# I Colored Light or Colored Object?

#### Odal

Is it possible to modulate light as it is to modulate sound.?That is, taking "white" light and changing its color without using a filter or another object like a prism?

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#### berkeman

Mentor
Welcome to the PF.

Filtering a color out of white light is not modulation, it is just selectively filtering the many colors of white light to get one particulat color.

Modulation has a technical meaning -- it means modifying a waveform in amplitude, phase or frequency:

https://en.wikipedia.org/wiki/Modulation

If you have a light wave or a sound wave propagating through your apparatus (pickup antenna, modulator, transmit antenna), you can do some things to it, but other things are harder. For example, amplitude modulating a sound wave or a light wave with such a setup is straightforward. Phase modulating them is harder, and frequency modulating them is very specialized if possible at all. (Assuming you mean just doing something to the propagating waveform, as opposed to converting it to electrical signals that you process and re-transmit with an antenna or transducer)

Can you say more about the context of your question? That will help us to give you the best answers that we can.

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#### rootone

White light is what we see as a fairly evenly mixed wavelengths of light within the visible spectrum.
Not surprising really since we evolved on Earth and the peak wavelengths of the Sun are in that range.
The equivalent with sound would be 'white noise', a vague hiss comprising of many frequencies.
It is possible to filter both light and sound so that some chosen frequencies are more or less apparent.
That isn't modulation though. it's just cutting out the bits you don't want.

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#### sophiecentaur

Gold Member
Is it possible to modulate light
Did you do a search on "Optical Modulators"? Does this question come from personal curiosity or did you have an application in mind?
Light Modulation is more or less what the LCD cells in front of a conventional back lit LCD monochrome TV display do. The cells Modulate the Amplitude of the light.
But modulating the amplitude is easy.
Modulating the frequency of a beam of light is much harder. It is possible to 'mix' optical signals to produce a sum or difference frequency (it's much easier to achieve this with Radio Frequency signals and that's done in radio and TV receivers).

#### Odal

Thanks everybody for the reactions. Here is what I mean as succinctly as possible.
The theory says that white light (or sunlight if you prefer) is a mix of (all) colors. Beyond all the technicalities I wonder how that can be proven otherwise than by letting it through a prism like Newton did.
As far as I know, colors from a tv screen for instance, come from Red Green Blue filters. That is, white light goes through these colored filters and get the color of the filter, and that is exactly what I find intriguing. Could it be possible, however that is done, to produce colors without those filters? I used the term modulation as an analogy, so please do not hold me to that term. The point is we can change how a sound sounds, can we then change the color of say a beam of light without letting it pass through filters which are already colored?

#### berkeman

Mentor
That is, white light goes through these colored filters and get the color of the filter, and that is exactly what I find intriguing. Could it be possible, however that is done, to produce colors without those filters?
That's how a color LCD display works, but not how an LED display works...

#### Odal

That's how a color LCD display works, but not how an LED display works...
I am afraid I do not see how that is relevant. I do not know enough about the differences between both. Do you mean to say that LED screens already do what I am asking about? If so, I would be very curious how it is done.

#### berkeman

Mentor
I am afraid I do not see how that is relevant. I do not know enough about the differences between both. Do you mean to say that LED screens already do what I am asking about? If so, I would be very curious how it is done.
Red, green and blue LEDs are used to generate the light. They do not start with white light and filter the light. So colored LEDs generate their colored light in a fundamentally different way compared to colored incandescent bulbs. And I thought that was what you were looking for...

Mentor

#### Odal

It is certainly very interesting, but what I make of it still does not answer my question. I may be wrong but this is how I understand electro-luminescence. Crystals when put under electric current produce colored light. The color is apparently linked to the kind of crystal. If I am right, then it is the same thing as having white light going through a colored filer, only in the case of leds, it is the filters themselves that give off light?
Did I understand it wrong?

edit: it is somehow comparable (only an analogy!) to cathode rays. They all have different colors depending on the gases in the tube, right?

#### berkeman

Mentor
Crystals when put under electric current produce colored light.
If I am right, then it is the same thing as having white light going through a colored filer, only in the case of leds, it is the filters themselves that give off light?
No, it is fundamentally different. Have you learned yet about how electron energy level transitions in atoms can give off photons?

#### Odal

No, it is fundamentally different. Have you learned yet about how electron energy level transitions in atoms can give off photons?
It would help if you explained how it is fundamentally different. The first thing that comes to mind are the black-body experiments that started the whole quantum adventure. But please let us keep to crystals and leds.

#### berkeman

Mentor
please let us keep to crystals
First can you say what you mean by "crystals"? Can you post some links to the reading you have been doing about crystals generating light? Thanks.

#### Odal

First can you say what you mean by "crystals"? Can you post some links to the reading you have been doing about crystals generating light? Thanks.
The Wikipedia article in your previous posts. Again I apologize for my imprecise terminology, it refers to different materials used in the diodes. Here is a quote from this article:
"

Green electroluminescence from a point contact on a crystal of SiCrecreates Round's original experiment from 1907.
Electroluminescence as a phenomenon was discovered in 1907 by the British experimenter H. J. Round of Marconi Labs, using a crystal of silicon carbide and a cat's-whisker detector

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#### rootone

You are not seriously expecting an answer?
I was expecting a question,

#### Odal

I was expecting a question,
I have trouble taking your remark seriously, and I would not know how to respond without rehashing the whole thread.

#### berkeman

Mentor
You are not seriously expecting an answer?
Well, we are trying to help you, and trying to determine your background and level of knowledge.

Please do some reading and then tell us why incandescent lights are fundamentally different from LED lights. That's a good start. Thanks.

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#### Odal

Well, we are trying to help you, and trying to determine your background and level of knowledge.

Please do some reading and then tell us why incandescent lights are fundamentally different from LED lights. That's a good start. Thanks.
I have been reading for years on the subject, even though the last time I had formal lessons in Physics was in High school, in the 60's and 70's.
I have read the article you kindly linked and gave my opinion. I take your remarks very seriously, but I cannot take any remark seriously, not when it shows a complete disregard of what has been written in the thread.

#### pinball1970

Gold Member
the last time I had formal lessons in Physics was in High school, in the 60's and 70's.
I want to take you to the start then

There is a difference between "colour" and "light" in the sense you are talking about - they are not the same thing. Colour is a process not a thing in itself.
A light source (say the sun) hits an object (say a flower) some of that light is absorbed and some is reflected into your eye which are received by specialized cells in your eye, this stimulation is relayed to the brain where a "colour" is assigned. You "see" it. There are many steps to get from the light source to the picture in your brain. White light is all the visible frequencies you are able to detect ROYGBINV. To change the "colour" of any individual light you would have to change the wavelength as has been mentioned

#### Odal

I want to take you to the start then

There is a difference between "colour" and "light" in the sense you are talking about - they are not the same thing. Colour is a process not a thing in itself.
A light source (say the sun) hits an object (say a flower) some of that light is absorbed and some is reflected into your eye which are received by specialized cells in your eye, this stimulation is relayed to the brain where a "colour" is assigned. You "see" it. There are many steps to get from the light source to the picture in your brain. White light is all the visible frequencies you are able to detect ROYGBINV. To change the "colour" of any individual light you would have to change the wavelength as has been mentioned
And I take you to use your logic instead of mindlessly repeating what is in textbooks. My question is very simple and you did not answer it.
You can chant all the conjurations you can think of, but here is again my simple question: is it possible to produce colored light without already colored objects or filters?

edit: to use your own expression, can you change the wavelength of light (what I meant non-technically by modulation) and therefore change how we see its color, and this once again, without making use of colored objects or filters?

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#### sophiecentaur

Gold Member
I have been reading for years on the subject, even though the last time I had formal lessons in Physics was in High school, in the 60's and 70's.
I have read the article you kindly linked and gave my opinion. I take your remarks very seriously, but I cannot take any remark seriously, not when it shows a complete disregard of what has been written in the thread.
@Odal It may be a good idea for you to rewind and start again here. I know of no one on PF who's more helpful and co operative than @berkeman so I can assure you he is trying to help and not just to argue.
Your OP has only one real answer to it and that answer is no. That is not to say that there is not a process that takes light in of one frequency and dishes out the 'same' light with a different frequency (The word Colour only has meaning in the context of Human Vision; you have to use the words Spectrum, Frequency or Wavelength, which can a actually be measured.

#### sophiecentaur

Gold Member
You can chant all the conjurations you can think of, but here is again my simple question: is it possible to produce colored light without already colored objects or filters?
That question is actually meaningless; you would need to make it much more clear what you actually want to know.. You are annoyed because you are dissatisfied with the answers you are getting.
Firstly, you have to realise that you are highly unlikely to have found an error in established Science. Secondly, a more polite approach will yield a better response.

#### Odal

@Odal It may be a good idea for you to rewind and start again here. I know of no one on PF who's more helpful and co operative than @berkeman so I can assure you he is trying to help and not just to argue.
Your OP has only one real answer to it and that answer is no. That is not to say that there is not a process that takes light in of one frequency and dishes out the 'same' light with a different frequency (The word Colour only has meaning in the context of Human Vision; you have to use the words Spectrum, Frequency or Wavelength, which can a actually be measured.
I am very grateful to you for answering my question directly instead of hiding behind common knowledge that is quite irrelevant to the issue here.
The answer is therefore: no, it is not possible to produce colored light without using already colored objects or filters?
Or, to put it in non-anthropomorphic terms: no, it is not possible to change the wavelength or frequency of a light beam without using filters or objects that absorb different wavelengths and frequencies?
This is a very serious matter with far reaching theoretical consequences, even if the practical consequences are inexistent.

ps: I certainly do not deny the willingness of @berkeman to help, and I do not think I have in any way shown any lack of respect. I did get irritated by some rude remarks from others and reacted accordingly.

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