# The Naked Scientists Forum

### Author Topic: what shape is light?  (Read 6151 times)

#### thebrain13

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##### what shape is light?
« on: 25/09/2015 00:37:40 »
What is meant by the whole transverse wave thing? What does that mean in reality? Does this suggest that light as a particle traces out a transverse wave, or that it's probability cloud does? Or does it have nothing to do with where it physically is, and simply mean that it's electric charge rises and falls along with its magnetic field rising and falling? And then what does that mean with fields rising and falling? Can somebody explain what this metaphor means experimentally?

#### PmbPhy

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##### Re: what shape is light?
« Reply #1 on: 25/09/2015 02:01:22 »
What is meant by the whole transverse wave thing? What does that mean in reality? Does this suggest that light as a particle traces out a transverse wave, or that it's probability cloud does? Or does it have nothing to do with where it physically is, and simply mean that it's electric charge rises and falls along with its magnetic field rising and falling? And then what does that mean with fields rising and falling? Can somebody explain what this metaphor means experimentally?
Light is an electromagnetic wave. That means that there is an electric field oscillating perpendicular to the direction of motion and a magnetic field  oscillating perpendicular to both the direction of motion and the direction of the electric field. For example; if the direction of motion is the x-direction and the electric field is parallel to the y direction then the magnetic field is parallel to the z-direction.

#### thebrain13

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##### Re: what shape is light?
« Reply #2 on: 25/09/2015 02:26:51 »
As a junior physics major at michigan state University, I've probably heard that statement 100 times. I still have no idea what that means though. What can you conclude from that experimentally?

#### PmbPhy

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##### Re: what shape is light?
« Reply #3 on: 25/09/2015 05:21:02 »
Quote from: thebrain13
As a junior physics major at michigan state University, I've probably heard that statement 100 times. I still have no idea what that means though. What can you conclude from that experimentally?
That question makes no sense to me. It's says exactly what it means. Experimentally the direction of the fields plays a major role when it comes to what's observed in nature. If you're asking what experiments have been done to show this then you can't really expect physicists to keep a mental record of all possible physics experiments which test every conceivable fact in physics. But one way to think of this is having a linear antenna sticking parallel to the z-axis. If a EM wave whose electric field is parallel to the z-axis approaches it along the x-axis then since the electric field is in the direction that currents can be established along the length of the antenna then the strength of the reception of the EM wave is much greater than if the antenna is not parallel to the line in which the electric field oscillates. Can you see why?

#### alancalverd

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##### Re: what shape is light?
« Reply #4 on: 25/09/2015 09:18:17 »
As a junior physics major at michigan state University, I've probably heard that statement 100 times. I still have no idea what that means though. What can you conclude from that experimentally?

The wave model neatly predicts dispersion, reflection, polarisation, refraction, diffraction and interference, and through the concept of coherence, the properties of lasers and masers.

Using Maxwell's wave equations, i.e. the mathematical formalism of Pmb's conceptual model, we can see that the speed of light in a vacuum is constant, which is rather important.

Remember throughout your career, that physics is the means by which we build predictive models of what actually happens. Wave mechanics is the preferred model for most optical phenomena.

#### Bored chemist

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##### Re: what shape is light?
« Reply #5 on: 25/09/2015 10:30:57 »
Light is the same shape as  being late for a meeting or the value of fine art to society.
Some things just don't have a shape.

#### Colin2B

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##### Re: what shape is light?
« Reply #6 on: 25/09/2015 10:42:53 »
..........And then what does that mean with fields rising and falling? Can somebody explain what this metaphor means experimentally?
This isn't a metaphor. The strength of the electric and magnetic fields actually vary with time and this can be measured.
Reread PmbPhy's first reply to you, it's all there in a nutshell.

#### mathew_orman

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##### Re: what shape is light?
« Reply #7 on: 27/09/2015 10:05:42 »
Photon is spherical and its diameter defines the wavelength...

#### Bored chemist

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##### Re: what shape is light?
« Reply #8 on: 27/09/2015 10:44:42 »
Photon is spherical and its diameter defines the wavelength...
Nope, it can't be.
Consider polarisation effects.

#### mathew_orman

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##### Re: what shape is light?
« Reply #9 on: 27/09/2015 10:58:31 »
Consider the process of creation...

#### Bored chemist

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##### Re: what shape is light?
« Reply #10 on: 27/09/2015 12:59:12 »
Consider the process of creation...
Do you realise that your statement makes no sense?

#### evan_au

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##### Re: what shape is light?
« Reply #11 on: 27/09/2015 22:24:54 »
Thinking about the "shape" of photons...

In the early days of applying shielding to prevent ElectroMagnetic Interference (EMI), it was found that if a door did not make good electrical contact all around, it could emit a lot of interference.

A crack in the door, perhaps 1 meter long, and < 1mm wide could emit a lot of interference in a frequency band of 100's of MHz.

So, in a sense, you could describe these linearly-polarized, VHF photons as being "long and skinny", in that they could "fit through" a slot of 1m x 1mm.

#### mathew_orman

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##### Re: what shape is light?
« Reply #12 on: 28/09/2015 07:59:48 »
Consider the process of creation...
Do you realise that your statement makes no sense?
The creation process is where the shape of EM wave is defined...
It can be diverging, converging and constant (photon)...

#### Bored chemist

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##### Re: what shape is light?
« Reply #13 on: 28/09/2015 10:47:42 »
And the creation process starts with moving an electron in a direction perpendicular to the propagation direction of the photon. That makes it clear that there's a lack of symmetry to the system.
As I said, it can't be spherical, or it wouldn't "know" whether to go through a polarising filter or not.

#### lightarrow

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##### Re: what shape is light?
« Reply #14 on: 28/09/2015 11:09:45 »
Consider the process of creation...
Do you realise that your statement makes no sense?
If he realized it, he wouldn' write that bull...t.

--
lightarrow

#### mathew_orman

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##### Re: what shape is light?
« Reply #15 on: 29/09/2015 09:27:52 »
And the creation process starts with moving an electron in a direction perpendicular to the propagation direction of the photon. That makes it clear that there's a lack of symmetry to the system.
As I said, it can't be spherical, or it wouldn't "know" whether to go through a polarising filter or not.
It is a tip based on experimental results and  not an argument...
You can setup a source of photons with arbitrary polarization and they will pass trough any polarization filter without been polarized...
Also, if you know the process of photon creation then please show it to me on a example with single electron of atomic Hydrogen which spectrum shows several different wavelength photons:  http://ch301.cm.utexas.edu/section2.php?target=atomic/H-atom/line-spectra.html
« Last Edit: 29/09/2015 09:37:25 by mathew_orman »

#### PmbPhy

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##### Re: what shape is light?
« Reply #16 on: 29/09/2015 09:58:52 »
Quote from: evan_au
Thinking about the "shape" of photons...
Photons don't have a shape. They're point particles. Read the Feynman Lectures on quantum mechanics and you'll see this explained quite well there.

Quote from: evan_au
So, in a sense, you could describe these linearly-polarized, VHF photons as being "long and skinny", in that they could "fit through" a slot of 1m x 1mm.
That is not a result of shaped photons but is due to the nature of the electromagnetic wave itself.

#### evan_au

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##### Re: what shape is light?
« Reply #17 on: 29/09/2015 11:31:42 »
Quote from: mathew_orman
You can setup a source of photons with arbitrary polarization and they will pass trough any polarization filter without been polarized...
I am puzzled by this statement - please clarify.

My understanding was that if you had a non-coherent light source (eg a tungsten-filament light globe), about half of the photons would not make it through a polarizing filter (because they had the "wrong" polarization).

Those that do make it through will then be polarized.

#### mathew_orman

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##### Re: what shape is light?
« Reply #18 on: 29/09/2015 12:24:55 »
Quote from: mathew_orman
You can setup a source of photons with arbitrary polarization and they will pass trough any polarization filter without been polarized...
I am puzzled by this statement - please clarify.

My understanding was that if you had a non-coherent light source (eg a tungsten-filament light globe), about half of the photons would not make it through a polarizing filter (because they had the "wrong" polarization).

Those that do make it through will then be polarized.
Yes, but if you arrange the mirror copy 180 deg in phase and arrange geometry that both beams cross at polarizer then by the law of superposition, superimposed photons will be invisible at the intersection thus unaffected by the polarizer...
In short, if photon is inside his copy which is 180 deg in relative phase it is invisible and both become visible after they cross each other...

#### PmbPhy

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##### Re: what shape is light?
« Reply #19 on: 29/09/2015 13:03:44 »
Quote from: mathew_orman
Yes, but if you arrange the mirror ...
What mirror are you talking about. I see nothing in this thread that has anything to do with mirrors. You must have a thought experiment in mind that you haven't told us about.

#### Bored chemist

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##### Re: what shape is light?
« Reply #20 on: 29/09/2015 13:16:08 »

In short, if photon is inside his copy which is 180 deg in relative phase it is invisible and both become visible after they cross each other...
Short, but meaningless.
And, as has been pointed out, if you put light through a polarising filter, it comes out polarised- whether you like it or not.

#### evan_au

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##### Re: what shape is light?
« Reply #21 on: 29/09/2015 13:17:32 »
Quote from: PmbPhy
That is not a result of shaped photons but is due to the nature of the electromagnetic wave itself
But propagating photons are an electromagnetic wave. This is true whether it is propagating through free space, through a transparent medium, or through a waveguide.

Quote
Photons don't have a shape. They're point particles.
If I take the previous example of a 1000mm x 1mm slot, linearly polarized VHF photons≈VHF electromagnetic waves pass through quite easily.

If I now rearrange this slot into 10 circular holes with the same total area, these VHF photons≈VHF electromagnetic waves do not propagate beyond the holes (or at least, with greatly reduced intensity=quantity).

If a propagating photon were truly a mathematical point, it would not matter whether I had one long slot, or 10 small circles; if the area is the same, there should be the same number propagating on the other side. However, this is not what we see.

So I suggest that in identifying a size and shape of hole that lets linearly-polarized VHF photons pass easily, and a different size and/or shape of hole that does not let them pass, then we have (loosely speaking) constrained the size and shape of these VHF photons.

#### lightarrow

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##### Re: what shape is light?
« Reply #22 on: 29/09/2015 21:04:18 »
Quote from: PmbPhy
That is not a result of shaped photons but is due to the nature of the electromagnetic wave itself
But propagating photons are an electromagnetic wave.
I have objections about it, but let's discuss it another time.
Quote
Quote
Photons don't have a shape. They're point particles.
If I take the previous example of a 1000mm x 1mm slot, linearly polarized VHF photons≈VHF electromagnetic waves pass through quite easily.
If I now rearrange this slot into 10 circular holes with the same total area, these VHF photons≈VHF electromagnetic waves do not propagate beyond the holes (or at least, with greatly reduced intensity=quantity).
If a propagating photon were truly a mathematical point, it would not matter whether I had one long slot, or 10 small circles; if the area is the same, there should be the same number propagating on the other side. However, this is not what we see.
So I suggest that in identifying a size and shape of hole that lets linearly-polarized VHF photons pass easily, and a different size and/or shape of hole that does not let them pass, then we have (loosely speaking) constrained the size and shape of these VHF photons.
Photons are point like just when they are detected.
Anyway your reasoning doesn't seem correct to me, because photons are not like tiny grains of sand, the fact they pass or not through a slit doesn't depend on their dimensions (if they have) but on the wave properties of the em field too.

--
lightarrow
« Last Edit: 29/09/2015 21:08:05 by lightarrow »

#### thebrain13

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##### Re: what shape is light?
« Reply #23 on: 30/09/2015 02:49:05 »
Quote from: thebrain13
As a junior physics major at michigan state University, I've probably heard that statement 100 times. I still have no idea what that means though. What can you conclude from that experimentally?
That question makes no sense to me. It's says exactly what it means. Experimentally the direction of the fields plays a major role when it comes to what's observed in nature. If you're asking what experiments have been done to show this then you can't really expect physicists to keep a mental record of all possible physics experiments which test every conceivable fact in physics. But one way to think of this is having a linear antenna sticking parallel to the z-axis. If a EM wave whose electric field is parallel to the z-axis approaches it along the x-axis then since the electric field is in the direction that currents can be established along the length of the antenna then the strength of the reception of the EM wave is much greater than if the antenna is not parallel to the line in which the electric field oscillates. Can you see why?

so pmbphy, when you say the strength of the reception of the EM wave is much greater than if the antenna is not parallel to the line in which the electric field osilates, do you mean "more photons will be absorbed, hence the stronger strength" or do you mean "photons whose electric fields align with the rod will impart more force?"

#### mathew_orman

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##### Re: what shape is light?
« Reply #24 on: 30/09/2015 08:19:36 »

Light is an electromagnetic wave. That means that there is an electric field oscillating
Propagating waves or photons do not oscillate...
Only standing waves oscillate...

#### The Naked Scientists Forum

##### Re: what shape is light?
« Reply #24 on: 30/09/2015 08:19:36 »