# Size of a photon

Maybe it's been asked before but:
-What's the size of a photon
-What happens to it when it is 'absorbed' by a surface (like an opaque surface)
-What's the mass of a photon

That's what I can think of right now..

Thanks

:-)
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Commented:
A photon can be considered as particle associated to some electromagnetic radiation.

So some of its properties depend on the electromagnetic radiation concerned (eg. visible light, radiation with a wavelength between 400..800 nm)

Size : cann not be determined. In fact it has been proved that it can not be determined (principle of Heisenbergh).

Mass : with each photon there is some energy related. If energy would be expressed in J (Joules), you have normally very small numbers eg 10^ -19. Substituting those numbers in the well known relation of Einstein

E = m c ^ 2

and solving for m : m = E / c ^2, you can calculate the mass. (in kg per photon)

Suppose a photon has an energy of 1 * 10 ^ -19 Joules (somewhere in the visible light). The corresponding mass is then :

1*10^ -19 * (3 * 10 ^5) ^ 2 =

9 * 10 ^ - 9 kg

At the moment a photon interacts with some matter, if the energy of the photon is not similar to the difference in energy levels of electrons in an occupied orbital and an other non occupied orbital, nothing happens. So the matter is transparant to that type of light (or more general electromagnetic radiation).

If however the difference corresponds to the energy of the photon, then an electron is promoted towards a higher energy level using the energy of the photon (which ceases to exist ).

The next question is : what happens to the electron with the extra energy?

It can do several things :

1. remain some time in this energy and then fall back to the previous level, emitting light of the same wavelength as the original (fluorescence)
2. fall back to some other energy level, emitting light of a longer wavelength thanthe original (fosforescence)
3. induce chemical reactions.
4. transfer this energy into heat.

Hope this helps.

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Commented:
1) What is he size of a wave? Depends on how you define it. If you only take the upper par of it, is is small, but the wave really occupy a lot of water surface. The photon is the same think, you have a 90% of finding it in a ball of radius R, but the photon it self is not confined inside, and if you relax it to 80% of probabilities to find it inside, the radius is much bigger.

Even, a photon can be considered bigger when vibrating at slower rates given it has the same energy with it, as the amplitude of the wave is bigger.

So, the size of a photon is almost a nonsenese to ask... but well, here you have a number:

The characteristic size of a photon is inversely related to its frequency:
Lambda      = c/f

where

Lambda      = photon size
f      =      photon frequency
c      =      speed of light
=      3 × 108 m/s

2) The photon it self melt into a electron in the surface of some atom and gives all its energy to this electron -the mass, if any, is converted to energy, so assume no mass is present- that usually uses it to jump to a higer layer in the atom structure. If it can not stay there for long, as this layer may be unstable, it decay again to the layer where its belong, releasing a new photon that carries the energy i took.

3) Nothing. As it travels a speed of light, any mass would be bosted to infinite, so only energy is present. You can convert energy to mass if you want, but you shoul have to "stop" the photon, and you can't. Anyhow, energy is mass, so the answer is confusing, i know, but it has to be so, reality is confusing!
Commented:
The wavelength of a photon is inversely propotional to its energy.
The 'size' of a photon, or how precisely you can know its position, is inverely proportional to how precisely you know its momentum.

The term 'mass' is now generally taken to mean what was known as 'rest mass',
and the term "relativistic mass" has been replaced in favor of the term "energy"
So while a photon has energy and momemtum proportional to its frequency,
its 'mass' is 0 (but since a photon is never at rest, it's usually more useful to talk aout its 'energy' or 'momentum')
Commented:
Commented:
If you could know precisely the momentum of a photon, its "size" would be infinite, since could be located anywhere in space.
If you restrict the 'size' of a photon by puting it in a box or squeezing it through a hole,
you create uncertainty in its momentum.

Commented:
The size of a photon is a really hard question, perhaps meaningless.

By one measure, they are very small, as two beams of photons can cross and there are very very few interactions.

By another measure, if you put one photon through a device that can measure its momentum very accurately, then its position uncertainty, and therefore its extent, is very large.
Commented:
Since photons ony interact when they can exchange charged particles or gravitons, I don't think interaction cross section is a meaningful measure of size.
If size is defined as its physical extent, the answer is simple, it's just
(h bar)/(2*momentum uncertainty)
Which can be very small if you put the photon in a very small box, or very large if you measure its momentum very accurately (using a very large box)
Commented:
>Since photons ony interact when they can exchange charged particles or gravitons, I don't think interaction cross section is a meaningful measure of size.

Righto, but most of us (with a classical view of things) have the intuition that if two things pass by each other without (classically)  hitting, they've not intruded on each other's (classical) extent.

Commented:
Ocean wave sets can intrude on each other's extent without hitting.
Author Commented:
What is this "principle of Heisenbergh"?
I thought it must have a size, since I can see light through a hole in the wall, but not through a wall.. But I'm always overseeing laws of physics..

For the mass, does it mean that there are different masses in photons, depending on its wavelength? I would like some concensus if possible.. I've been trying to understand photons forever..

And why do they bounce anyway? Are they elastic? How come they don't just crash into surfaces like most other particles would? Or just go through, like most waves?

I need a brain..

:-)
Commented:
The principle of Heisenbergh states that it is impossible to measure or to define at the same time the speed and the place of any object. This is the case for any object (including macroscopic ones such as cars). However the uncertainity is for eg. cars so small that we can state that we can actually measure their speed and place at the same moment.

However for small particles this principle becomes very important.

As a collateral effect of this principle, for extremely fast objects (eg. photons) this means that the uncertainity on their position (and their size) becomes huge.

By the way, photons have a dual caracter. At the one hand they behave as particles, at the other as waves. So it the hole in your wall is small enough (in the order of magnitude of the wavelength) other fenomena such as diffraction occur.

Reflection on metals is due to the fact that the electrons in metals are not really bound to a specific atom. This makes that there are a lot of different energy levels available. The electron goes immediately to an other energy band (in metals we speek about energy bands and not about orbitals) and immediately fall back.

Hopes this helps
Commented:
>>I thought it must have a size, since I can see light through a hole in the wall, but not through a wall.
So you mean, that you can see through a hole, because there is empty space? Then why can you see through glass?
The space between atoms are filled by a lot of nothing .. only a neutron star (where the masses collapsed to a very dense matter by overwhelming the nucleus forces) is so dense, that i would dare to even try to say: there is no space for light to go through

Light doesn't go through the wall, because it interacts with the surface-electrons... the light is reflected or absorbed, but not lead through because there is a hole in it but the interaction doesn't fit to the specified wavelength

What may by declared as size of a photon?
Lets say what can't ever be declared as a size of it: Anything smaller than lambda/2 won't be the size of the photon, as you will never see any difference in beeing at any position +/- lambda/4 around the maximum of the amplitude of the wave... so if you want to measure something (with respect to length) with light, you have to measure things greater than lambda/2 ... so i would try call this the size...

regards Holger
Commented:
>>The principle of Heisenbergh states that it is impossible to measure or to define at the same time the speed and the place of any object.

Right. Down to physical measurement, it indicates that even the most accurate measuring device will alter, even slightly, the object measured, generating therefore innacurate results. For instance, when you use the ruler to know the length of a sheet of paper, the weight of the ruler will expand the surface of the paper, altering the length you wanted to measure. Same happens when you want to know the lbs of a tyre. You let some air go, and thus the lbs you obtained are not equal to those existing prior to your measurement. Even when you look at something, the object is altered, in the smallest detail, by the photons which hit the object and reflect to your eyes.

That is why some people consider that the conclussion of Heisenbergh's principle of uncertainty is that the truth is impossible to reach, scientifically proving Gorgias' (an ancient greek nihilist philosopher) statement that "nothing exists, if anything does exist it cannot be known, if anything exists and can be known, it cannot be communicated".

Brgds.,

germanpenn
Commented:
>>I thought it must have a size, since I can see light through a hole in the wall, but not through a wall.
It will be difficult to see light through a hole smaller than the wavelength of the photon.

>> For the mass, does it mean that there are different masses in photons
All photons have zero 'rest mass', which is what the word 'mass' generally means to physicists.
Photons have a 'relativistic mass', which is an outdated term meaning 'energy', that is proportional to its frequency (Planck's constant times the frequency)

>> And why do they bounce anyway?
They don't really bounce, they are absorbed and re-emited.
Commented:
What's the size of a photon?

It has no defined size

-What happens to it when it is 'absorbed' by a surface (like an opaque surface)?

Its energy usually show up as heat

-What's the mass of a photon?

It has no rest mass
Commented:
>>I thought it must have a size, since I can see light through a hole in the wall, but not through a wall.. But I'm always overseeing laws of physics..

Particles are not small balls, and aren't just like ocean waves, it is a mixture of both... well, the fact is that particles are not neither of this, they are other thing, but so separate from our daily experiences that we can not compare it with macroscopic things like balls or waves and understant the particles.

Thus, some time they act as balls, bouncing on surfaces, other, they act as waves, turning on the corners as the wave spread and reaching places not visibles from the origin of the light.

>>For the mass, does it mean that there are different masses in photons, depending on its wavelength? I would like some concensus if possible.. I've been trying to understand photons forever..

Particles are said to be more like a function of probability of finding it like a small ball in a given position. Like a normal bell that shows you that the most probable value to be measured by a experiment is around an average one, but can spread along with less probability as you move away from the central point.

What is the size of the bell? It covers all the posibles values for the experiment, so it's size is maximum posible. But, at the same time, if you only take as size the parts of the bell where probability to find the result of a single experiment is greater than 0.1, then the "size" of the bell can be a given number.

Imagine the experiment is to find a moving particle in the place where you left it one milisecond ago. The more heat this particle has, the more it moves, so the biger the "size" becomes, but if the particle is cold and frozen, it won't move a lot, so probasbilities are just around central point, and the size get smaller.

So "size" of a photon is like this, infinitum in one way, finite and smaller as less energy it has in other.
Commented:
> smaller as less energy it has in other.
Actually, with less energy it would tend to be larger.
Commented:
Yes, ozo, i was wrong in this point, but the general idea reamains, only that a less energic photon look less like a normal bell because it is more flat and less tall in its central point, so the places where prob. is > 0.1 becomes wider, but shorter, so the "size" becomes bigger, while the "density of probability of finding the ball" in the central point (if it makes sense to have a central point in a wave-like object) is lower.

With a lot of energy: Very concentrated Gauss bell. Portion above 0.1 is small (radius)

^
/    \
/      \
--          --

With less energy: Gauss bell very dispersed. Portion obove 0.1 wider but not as high.

___----¨¨¨¨¨¨----____
__---¨¨                                 ¨¨---___

Hope the graphs display OK in everyone's screen and font!
Commented:
The width of the Gauss bell is not so much a function of the amount of energy, as the uncertainty in the momentum.  (which, for photons, is proportional to the uncertainty in the energy)
Commented:
My understanding of all this stuff is more intuitive than theoretical, I didn't follow any theoretical phisic degree, but it interest me a lot, so believe ozo about any tech detaill!
Commented:
> (which, for photons, is proportional to the uncertainty in the energy)
That may be misleading, so perhaps I should try to clarify.
Total momemtum is proportional to total energy, but momentum las components in different directions, each of which can have its own uncertainty, so the extent of the photon can be different in different directions.
You might squeeze a photon through a hole in a wall, restricting its x,z extent,
and causing uncertainty in its x,z momentum, but it may still have a large y extent,
with less uncertainty in its y momentum.

Visible light can go through glass because the electons in the glass are too tightly bound to be easily shaken loose by the photons, so the light travels through without much interaction.
Commented:
>>Visible light can go through glass because the electons in the glass are too tightly bound to be easily shaken loose by the photons, so the light travels through without much interaction.
Nope - the resonating frequencies of glass-electrons (to shift them in higher energy levels) doesn't match the frequencies of visible light

take water - light goes through water, microwaves not (they boil the water) .. but microwaves are just same "light"-waves with a different frequency that matches the water.. so the "strong" boundage couldn't be the issue
Commented:
Bond stength affects resonant frequency, and I specified visible light, because ultraviolet light is absorbed by glass.
Micowaves do not match the resonant frequencys of water.  The water molecules heat up because they rotate in response to the changing E fields.
Author Commented:
What would be the smallest z and x that we could restrict the photons to? Has this been tried?

I heard that somewhere they can send ONE photon (fiber optics??) at the time.. That sounds to me that a photon is a unit (as opposed to a wave; we can't send ONE radio wave, can we?)

I'm still cracking my head here, all this input is great!

:-)
Commented:
Thats a prob for all physicists ...  wave or unit ... you can't decide as long as you don't have a means to detect... and then the photon will rturn to a state, that you want to detect ;-) .. because it IS both ... it's neither wave nor unit, just both

holger
Commented:
Like he said:

There's experiments that only make sense if photons are particles (like the photoelectric effect-- electons are knocked out of metals in milliseconds, even if the light intensity is waay too weak to send that amount of energy in a century).

Other experiments only make sense for waves (interference-- how could a single photon interfere with itself?)

Very peculiar stuff.   I think about it on my commute.  I'm surprised I havent rear-ended another car while pondering this stuff.

Commented:
>>I'm surprised I havent rear-ended another car while pondering this stuff.
That's only, because you were diffracted from both sides of the car in front, and your particles met each other again undestryed ;-))
Author Commented:
"The characteristic size of a photon is inversely related to its frequency:
Lambda     = c/f "

"E = m c ^ 2
and solving for m : m = E / c ^2, you can calculate the mass. (in kg per photon)
Suppose a photon has an energy of 1 * 10 ^ -19 Joules (somewhere in the visible light). The corresponding mass is then :
1*10^ -19 * (3 * 10 ^5) ^ 2 =
9 * 10 ^ - 9 kg"

The above statements are telling me that a photon has a mass and a size: are those statements accurate?

:-)
Commented:
If by "mass" you mean the old term "relativistic mass" (which has been replaced by the equivalent but clearer term "energy") then yes.
If by "mass" you mean the current usage of the word, which is equivalent to the old term "rest mass", then no.

If by "size" you mean a hole which it is difficult for it to fit through, then yes.
Commented:
I wonder if this old statement from A.E. Eddington in the 1930's is still considered correct, or was this just limited to the non-coherent experiments of that age.

something like:

If you have two electrons A and B, you can say there is a certain diistance between them,
but you can't say A is two furlongs west of B, because electrons are indistinguishable from one another.

The idea being if you arent staring at each one, they could swap places on you.

Commented:
@andreba

I can agree with your statement of the mass.

However for the size things are a bit more complicated. You correctly calculate the wavelength as a function of the frequency and so wavelength could definitively be related to the size.

However the wavelength is not the size. There are some problems : How many periods of this wave is one photon? What about diffraction  ? ...

Also take in minf that uptill now everything has assumed the photn in a vacuum. However, if we are not in a vacuum the frequency remains the same but c diminishes, so lamba becomes also smaller ...

Hopes this helps
Author Commented:
Can we 'see' a photon then? How about particles smaller than photons?

:-)
Commented:
Photons are all we see.  (or what do you mean by 'see'?)
What do you mean by particles smaller than photons?
An electron has no measurable "size" (although classically it might be said to have a radius between 3*10^-15m and 3*10-18m)
But most of the photons we see were emited from electrons on the surfaces of objects.
Author Commented:
I mean, even if we develop the HIGHEST MAGNITUDE microscope, would we be able to see a photon, given that we need photons to see things? Not clear, I know.. let me rephrase.. Can a photon A bounce on other photon B, thus allowing me to 'see' B? How about things smaller than the photon itself? (assuming it has some size, as depicted above)
I figure that, if my soccer ball is the photon, I can never see a tennis ball..

:-)
Commented:
Photons don't bounce off other photons, but they do bounce off electrons.
(3*10-18m should have been 3*10^-19m)
You can get photons to create a pattern of electrons, off which you can bounce other photons to see the pattern.
Author Commented:
Just one last question, before I give up:
what's the color of a photon? Is there anything that we can call 'natural state' of a photon(before it bounced off anything at all?)

Thanks for all the patience..

:-)
Commented:
Colour depends on wavelength, from about 780nm deep red to about 390nm deep purple
Author Commented:
Okies.. Now I can go back to my lair and accept the fact that the Universe can't be comprehended by my brain!

Thanks all!

:-)
Commented:
I meant violet not purple.
Although in the vitreous humour the wavelength would be shorter.
It may be better to describe the frequency,
384THz red to 769THz violet.
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