# How are photons coloured

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##### Guest
What determines the colour of a photon & how is it shifted as in another topic. Now I think of it: what is a photon

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#### nacnud

##### Guest
A photon is a packet of electromagnetic energy. The amount of energy the packet has determines its colour. In fact a photo can have any 'colour' in the electromagnetic spectrum.<br /><br />For example a blue photon has more energy than a red photon.<br /><br />The amount of energy in the photon is linked to the frequency of the photon through the formula<br /><br />E=hf<br /><br />Where:<br />E=Energy<br />h=planks constant<br />f=frequency<br /><br /><br />See also the wikipedia entry for a photon<br />

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##### Guest
The frequency of the photon determins its color. Since wavelength is inveresely proportional to the frequency, you could just as well say that the wavelength of the photon determins its color.

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#### centsworth_II

##### Guest
Both nacnud and maddad are correct. Increased energy and increased frequency (decreased wave length) go hand in hand. But remember, color is only the brain's way of interpreting the environment. The photon no more has color as a property than a hot plate has pain as a property, even though when you touch it, the brain interprets the heat as pain. <br /><br />Of course it's useful to describe photons according to the color perception they would stimulate in the human brain as it interprets nerve signals from the eye's retina. But based on this criterion, not all photons have color. <br /><br />Photons having wavelengths corresponding to the infrared or ultraviolet are not registered by the human brain and so to us are "colorless". And what about photons with wavelengths (or frequencies, or energies) corresponding to radio waves, or x-rays. We definately do not assign them a color even though they are the same photons that we interpret as colors when their energy places them in the visible part of the spectrum. <br /> <div class="Discussion_UserSignature"> </div>

S

##### Guest
Well there's no way to formulate my question as all the words tend to circulatory answers. <br />My problem, in one instant, gravity changes the frequency but as the speed of light is constant the change can't be by acceleration & neither by mass since photons are massless.<br />So how are photons colours changed?

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#### nacnud

##### Guest
By changing the energy of the photon which effects the frequence of the photon.

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#### paintwoik

##### Guest
<blockquote><font class="small">In reply to:</font><hr /><p>The frequency of the photon determines its color. Since wavelength is inveresely proportional to the frequency, you could just as well say that the wavelength of the photon determins its color.<p><hr /></p></p></blockquote><br /><br />This is the same thing as saying that the color red is red. So the question remains unanswered.<br /><br /><br />

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#### centsworth_II

##### Guest
<font color="yellow">"...in one instant, gravity changes the frequency....<br />So how are photons colours changed?"</font><br /><br />The "color" changes <b>whenever</b> the frequency changes. So if you understand, or accept, the frequency change, then the reason for the color change goes along with, is the same as, the reason for the frequency change.<br /><br />If you don't understand the reason for the change in frequency, then its better to try to understand that first, rather than bringing up the question of color. Also, are you refering to a specific instance, such as the red shift of light as seen in rapidly receding galaxies? If so, maybe others can try to explain that particular situation to you. I don't feel qualified to do it off the top of my head. <div class="Discussion_UserSignature"> </div>

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#### nacnud

##### Guest
Ok try this though experiment:<br /><br />Standing on the Earth and shining a torch upwards, the light leaves the Earth and travels off into space, we know that it takes energy to climb out of the Earth gravity but we also know that the speed of light is constant in a vacuum (sorry everyone I’m assuming the earth has no atmosphere for this so you had better hold your breath for the next five minutes <img src="/images/icons/smile.gif" /> )<br /><br />So how does the photon loose energy as it climbs?<br /><br />The answer is that the frequency of light changes, it becomes slower. Note that this is the speed of oscillation of the light wave/particle not the velocity with which that wave/particle moves. Equating this to colours gives the appearance that the light has been shifted towards the red or lower frequency part of the spectrum.<br /><br />In a similar manner shinning the torch at the ground would cause the light to become blue shifted as it gains energy.<br />

S

##### Guest
<blockquote><font class="small">In reply to:</font><hr /><p>So how does the photon loose energy as it climbs?<p><hr /></p></p></blockquote><br />Exactly! that's all I'm after. I've already tried two mechanisms above that don't seem reasonable.

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#### unclefred

##### Guest
This thread has evolved a lot since its start. However one can at least talk about the latest formulation of the question: SO HOW DOES THE PHOTON LOOSE ENERGY AS IT CLIMBS? <br /><br />There are several ways to look at it:<br /><br />Possible Explanation #1: Time is slower in the high gravitational field, thus the frequency (and wavelength) just change automatically as the photon moves to a different gravity field See: http://instruct.tri-c.edu/fgram/web/rel-gen.htm . A web search will give dozens more.<br /><br />Possible Explanation #2: Apply the equivalence principle. Consider light propagating from the floor to the ceiling of an elevator (which is assumed to be in free space away from any significant mass) . If the elevator is moving at constant speed, the light received is the same frequency as the light sent. If the elevator is accelerating, the light will be a Doppler shifted to the red. To produce the effect of gravity, the elevator must be accelerating. The elevator and a normal gravity equivalent way at looking at the situation. See: http://www.physics.utoronto.ca/~dey/GR.html and lots of other web sites.<br /><br />Possible Explanation #3: Solving for mass from the equation E=mc2, and computing the energy lost as if it were a simple Newtonian problem, gives the correct result. This will really rile some relativity buffs that the kiss (keep it simple) approach works. See: http://arxiv.org/ftp/gr-qc/papers/0403/0403082.pdf and other web sites.<br />

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#### Leovinus

##### Guest
One interesting thing about colors that I learned from my days in Physics classes:<br /><br />Electrons are at various fixed energy levels in an atom. When an atom's electron moves from a high energy level to a non-so high energy level, it gives up a photon which carries that energy away. The energy of the photon, as described above, determines its frequency and hence its color if it is in the visible spectrum.<br /><br />In this way, certain atoms give off only certain colors. One can look at a spectrum and determine what atom the color came from. For example, sodium has a unique yellow color which identifies it. <div class="Discussion_UserSignature"> </div>

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#### igorsboss

##### Guest
This is called "Fluorescence".<br /><br />This is how Neon, Sodium, and Florescent light bulbs work. Electrical energy excites the atoms, which, at a slightly later time, give up that energy again in the form of photons.<br /><br />These photons each have the same energy, hence the same frequency, hence the same color.<br />

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