hi everyone,
After getting over the photon path misconception, the error of the third question makes much more sense to me.
And, after remembering the double slit experiment, the second question is answered.

Just 1 and 4 left. Thanks for pointing me in the right direction.

Thanks for the responses. Yes, the Jackson text seemed fairly dense (which I liked). I am familiar with a bit of Feynman's work already (including the bongos). I've watched quite a few lectures on youtube (Feynman, Susskind and others) on General and Special Relativity, QM, and some field theory over the years and read a lot on Wikipedia. So, I do have some background.

The main issue I had with the Jackson text (based on a few minutes of looking through it) was understanding the equations more fully and how they describe what is going on. I had a goal at one point where Michio Kaku had showed an equation that, he said, most completely explained the universe as scientists currently understand it. I wanted to get to the point where I understood at least the basics of all of the parts and what they represent (Ricci Tensors, etc..).

Then I got sidetracked with work and moving and haven't got back to it. Maybe after this project that one will happen again. I tend toward the physics books with less math for easy reading (enjoyed Fabric of the Cosmos by Brian Greene quite a bit) and occasionally dig into the more technical areas if they're in line with my research. Thanks for the recommendations. Though, if there is a good "math for physics" type book, I'd be interested in that as well.

The more general mathematics texts seem to lead into areas that, although are interesting, do not seem as practical for these kinds of problems. I understand what derivatives do, and what Lagrangians are and why they are used. I topped out at second year calculus in college for math, but have also done some research in math on my own as well (mainly, number theory).

Speaking of physics math, what is the upside down delta triangle and what does it represent? It's always in physics texts but I have never seen it in anywhere else with any explanation. I've assumed it to be a variable, but that might be wrong.

 

[jtbell]

The main issue I had with the Jackson text (based on a few minutes of looking through it) was understanding the equations more fully and how they describe what is going on.

Most physics students don't touch Jackson until they're in graduate school, doing their MS or PhD coursework. By that time, they've already studied electromagnetism at least twice: in a first-year university physics course at the level of a textbook like Halliday/Resnick/Walker or Young/Freedman (both of which assume knowledge of introductory calculus), then in an upper-level undergraduate course at the level of Griffiths (which assumes knowledge of multivariable a.k.a. vector calculus). Jackson assumes that level of previous knowledge.

 

[bhobba]

Though, if there is a good "math for physics" type book, I'd be interested in that as well. The more general mathematics texts seem to lead into areas that, although are interesting, do not seem as practical. I topped out at second year calculus in college for math, but have also done some research in math on my own as well (mainly, number theory).

I think the following will greatly interest you as well as provide the necessary math to understand the view of modem physics:
http://physicsfromsymmetry.com/

It's more general than others mentioned. Sure - it covers EM - but from a viewpoint different to what you would have seen before - a viewpoint that underlies much of modern physics.

After reading it write back with what you think.

Just as an example EM becomes nearly trivial. The underlying essence of EM is what's called gauge symmetry - specifically if you add ∂uΛ where Λ is any function to a four vector called Au then the physics remains the same. That means Au is not observable so you really want an equation expressed in things that can be observed. We will see if we can find a reasonable function that does not have this issue so can be observed. We will start by looking at ∂vAu. Physically from gauge invariance this is the same as ∂vAu +∂v∂uΛ. So that's not a good choice - however it suggests it would be if we can cancel ∂v∂uΛ. Note if you do the same thing to ∂uAv you get ∂uAv +∂u∂vΛ = ∂uAv +∂v∂uΛ. Same problem - but wait - the pesky term we do not want is the same for both. So if we subtract one from the other it would disappear - meaning the equation is invariant to a gauge transformation so would be measurable.

Define Fuv = ∂uAv - ∂vAu. It is invariant to gauge transformations and its components are defined as follows:
https://en.wikipedia.org/wiki/Electromagnetic_tensor

Note ∂u∂vFuv = ∂u∂v(∂uAv - ∂vAu) = ∂u∂v∂uAv - ∂u∂v∂vAu = ∂u∂v∂uAv - ∂v∂u∂vAu = ∂u∂v∂uAv - ∂u∂v∂uAv = 0 - in the last step we simply relabeled dummy variables.

Define ∂vFuv as Ju so ∂vFuv = Ju. Ju is called the 4 current. And you have ∂uJu =0. ∂vFuv = Ju is called Maxwell's equations although you probably have not seen it in that form, but it is a medium difficulty exercise to put it in the usual form. And we have shown the 4 current is conserved ie charge is conserved.

You probably have not seen this before, and maybe not follow the tensor notation used. The suggested book will fix that. The thing is its an example of the power of symmetry. You take gauge symmetry and EM more or less falls out. Why gauge symmetry? - well actually it's because of the requirements of symmetries to be local:
https://quantummechanics.ucsd.edu/ph130a/130_notes/node296.html

That's part of the real beauty of modern physics.

Thanks
Bill

 

[Drakkith]

After getting over the photon path misconception, the error of the third question makes much more sense to me.
And, after remembering the double slit experiment, the second question is answered.

Just 1 and 4 left. Thanks for pointing me in the right direction.

First, what is it? Is white light the result of all color wavelengths present in an area?

White light is the result of certain combinations of wavelengths being detected by the eye. A broad spread of wavelengths across the visible spectrum will appear white, as will certain combinations of narrow "slices" of the spectrum. A CFL appears white, but its spectrum is very different than that of the Sun (which is also white, not yellow). The eye is constrained by the fact that it only uses three different types of color receptors, preventing it from being an accurate spectroscope.

Fourth, why is a photon absorbed or reflected? In other words, what happens, in terms of energy, that causes a photon to bounce, be absorbed, or to have partial reflection/absorption?

There's no way to answer this in terms of energy. An accurate answer is difficult to give, but it involves the way that atoms and molecules interact with incoming EM waves. Classically, if the frequency is in the right range, the electrons in the atoms of the material can oscillate in such a way as to generate an EM wave that almost exactly matches the incoming wave, differing in perhaps phase or polarization. The energy of the incoming EM wave is transferred to this outgoing wave with varying amounts of efficiency that depend on a number of details that I won't get into.

An accurate explanation involving photons is much more complicated and well beyond my expertise.