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a1call
2008-Feb-13, 02:16 AM
*- What is the size of a Photon?
*- is there a minimum hole/aperture size (exact or formed at some angle by some objects), below which no photon/light can pass through?

Thanks

Jerry
2008-Feb-13, 02:34 AM
Yes, but the size of the hole depends upon the wavelength of the photon. Gamma rays can penetrate what, up to a half meter of lead, but this is another way saying is that the wavelength of gamma rays is so small, they slide between and even through the nucleus of an atom in some cases without splitting the atom and/or being absorbed.

For ordinary light, defraction gratings are designed that allow specific frequencies to pass, while reflecting or absorbing others. The sized of the gap in the grating is directly related to the wavelength of the light.

Remember, the wave mechanic world is quite different from say, screening marbles - think of photons as carrying information the same way the wake of a boat ripples through a lake, and that information is dampened as the wave enters the narrow opening to a harbor.

a1call
2008-Feb-13, 02:40 AM
Thank you Jerry,

So can we say that:

*- There is a limit to the minimum possible width of a beam of laser light of a given frequency.
?

Ara Pacis
2008-Feb-13, 04:26 AM
Are you referring to amplitude?

Think of a microwave oven. The reason you can see inside of it when it's on is because visible light can pass through the holes in the screen, but the microwaves can't.

a1call
2008-Feb-13, 04:41 AM
Interesting point. Are you implying that microwave photons are millimeters in diameter wide?

What I am trying to figure out is that not considering diffraction based limitations of a lens you could concentrate any given light to any intensity if you could concentrate it to an infinitely small area.

I am trying to establish that again aside from diffraction which theoretically can be indefinitely minimized by enlarging the lens indefinitely, there is a photon size limitation as well (or perhaps not).

Ara Pacis
2008-Feb-13, 07:07 AM
I think there is a limitation on focus based on the wavelength of the chosen frequency. This is why optical microscopes have limitations in how small they can see. At a certain point, light does not interact with the resolution of what they want to image, so they use electrons microscopes instead.

Neverfly
2008-Feb-13, 07:26 AM
Interesting point. Are you implying that microwave photons are millimeters in diameter wide?

I'm not aware of microwaves being a photon.
Microwaves also are not very micro... At I THINK 1200 megahertz it has a wavelength of about 12 cm.

Noclevername
2008-Feb-13, 07:41 AM
I'm not aware of microwaves being a photon.
Microwaves also are not very micro... At I THINK 1200 megahertz it has a wavelength of about 12 cm.

Light, heat, microwaves, X-rays, all part of the EM spectrum.

Neverfly
2008-Feb-13, 07:51 AM
Light, heat, microwaves, X-rays, all part of the EM spectrum.

So are they all comprised of photons?

What is a wave then?

Noclevername
2008-Feb-13, 08:24 AM
So are they all comprised of photons?

What is a wave then?

Wave-particle_duality (http://en.wikipedia.org/wiki/Wave%E2%80%93particle_duality)

Ken G
2008-Feb-13, 12:37 PM
What I am trying to figure out is that not considering diffraction based limitations of a lens you could concentrate any given light to any intensity if you could concentrate it to an infinitely small area.

The problem is with your caveat that you are "not considering diffraction". Diffraction is precisely the place where the wavelength of the wave that tells the photons where to go comes into play, so if you are not considering that, you must be dealing with photons of effectively small wavelengths-- so you miss what the microwaves do when they encounter the screen. Note that the wavelength is not really "the size of the photon", but it does limit the scale on which the photon can exhibit interference effects like diffraction. That is a lot like the "size of the photon" because you can't use interference to localize the photon any more than that.

Ken G
2008-Feb-13, 12:44 PM
Think of a microwave oven. The reason you can see inside of it when it's on is because visible light can pass through the holes in the screen, but the microwaves can't.That's an interesting juxtaposition, can you say more about how those holes work? They must set up a diffraction pattern that strongly limits the microwave leakage, but how do they avoid interference fringes like a diffraction grating?

Jeff Root
2008-Feb-13, 01:11 PM
Does a photon actually have extension in space perpendicular to its
direction of motion? If so, how is it defined? How can it be measured?

What defines the length of a photon? How can it be measured?

-- Jeff, in Minneapolis

jlhredshift
2008-Feb-13, 01:36 PM
That's an interesting juxtaposition, can you say more about how those holes work? They must set up a diffraction pattern that strongly limits the microwave leakage, but how do they avoid interference fringes like a diffraction grating?

I can tell you that in America microwave ovens run at 2650 MHZ.

Ken G
2008-Feb-13, 03:43 PM
I can tell you that in America microwave ovens run at 2650 MHZ.
That's useful to know-- the wavelength of that is about 11 cm.

alainprice
2008-Feb-13, 03:56 PM
I thought microwave ovens operated at 2.45 GHz (2450 MHz or lambda = 12.25 cm) . Nonetheless, they are photons of a longer wavelength than the door screen.

Jeff Root
2008-Feb-13, 04:16 PM
Confirming 2450 MHz rather than 2650 MHz.

-- Jeff, in Minneapolis

jlhredshift
2008-Feb-13, 04:20 PM
Confirming 2450 MHz rather than 2650 MHz.

-- Jeff, in Minneapolis

agreed sort of dyslexia sorry

Noclevername
2008-Feb-13, 08:09 PM
Does a photon actually have extension in space perpendicular to its
direction of motion? If so, how is it defined? How can it be measured?

What defines the length of a photon? How can it be measured?

-- Jeff, in Minneapolis

As a wave, rather than a particle. It can also be treated as a particle for some measurement purposes.

Confusing, isn't it? That's one of the things about quantum mechanics-- it works differently than our "common sense" tells us things work.

Jeff Root
2008-Feb-14, 03:24 PM
Does a photon actually have extension in space perpendicular to its
direction of motion? If so, how is it defined? How can it be measured?

What defines the length of a photon? How can it be measured?
As a wave, rather than a particle. It can also be treated as a particle
for some measurement purposes.
That doesn't answer any of my questions. It isn't even clear that it

Confusing, isn't it? That's one of the things about quantum mechanics--
it works differently than our "common sense" tells us things work.
No, it isn't confusing. Wave properties of light such as diffraction
and interference patterns are shown in some situations, while
particle-like properties of light such as the photoelectric effect
and photochemical reactions are shown in other situations. They
can be combined by, for example, projecting an interference pattern
on a photomultiplier tube or CCD array one photon at a time, in which
it takes a while for the pattern to emerge from what at first appears
to be a random scatter of hits.

Now: Does a photon have extension in space perpendicular to its
direction of motion? If so, how is it defined? How can it be measured?

What defines the length of a photon? How can it be measured?

-- Jeff, in Minneapolis

Ara Pacis
2008-Feb-14, 03:37 PM
I don't think there is a way to measure the length of a photon. Once it gets where it's going and can be detected, it's all there. Everything not a photon has to travel slower than a photon, which makes it impossible to grab the tail and wag the dog, so to speak.

Ken G
2008-Feb-14, 03:45 PM
Now: Does a photon have extension in space perpendicular to its
direction of motion? If so, how is it defined? How can it be measured?
This is the kind of question where the answer depends on what you really mean by the words you are using. A photon is a "point particle", so it has no inherent size at all, at any wavelength. However, the waves that tell that point particle where to show up do have wavelengths associated with them, and depending on those (which you can think of as a little group of "k vectors", possibly centered tightly on one direction and one magnitude as with a laser, that constrain the motion of that quantum), there is only so much you can know about where the quantum will show up. If you want to call that uncertainty about where quantum can show up an "extension in space", then your answer is governed by the wavelength and the spread in that little group of k vectors.

Note that above I am using the meaning for "photon" that is "quantum emitted by a quantum mechanical process". As I've mentioned before, there is a very different meaning for the word "photon" that is probably even more common, which is to treat each one of those k vectors as a "photon mode" or a "quantum of energy", and think of the one quantum I was talking about above as being ruled by a superposition of these possible photon states. In that case, the photon has infinite extent in all directions, but that's not what one measures, it is a mental construct-- one measures the quantum process.

a1call
2010-Dec-20, 05:28 AM
Thank you Jerry,

So can we say that:

*- There is a limit to the minimum possible width of a beam of laser light of a given frequency.
?

Remembered this question, did some reading and got the answer. With an ideal/perfect lens/mirror the smallest area of a "focal point" is an Airy Disk (http://en.wikipedia.org/wiki/Airy_disk) which is limited by diffraction aka wavelength/lens-size(aperture). So there is no theoretical limit to the smallness of focal point not withstanding diffraction and aberrations(lens imperfections).

Focused laser beam

A circular laser beam with uniform intensity across the circle (a flat-top beam) focused by a lens will form an Airy disk pattern at the focus. The size of the Airy disk determines the laser intensity at the focus.

Also see Focus (optics) (http://en.wikipedia.org/wiki/Focus_%28optics%29)