Does light have a temperature?

[Natski]

Does light have a temperature? Often in photography they discuss color temperature, but does this mean light has a measureable, real temperature?

 

DOES LIGHT ITSELF HAVE A TEMPERATURE?

[danny8522003]

Im fairly sure the answer is no. Objects have temperature because they vibrate (or wobble as someone on here used to say) at a certain rate. Light however is a wave and doesnt vibrate.

 

I think the temperature they refer to is how the light is perceived within the picture, like how soft light differs from spotlights. A bit like how orange is a warm colour and blue is a cold colour.

[Martin]

Does light have a temperature? Often in photography they discuss color temperature' date=' but does this mean light has a measureable, real temperature?

 

DOES LIGHT ITSELF HAVE A TEMPERATURE?[/quote']

 

most of the light you see is thermal (I will define later) radiation and has a certain temperature

which is defined as the temp of a blackbody radiator that would produce that mix of wavelengths

 

it is not as weird as it sounds

 

I cant answer completely now but will get back to it, its just a matter of defining how you use the words

[Martin]

temperature is a characteristic of a system in equilibrium---you have to be able to imagine putting a box around something and waiting until it "equalizes" inside the box

 

if it hasnt equalized yet----if there is an ice-cube melting at one end of the box and a candle burning at the other end of the box----then it doesnt have a well-defined temperature yet.

 

so there is actually a common sense aspect to the idea: you have to know when you have waited enough that things have reached (approximate) equilibrium

 

please imagine a box from which everything has been removed that you possibly can remove, except some light

 

there is no air in the box, only light

 

is there a temperature inside the box?

[Martin]

you can imagine the inside walls of the box are perfect mirror and the light is bouncing around in there for all eternity

 

but I rather think of it with the box insulated from outside and the inner walls black, and the light in the box is absorbed and re-radiated over and over

 

and I wait until the light in the box has reached (approx) EQUILIBRIUM with the walls, which is necessry for the concept temp to even be defined, and I imagine magically sticking a thermometer in thru one of the walls of the box

 

so now the bulb of the thermometer is in the middle of the light inside the box and the bulb of the thermometer also comes into equilbrium with the light!

 

and YESSSS! it turns out the light does in fact have a temp

[Martin]

BTW the temperature of unobstructed sunlight is the same as the temperature of the surface of the sun

 

BTW the temperature of the cosmic microwave background is about 2.7 degrees kelvin

 

 

because the mix of wavelenghts of the CMB is almost exactly the same mix as would be radiated by a dark object which was at 2.7 degrees kelvin

 

 

it would be ok if you want to say to yourself that the light doesnt really have a temperature that this is just ARTIFICIAL TECHNOSPEAK scientific jargon. you can say that the light does not have any temperature it is only the WALLS OF THE BOX that have temperature and what they call the temperature of the light is really MERELY THE TEMP OF A DARK WALL OR OBJECT WHICH WOULD RADIATE THE SAME MIX OF WAVELENGTHS

 

it is fine with me if you want to think that way, that is just a difference at the level of language.

 

but for me I think of everything including the light, in an isolated system at equilibrium, as having the same temp. so i think of the light has having that temp. that the other stuff does. so for me, it really does have a temperature

 

there is a catch though. light from a LED or a laser, that is a single wavelength, is not a THERMAL MIX of wavelengths. what I am talking about is light that has the kind of lopsided bellcurve distribution of wavelengths that you get from a glowing object, or a litebulb filament, or the surface of the sun, or a brick in your fireplace

 

monochromatic, or other kind of nonthermal light is a more complicated thing to talk about the temperature of. somebody else can discuss that, not me.

[J.C.MacSwell]

BTW the temperature of unobstructed sunlight is the same as the temperature of the surface of the sun

.

 

Martin, distance from the sun must have some effect. Like an inverse squared rule. Wouldn't we be in trouble otherwise? (atmosphere notwithstanding)

[J.C.MacSwell]

there is a catch though. light from a LED or a laser' date=' that is a single wavelength, is not a THERMAL MIX of wavelengths. what I am talking about is light that has the kind of lopsided bellcurve distribution of wavelengths that you get from a glowing object, or a litebulb filament, or the surface of the sun, or a brick in your fireplace

 

monochromatic, or other kind of nonthermal light is a more complicated thing to talk about the temperature of. somebody else can discuss that, not me.[/quote']

 

If you summed the energy over whatever MIX of wavelengths you had you would have an equivalent of some black body radiation temperature.

 

This could be considered the temperature in some respect, although I don't think it would be at the same entropy.

[Martin]

Martin, distance from the sun must have some effect. Like an inverse squared rule. Wouldn't we be in trouble otherwise? (atmosphere notwithstanding)

 

MacSwell, good to see you! From general quality of your posts I think you would find it easy to write a short essay about this (presumably noobie) question "does light have a temperature?"

 

If you would like to at all, I would certainly appreciate it.

 

My attitude (which you dont have to agree with) is that much of the light we experience around us is approximately thermal or blackbody and

 

(1) physicists and astronomers are always talking about the temperature of BB radiation-----e.g. the microwave background----so the idea of light having a temperature is pragmatically meaningful

 

(2) there are two ideas, roughly corresponding to the terms "color temperature" and "brightness temperature"-----you can measure the temp of BB radiation using the fourth power law, by its ENERGY DENSITY or irradiance or something along those lines, or you can measure it by fitting a planck curve to the wavelength MIX

 

What I am hoping for is, if possible, a simpatico explanation of this, that gets across the idea that it is meaningful to talk about the temperature of light, without making it sound too complicated.

 

I wrote some immediate responses to the original poster (who didnt reply) and may not have been clear enough. If you feel the urge, I'd be happy if you would make a fresh start.

 

the Stefan-Boltzmann fourth power law is (like maxwell equations) one of the most beautiful, classically elegant, things in physics. I think had you not chosen the handle "MacSwell" you might have done well to choose the name "BoltsMan".

 

Someone should discuss the fourth power law for noobs. Want to?

[Martin]

the original poster Natski

asked the identical question on physicsforums

http://www.physicsforums.com/showthread.php?t=85728

and got essentially the same answer

 

"Does light have a temperature? Sure does!"

 

I dont think we have to worry about Natski

[J.C.MacSwell]

MacSwell' date=' good to see you! From general quality of your posts I think you would find it easy to write a short essay about this (presumably noobie) question "does light have a temperature?"

 

If you would like to at all, I would certainly appreciate it.

 

My attitude (which you dont have to agree with) is that much of the light we experience around us is approximately thermal or blackbody and

 

(1) physicists and astronomers are always talking about the temperature of BB radiation-----e.g. the microwave background----so the idea of light having a temperature is pragmatically meaningful

 

(2) there are two ideas, roughly corresponding to the terms "color temperature" and "brightness temperature"-----you can measure the temp of BB radiation using the fourth power law, by its ENERGY DENSITY or irradiance or something along those lines, or you can measure it by fitting a planck curve to the wavelength MIX

 

What I am hoping for is, if possible, a simpatico explanation of this, that gets across the idea that it is meaningful to talk about the temperature of light, without making it sound too complicated.

 

I wrote some immediate responses to the original poster (who didnt reply) and may not have been clear enough. If you feel the urge, I'd be happy if you would make a fresh start.

 

the Stefan-Boltzmann fourth power law is (like maxwell equations) one of the most beautiful, classically elegant, things in physics. I think had you not chosen the handle "MacSwell" you might have done well to choose the name "BoltsMan".

 

Someone should discuss the fourth power law for noobs. Want to?[/quote']

 

Thanks Martin, I'm intrigued but not well grounded. If this is a noobie question then I am certainly a noobie. I usually post trying to gain insight or add something without misguiding anyone. If my posts are short it is easier for someone like Swansont or yourself to set things straight. I may be "Macswell" but I'm certainly no Maxwell.

 

In short, this is an area where I see a lot happening and appreciate some of the concepts but have trouble separating the "music" from the noise.

[swansont]

the original poster Natski

asked the identical question on physicsforums

http://www.physicsforums.com/showthread.php?t=85728

and got essentially the same answer

 

"Does light have a temperature? Sure does!"

 

I dont think we have to worry about Natski

 

Well, that discussion wasn't exactly clear-cut. The notion that a bunch of photons, in and of themselves, won't tend to equilibrate clouds the issue a little but I have to agree that the temperature of light is the same as its source, if it's a blackbody, and that not all light is in thermal equilibrum.

[J.C.MacSwell]

Martin, distance from the sun must have some effect. Like an inverse squared rule. Wouldn't we be in trouble otherwise? (atmosphere notwithstanding)

 

Another thought on this. Does focussed light gain temperature?

[MIG]

First we must see what is the temperature and then to discuss

[MIG]

temperature: A measure of the average energy of a system of atoms. or:

 

A measure of the speed of motion of a typical atom or molecule in a substance.

or:

the degree of hotness or coldness of a body or environment (corresponding to its molecular activity)

the somatic sensation of cold or heat

http://www.cogsci.princeton.edu/cgi-bin/webwn2.1

 

Temperature is the physical property of a system which underlies the common notions of "hot" and "cold"; the material with the higher temperature is said to be hotter.

en.wikipedia.org/wiki/Temperature

 

A measure of the energy in a substance. The more heat energy in the substance, the higher the temperature. The Earth receives only one two-billionth of the energy the sun produces. Much of the energy that hits the Earth is reflected back into space. Most of the energy that isn't reflected is absorbed by the Earth's surface. As the surface warms, it also warms the air above it.

eobglossary.gsfc.nasa.gov/Library/glossary.php3

 

The temperature is a measure of the internal energy that a substance contains. This is the most measured quantity in the atmosphere. 1

gcrgweb.sdsu.edu/rosaparkswx/glossary.html

 

The temperature of a sample of matter is a measure of the average kinetic energy of the molecules in that sample.

http://www.physchem.co.za/Common%20Files/Glossary.htm

 

a measure of how hot or cold a place is

http://www.nationmultimedia.com/edu/edu_back/may_8_14/vocab.html

 

A degree of hotness or coldness the can be measured using a thermometer. Also a measure of how fast the atoms and molecules of a substance are moving (see Kinetic energy). Temperature is measured in degrees on the Fahrenheit, Celsius, and Kelvin scales.

http://www.ucar.edu/educ_outreach/webweather/glossary.html

 

the degree of hotness or coldness as measured on some definite temperature scale

k12.ocs.ou.edu/teachers/glossary/t.html

 

A physical quantity characterizing the mean random motion of molecules in a physical body.

http://www.austin360.com/weather/content/shared/weather/weather_glossary.html

 

measurement of the average kinetic energy of the individual atoms of a substance or object.

http://www.ocean-institute.org/edu_programs/materials/P/Glo/S_Glos.htm

 

a measure of the random motion energy (the average kinetic energy) of a group of particles in a gas, liquid, or solid. The temperature is higher if the particles are moving faster.

http://www.astronomynotes.com/glossary/glosst.htm

[swansont]

Another thought on this. Does focussed light gain temperature?

 

No, just like a bucket of water doesn't "gain temperature" when compared to a spoonful.

[MIG]

if this definitions are ok then it's impossible to talk about temperature of photons (light)

 

-When concentration of photons hit some atom it will cause movement of the atom

-Bigger the concentration faster hi will travel ( or hi will have bigger temperature )

[J.C.MacSwell]

No, just like a bucket of water doesn't "gain temperature" when compared to a spoonful.

 

Then (if my thinking is correct) the idea I suggested in post 7 is wrong then and the temperature of unobstructed sunlight, as Martin said, is at the same temperature as the sun.

 

So it's the same whether measured here or on Pluto?

 

Obviously it is "diluted", but not in temperature?

[swansont]

Then (if my thinking is correct) the idea I suggested in post 7 is wrong then and the temperature of unobstructed sunlight' date=' as Martin said, is at the same temperature as the sun.

 

So it's the same whether measured here or on Pluto?

 

Obviously it is "diluted", but not in temperature?[/quote']

 

 

Right. Same temperature, but fewer photons. Temperature tells you the average energy of the ensemble. So just like a drop of water and a bucket of water can be at the same temperature, the bucket contains more thermal energy because there's more water. If you added a fixed amount of energy to either, the drop's temperature would go up more.

[YT2095]

just a side question for clarity (I think it Might relate to this thread a little too).

 

if you have a 1cm width beam from a 5 milliwatt laser and the same from a 500milliwatt laser, both the same color frequency, what changes or rather what`s the difference?

 

my guess is that the more powerfull laser won`t have faster photons as that`s a fixed speed, and the distance per peak to peak cycle will be the same for both too, the only other variable would be photon density wouldn`t it?

 

as in more photons per given area hitting a target, and THAT is when the temp thing is detectable.

 

yes/no?

[x__heavenly__x]

hi martin,

there was an idea i posted looong back, abt a shpere of glass reflecting light inf, but the glass is special since it should not!!! absorb the vibrational energy of the photons(a kindof temp?)...

 

and it seems that the light would fade in the sphere after some tmime(get absorbed)...well the post was entitled light storage something, i have forgotten sorry

[Martin]

just a side question for clarity (I think it Might relate to this thread a little too).

 

if you have a 1cm width beam from a 5 milliwatt laser and the same from a 500milliwatt laser' date=' both the same color frequency, what changes or rather what`s the difference?

 

my guess is that the more powerfull laser won`t have faster photons as that`s a fixed speed, and the distance per peak to peak cycle will be the same for both too, the only other variable would be photon density wouldn`t it?

 

as in more photons per given area hitting a target,...

 

yes/no?[/quote']

 

I would defer to swansont about this, but yes I think that says exactly what the difference is-----the beamwidth is the same in both cases, and one laser is more powerful----and the energy carried by each photon is the same in each case. So the difference has to come down to sheer number of photons hitting the target per unit time.

[Martin]

this doesnt have too much to do with temperature, because nothing is in equilibrium (the light hasnt settled down to a thermal mix by being absorbed and re-radiated and it hasnt gotten into a comfortable balance with the surroundings) and we arent studying an isolated system of some light in some definite box

 

the picture here, with the laser, is more analogous to the box with a candle burning at one end and an ice-cube melting at the other----the over all temperature of the system is not well-defined. lasers are very non-equilibrium-type creatures. (swansont please correct if wrong)

 

but YT even though this is not directly about temperature, it is interesting.

 

One could, out of simple curiosity, ask HOW MANY photons per second are coming from the half-watt laser?

 

You YT, who hate all arithmetic with great vehemence, should acquire the ability to estimate this

 

I will explain how, unless someone else gets to it first. It is easy.

[MetaFrizzics]

The picture seems too literal for me.

I would postulate that the issue isn't # of photons per se, but some percentage of efficiency in the laser indicating how many of the photons were actually ordered and polarized as desired. Probability functions would come into play, and indicate the 'purity' of the laser beam.

 

Secondly, the milliwatt rating of the laser beam unit would not be an accurate estimator of the number of photons in the beam either. These numbers (usually on the housing of the laser unit) are indications of power consumption and suppy alone, and give no indication of the efficiency of the laser design.

[Martin]

how many photons does a half-watt laser make?

it will depend on the wavelength, which we decide on later

 

For the sake of discussion I shall assume that the half-watt is NOT the power consumption (in my experience with lasers that is NOT what the number on the case refers to) but is the light power output. For this calculation, the nice simple number you have to look up in the HandBooke (and which Severian knows by heart) is 2E-25 that is 1.98 but call it 2, times 10^-25.

 

this is the metric value of hc, namely 2E-25 joule meter

 

what this means is very very simple and should be learned in grammar school namely that IF YOU HAVE A PHOTON WITH WAVELENGTH ONE METER then the blooming photon has an ENERGY OF 2E-25 JOULES.

 

after that it is common sense to find out how many photons come in one second because say the light is some say GREEN light of half a micron wavelength, then it is shorter by a factor of 0.5E-6

which makes the photon more energetic by the reciprocal, a factor of 2E6

 

The resourceful arithmetic-loathing YT can see immediately that a single green photon must have energy 4E-19 JOULES

 

But in one second the 500mW laser puts out HALF A JOULE

 

and so it is no sweat to find how many photons in one second.