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Author Topic: Do photons have colours?  (Read 8250 times)

Offline chiralSPO

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Re: Do photons have colours?
« Reply #25 on: 10/07/2015 03:31:56 »
light is em wave? or photon particle? or both?

both, just like electrons are particles and waves.

why is powerful laser particle beams able to cut metal but not bend flame?

I don't understand why you think being able to bend a flame is important or relevant. Of course a laser isn't going to move a flame, just like shooting the ocean with a machine gun doesn't move the ocean. There are still interactions between the flame and the photons, if you just look for it (for instance, a candle flame casts a shadow if you shine a bright light on it. (
)

Light does have momentum, and it has been harnessed by a light sail:

how sunlight heats up water? photons hit water atoms so electrons in water jump to higher orbitals?

That's part of it, although, you'll notice that water is essentially colorless, so it's not visible light that's getting absorbed. Some is ultraviolet, but mostly it's infrared light that's getting absorbed by the water. Instead of exciting the electrons, it causes the water molecules to vibrate directly (get warm). https://en.wikipedia.org/wiki/Infrared_spectroscopy
This is also what makes water and carbon dioxide greenhouse gases.

if you answer those questions to understandable logic, i'll accept your theory of light.

Good. I guess my work here is done. Thanks for accepting my theory.

all scientist can be wrong,  before science able to explain everything in detail perfectly. like how electrons emit photons.

It's true, all scientists can be wrong. But it will be another scientist who disproves them.
 

Offline jccc

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Re: Do photons have colours?
« Reply #26 on: 10/07/2015 03:46:12 »
why is photon particle beam able to cut metal not move flame? not relevant?

shooting gun able to move water, produce water flow, observable. move a flame is not move ocean. your argument is pretty weak.

agree?
 

Offline jccc

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Re: Do photons have colours?
« Reply #27 on: 10/07/2015 04:17:38 »
is any light sail bigger than earth?

why is sunlight photons momentum not sailing earth? did we changed orbit?

if sunlight has momentum, the space station will be pushing away from the sun, did it happen?

 

Offline evan_au

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Re: Do photons have colours?
« Reply #28 on: 10/07/2015 11:40:04 »
Quote from: jccc
most photons are color less, like radio wave photons, x-rays.
I think you need to define what you mean by "color-less"; I assume you mean "invisible to humans".
  • Bees can use ultraviolet light to select flowers with nectar, even though it is invisible to humans.
  • Some snakes can use infra-red light to catch mice, even though it is invisible to humans.
  • An AM radio can detect frequencies around 1 MHz, even though it is invisible to humans
  • An X-Ray machine can detect X-Rays to detect broken bones, even though it is invisible to humans.
  • The mantis-shrimp can detect subtle hues that are invisible to humans.
  • But all of these are electromagnetic radiation.
  • So almost any form of electromagnetic radiation can be detected or "seen" if the radiation is powerful enough, and you have the right sort of detector. 
We unify these different electromagnetic phenomena by describing these different parts of the  spectrum by the photon energy, which can also be determined by photon wavelength and frequency. These concepts are a generalisation of the term "color".

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how does an electron knows what color of photons should emit?
When an electron jumps between two orbitals of an atom, the energy difference between the two orbitals determines the color of the photon emitted (or absorbed).

There are also other mechanisms of emitting photons, depending on the frequency. For example, a radio antenna emits photons at a frequency where the electrons in the wire are driven backwards and forwards by the RF amplifier. (And in this case, the RF amplifier often obtains this frequency from the mechanical vibration of a quartz crystal*.)

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what direction?
Apart from lasers, where the stimulated photon has the same direction as the incoming photon, most photon emission mechanisms are fairly random in their direction.

To some extent, you can steer where photons appear by using reflection, refraction and interference, despite the original random direction.

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how many?

One per orbital change. Sometimes a photon can quickly cascade down a number of orbital changes, producing a shower of photons.

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what speed?

c (in a vacuum). It is lower in a non-vacuum.

*Don't get your hopes up that every form of electromagnetic vibration can be produced from a mechanical vibration. By the time you get to visible light and shorter wavelengths, mechanical vibrations of atoms & molecules are too sluggish to make a strong contribution to light output.
 

Offline lightarrow

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Re: Do photons have colours?
« Reply #29 on: 10/07/2015 16:59:36 »
I don't understand why you think being able to bend a flame is important or relevant. Of course a laser isn't going to move a flame, just like shooting the ocean with a machine gun doesn't move the ocean.
But of course it depends on the laser power.
Try with a 500TW laser like this one and then tell me:
https://en.wikipedia.org/wiki/National_Ignition_Facility#Impetus

Quote
In total these amplifiers boost the original 6 J provided by the PAMs to a nominal 4 MJ.[10] Given the time scale of a few billionths of a second, the peak UV power delivered to the target is correspondingly very high, 500 TW.

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lightarrow
 

Offline PmbPhy

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Re: Do photons have colours?
« Reply #30 on: 10/07/2015 21:05:59 »
Quote from: lightarrow
But of course it depends on the laser power.
Try with a 500TW laser like this one and then tell me:
https://en.wikipedia.org/wiki/National_Ignition_Facility#Impetus

Quote
In total these amplifiers boost the original 6 J provided by the PAMs to a nominal 4 MJ.[10] Given the time scale of a few billionths of a second, the peak UV power delivered to the target is correspondingly very high, 500 TW.
That has absolutely nothing to do with bending a flame. Let us not forget that a flame is basically the visible part of a gaseous part of a fire, nothing more and nothing less. Think of it like this. Consider a laser beam passing through a spherical ball of hot gas, so hot as to be visible, i.e. a spherical flame (what happens when you try to light a match in a zero-g environment). The properties of the gas are uniform so if the flame was bent as it went in and was deflected then it would break the symmetry of the setup and there's nothing in the symmetry which would allow for that. That's a layman's description and one that's easy to see for anybody.
 

Offline chiralSPO

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Re: Do photons have colours?
« Reply #31 on: 10/07/2015 21:24:16 »
why is photon particle beam able to cut metal not move flame? not relevant?

shooting gun able to move water, produce water flow, observable. move a flame is not move ocean. your argument is pretty weak.

agree?


Imagine you are looking at the ocean from 4 ly away. With a big enough telescope you can see the Earth, and even see that it has oceans, but good luck seeing any effect the bullets have. Same with photons and flame. The photons move some of the gas particles around, but nothing you could see, even with the best microscopes and high-speed cinematography.
 

Offline PmbPhy

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Re: Do photons have colours?
« Reply #32 on: 10/07/2015 21:57:53 »
Quote from: jccc
shooting gun able to move water, produce water flow, observable. move a flame is not move ocean. your argument is pretty weak.
Oh good Lord. I swore that I'd ignore you forever but when you make such horrible claims I just can't let them pass. You're deduction is totally without any reason whatsoever. His analogy is quite accurate. A photon can interact with the flame by interacting with individual atoms by exciting them to higher energy levels. The amount of momentum transferred to them by the photon is so small that there is virtually no cumulative effect on the flame itself. This is exactly analogous to shooting a wave with a gun. Your deduction skills are still as horrible as they ever were. Is it no wonder to you why I won't converse with you anymore?
 

Offline jeffreyH

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Re: Do photons have colours?
« Reply #33 on: 10/07/2015 22:21:39 »
He doesn't understand that it is elementary physics my dear Watson.
 

Offline wolfekeeper

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Re: Do photons have colours?
« Reply #34 on: 11/07/2015 04:05:23 »
why is sunlight photons momentum not sailing earth?
It essentially does.
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did we changed orbit?
Earths orbit is slightly different than it would have been if the Sun wasn't shining, but it has a similar effect as if the Sun was a bit less massive (sunlight pressure goes inverse square law just like gravity does.)
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if sunlight has momentum, the space station will be pushing away from the sun, did it happen?
Oh yes, certainly. Allowance for light pressure is routinely made with satellites. It definitely changes orbits, albeit only very slightly, and more significantly, causes torques on satellites.
 

Offline lightarrow

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Re: Do photons have colours?
« Reply #35 on: 11/07/2015 07:58:36 »
Quote from: lightarrow
But of course it depends on the laser power.
Try with a 500TW laser like this one and then tell me:
https://en.wikipedia.org/wiki/National_Ignition_Facility#Impetus

Quote
In total these amplifiers boost the original 6 J provided by the PAMs to a nominal 4 MJ.[10] Given the time scale of a few billionths of a second, the peak UV power delivered to the target is correspondingly very high, 500 TW.
That has absolutely nothing to do with bending a flame. Let us not forget that a flame is basically the visible part of a gaseous part of a fire, nothing more and nothing less. Think of it like this. Consider a laser beam passing through a spherical ball of hot gas, so hot as to be visible, i.e. a spherical flame (what happens when you try to light a match in a zero-g environment). The properties of the gas are uniform so if the flame was bent as it went in and was deflected then it would break the symmetry of the setup and there's nothing in the symmetry which would allow for that. That's a layman's description and one that's easy to see for anybody.
?
I had understood nothing of what you have written.
Anyway if you don't know what is "bending a flame" take a look at this:

Question: can you do the same with a 500TW laser?

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Offline PmbPhy

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Re: Do photons have colours?
« Reply #36 on: 11/07/2015 15:02:35 »
Quote from: lightarrow
I had understood nothing of what you have written.
Why? It was so clearly written to. What I said was basically this; You asked why if a laser is able to cut metal but not bend flame? Suppose it did bend the flame. In what direction would the flame be bent? Let's use a spherical flame like that which is created when you first strike a match in a zero-g environment like they did aboard a shuttle flight at one time. It doesn't burn long because oxygen can't get to the match head where the fuel is. However it can be done if done properly. So if they have a spherical flame and they pass a laser beam through the center of the flame then in what direction is the flame bent and why?

Quote from: lightarrow
Anyway if you don't know what is "bending a flame" ..
Who said that I didn't know what a bent flame is? I was asking what you had in mind as far as a bent flame.
There's a big difference there.

Quote from: lightarrow
...take a look at this:
Unrelated since the flame isn't bending but is being spread apart. The part of the gas with positive ions are pulled in one direction while the negative ions are pulled in the opposite direction. Hence the flame is parted, not bent.

Quote from: lightarrow
Question: can you do the same with a 500TW laser?
No. Absolutely not. There is nothing to cause the flame to bend and if there's no cause for it then it doesn't happen. Understand now?
 

Offline lightarrow

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Re: Do photons have colours?
« Reply #37 on: 11/07/2015 19:38:51 »
Quote from: lightarrow
I had understood nothing of what you have written.
Why? It was so clearly written to. What I said was basically this; You asked why if a laser is able to cut metal but not bend flame?
Not at all, it was jccc to ask this, I replied to him.
Quote
. Suppose it did bend the flame. In what direction would the flame be bent?
Obviously in the direction of k, that is of the beam propagation: photons have momentum and a part of this is transferred to the air/flame molecules after scattering.
Quote
Unrelated since the flame isn't bending but is being spread apart. The part of the gas with positive ions are pulled in one direction while the negative ions are pulled in the opposite direction. Hence the flame is parted, not bent.
You mean it's a linguistic problem?
I don't know if it moves in the same direction of the positive or the negative ions, but it moves. I see a bent flame and this is enough for me.

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Offline evan_au

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Re: Do photons have colours?
« Reply #38 on: 12/07/2015 06:56:37 »
Quote from: jccc
if you mix red, blue and green photons, what color photons you get?
if you mix white and black photons, what color photons you get?

I think there is a confusion here between the color that can be expressed by a single photon, and the colors that can be perceived by a human when viewing a light beam containing many photons, usually of different colors/frequencies/wavelengths.

The human visual system has three color receptors producing conscious images, which are most sensitive to photons of red, green and blue*. However, each receptor is also partially sensitive to other colors.

In principle, a single photon could excite only a single cone cell, and produce only the sensation of red, green or blue.

However, light intense enough for humans to perceive color contains many photons. If these photons have the same color (ie same frequency and wavelength, such as from a laser), the many photons can excite many cone cells at once. Depending on the relative strength of excitation, additional colors may be perceived which includes all the colors of the rainbow, including orange & yellow (for partial excitation of both red and green receptors) or violet (which excites the blue receptor, but has almost no response from the red & green receptors). 

There are many additional colors which do not appear in the rainbow, such as white and magenta. These can only be generated by a mix of photons of different frequency; for example the sensation of white requires a mix of photons which excite the R, G & B receptors roughly equally.

The sensation of white can also occur when the intensity of light falls below the sensitivity of the cone cells, leaving the rod cells to detect light; the rod cells are only "black and white". The sensation of Black occurs when the intensity of light is below the sensitivity of the human rod cells.

So a "Starry, Starry Night" appears predominantly as white stars on a black background, even though the different stars have quite different colors which can be seen in a long exposure photograph of the night sky.

* The cone cells are not very sensitive to light, so it takes many photons to evoke a sensation of color. But I'll ignore that here.
 

Offline Thebox

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Re: Do photons have colours?
« Reply #39 on: 12/07/2015 11:00:01 »
A Boothroyd asked the Naked Scientists:
   
 I have been thinking about it......  And i reckon the "back" of a photon "looks like" is color.  Was wondering if?  That could be the answer because it explains much.

What do you think?

There is no ''colour'' in or of a photon, ''colour'' is a process of interpretation by the brain or use of device. ''Colour'' is frequency difference, an energy difference detected by us and device. No more than a sound is not a sound until it is heard.
The constant of light in space is colourless, only by interaction of Em radiation with matter is ''colour'' detected by us.
''Colour'' is a wave breaking on a surface, a spectral diffusion of thermal differences. Signatures we detect by sight through the otherwise colourless constant of Em radiation. A radiation we evolved to see through, making a clarity of space, crystal clear to the eye.
« Last Edit: 12/07/2015 11:19:21 by Thebox »
 

Offline Colin2B

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Re: Do photons have colours?
« Reply #40 on: 12/07/2015 17:47:08 »
''Colour'' is frequency difference
not quite. The colour of the light is determined ny it's frequency, not the difference between that and any other frequency.
This is the same with sound where frequency = pitch
 

Offline David Cooper

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Re: Do photons have colours?
« Reply #41 on: 12/07/2015 18:08:49 »
In Einstein's model of reality, a photon should not have a specific colour because it has no specific frequency - its frequency varies infinitely as you test it from different frames of reference. Colour can only come into being when the photon interacts with something else (which happens at either end of its journey, and the two colours it has may be very different).
 

Offline wolfekeeper

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Re: Do photons have colours?
« Reply #42 on: 12/07/2015 19:41:32 »
No, theBox is precisely correct, colour is a perceptual thing, and it is mostly differences between colours that is perceived; that's because lighting varies enormously, but our perception interpolates around that and gives things a (mostly) stable colour for us.

The wavelength or frequency of a photon are related by Einstein's equations.
 

Offline David Cooper

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Re: Do photons have colours?
« Reply #43 on: 12/07/2015 20:56:09 »
No, theBox is precisely correct, colour is a perceptual thing, and it is mostly differences between colours that is perceived; that's because lighting varies enormously, but our perception interpolates around that and gives things a (mostly) stable colour for us.

If you're talking about the colour qualia in our heads, that's fine, but it doesn't apply to photons and their frequencies which have measurable values - the only way for these to vary is by varing the speed of collision of the detector with them.

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The wavelength or frequency of a photon are related by Einstein's equations.

Lovely, but if you accelerate towards a laser which is sending a beam of light towards you, the colour you detect will change, and if you stop accelerating at various times and declare yourself to be stationary while you determine what colour the light is, you will produce a variety of different answers which demonstrate that the light is not tied to any specific colour (unless you're working under a different theory like LET where the colour of a photon has an absolute frequency.)
 

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Re: Do photons have colours?
« Reply #43 on: 12/07/2015 20:56:09 »

 

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