# Light is an oscillating elec field, But with respect to what

1. Sep 25, 2015

### SBNY444

I hear frequently that light is simply an oscillating electric field. But I have problems visualizing this. Mostly because I'm a mech. engr. and I have a tendency to always try to picture something physically in my head to help me understand it. I simply can't do it when I think about light this way. When you think of a mass on a spring, you can think of it's oscillating position with respect to some point in space. But when I try to translate that to light I always think, "well what's the actual quantity that is oscillating"? and also "what is it oscillating with respect to"? I'll typically think of it as a light bulb and an eye ball next to each other. If it is an oscillating electric field, then is the light bulb and eye simply point charges oscillating between positive and negative at the broad frequency of white light? This obviously sounds ridiculous when I repeat it to myself. Then usually I think of it as 3D wave, like sound, where the quantity that is oscillating in all directions is air pressure. Neither of these thought experiments help me understand what light actually is at all obviously. Help!

2. Sep 25, 2015

### Staff: Mentor

The electromagnetic field strength. It is not a mechanical oscillation in space.
I don't understand that questions.
Unrelated: There is no "frequency of white light". Light we perceive as white is a mixture of different wavelengths.

3. Sep 25, 2015

### SBNY444

Well what is the electromagnetic field oscillating with respect to?

Yes that last question was worded poorly. I'm just trying to understand if it is an electric field, then what quantity (already answered) is oscillating and with respect to what. Two point charges create a field. So in my uninformed efforts to try and visualize an oscillating "field" I will think of two point charges. Each point charge oscillating between positive and negative. I already know this is ridiculous. So my question was, is the light bulb (let's say its a blue light bulb) and eye simply these two point charges oscilating between postive and negative at 606–668 THz? (450–495 nm wavelength)

4. Sep 25, 2015

### Staff: Mentor

That question does not make sense.
A single one is sufficient.
Positive and negative what?

You can emit light if you have electrons in tiny antennas oscillating back and forth with the right frequency. That's not how light bulbs work, but the antenna is easier to understand so let's keep that model.

5. Sep 25, 2015

### Staff: Mentor

Space and time. I.e. $\nabla^2 E = \frac{\partial^2}{\partial t^2} E$

6. Sep 26, 2015

### SBNY444

Charge clearly. (coulombs if you like).

a field has field vector lines. These field lines would presumably be different from one observer to the next. This field and also field lines are oscillating in magnitude and direction. Are those field lines oscillating between up and down or forward and back or a combination of the two between the antenna and some detector, say my eye?

Ok, if i were to take a negatively charged end of say a capacitor and vibrate it up and down at 660 THz, would I create blue light?

7. Sep 26, 2015

### Staff: Mentor

A point charge oscillating between point charges? Like, charge A oscillates between charges B and C? Why? Where are B and C located and what is their relevance?
Only if the observers are moving relative to each other, but I don't think you want to include special relativity here.
The field lines are not oscillating. You can describe them as moving, but I don't think that model helps.

8. Sep 27, 2015

### vanhees71

The electromagnetic field is a fundamental quantity, which we cannot explain by simpler assumptions. They are just there and are described very well by Maxwell's equations. Their physical meaning is empircally defined by their influence of charged test particles, and electric charge is a fundamental property of matter too. So it is just there and cannot be explained by something more fundamental or simpler.

In a given inertial reference frame the electromagnetic field can be decomposed in electric and magnetic components, which are described as usual three-dimensional Euclidean vector fields $\vec{E}$ and $\vec{B}$. For a region in space without charges and currents, Maxwell's equations have solutions, describing electromagnetic waves like
$$\vec{E}=\vec{E}_0 \cos(\omega t-\vec{k} \cdot \vec{x}), \quad \vec{B}=\vec{B}_0 \cos(\omega t - \vec{k} \cdot \vec{x}),$$
where the relation between the frequency and the wave vectors is
$$\omega = c |\vec{k}|,$$
which means that these plane waves move with the universal constant $c$ appearing in the Maxwell equations (in a proper choice of units like Gaussian or Heaviside-Lorentz units; in SI units this is a bit hidden in the conversion constant between SI units and more natural units, but it turns out that the only physically relevant quantity is again a universal constant with the dimension of a speed, $c=1/\sqrt{\mu_0 \epsilon_0}$). Because this speed is the phase velocity of the plane electromagnetic wave in a vacuum, it's usually called the speed of light in a vacuum, although relativity teaches us that it has a far more general meaning as a conversion factor between time and distance units. In even more natural coordinates one would measure distances in terms of the travel time of light signals (and in the SI one does so in fact by defining $c$ to a certain value matching as precisely as possible the older definition of the time units second and distance unit metre).
Finally the Maxwell equations also tell us that we must have
$$\vec{k} \cdot \vec{E}_0=\vec{k} \cdot \vec{B}_0=0, \quad \vec{E}_0 \cdot \vec{B}_0=0.$$
One gets even more precisely
$$\vec{E}_0 \times \vec{B}_0 \propto \vec{k}.$$
So the electromagnetic waves are transverse waves, and what's oscillating is the electromagnetic field (or in the fixed reference frame, we've used above to describe this particular solution of the Maxwell equations, the electric and magnetic field components $\vec{E}$ and $\vec{B}$).

9. Sep 28, 2015

### SBNY444

No, not three charges. I gave the example of a light bulb and an eye; two things. I was imagining the light bulb had some charge and that charge created a field and the quantity of the charge oscillated which caused the strength of the field to change thusly creating light that the eye could detect. I'm not presuming the point charge way of thinking about light is even correct. I'm just creating a thought construct to lay out some ideas of how to think about light so I can try to understand it. Rather than saying that the way I'm thinking about it is wrong and doesn't make sense, would you like to offer another way to think about it?

The reason why I think about directionality of the field changing is because we have things like polarized light. Which is light that is oscillating in a certain direction in a sense. So then I'm imagining light as bunch of plane waves all interacting with each other to create things like circular polarized light. And when i put my polarized sun glasses on and stare at the light bulb, I'm only getting the photons that hit my eyes whos electromagnetic fileld is oscillating in, say, the transverse direction because all of the verical oriented waves are filtered out. Again, I don't know if this is true and I'm almost certain it is not but this is how I am imagining it. Rather than simply saying no, I'm wrong and I don't make sense, can you give me some indication where my thought process is breaking down?

10. Sep 28, 2015

### Drakkith

Staff Emeritus
Well, since its the field vectors, which represent the field strength and direction, which are oscillating, I'd guess they oscillate with respect to the local value of the field at that point.

11. Sep 28, 2015

### Staff: Mentor

The quantity of charge cannot oscillate. That would violate charge conservation. What you can have is that charges oscillate from one location to another, or that the magnitude of a charge dipole oscillates. In either case, you also have associated currents. The fields depend on both the charge and the current.

Regarding suggesting another way to think about it. I would recommend not attempting to develop a mechanistic mental model for it. The idea of the EM field oscillating in space and time is not that complicated and can be understood directly.

12. Sep 29, 2015

### SBNY444

Yes, this makes sense. I neglected to take this important fact into account. So a moving field and consequently light can be generated simply from a moving charge. Let's imagine for a moment that I have small steel 1 cm diameter sphere and it was charged with some arbitrary amount of coulombs. I then vibrated this sphere up and down at 606–668 THz (450–495 nm wavelength which is in the range of blue light). Would I see blue light being emitted from the sphere?

This is all well and good but it does not help me to understand the concept. Saying things like this is how you dismiss people from the conversation rather than making it more accessible to them. This is one of the reasons why most of our culture is not scientific literate - educators fail to make the subject matter accessible. I digress...

Perhaps a picture would be better. Attached I have a picture of a light bulb emitting some light and I have some red arrows that, I think, represent the orientation of the electric field. Would you say this is accurate description? Of course, I'm depicting one plane of waves. Is it safe to assume that there are move light waves are propagating at any and all planes? - rotating the blue wave 360 degrees into the page.

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13. Sep 29, 2015

### Staff: Mentor

Ignoring the obvious impracticalities, yes.

Regarding your feeling of dismissal, I am not dismissing you as a person or as a student. I am, however, trying to get you to dismiss your line of questioning. Not all questions that a student asks are useful to answer if the goal its to learn. In particular, a line of questioning intended to provide a mechanical analogy of light to a mass on a spring, or an acoustic wave is fundamentally problematic.

A better approach is to try to understand light as it is described by Maxwell's equations. I am glad to help if that is the goal.

Regarding the picture, if the source of light were a laser that would be reasonable. Incandescent bulbs produce incoherent waves so it wouldn't be nice and neat as drawn.

14. Sep 30, 2015

### SBNY444

Great. Let me take a step further. I have a picture attached that I will refer to. If I impractically vibrated this sphere up and down with respect to your computer screen indicated by the green arrow in the lower left hand corner, would the two theoretical detectors see the same thing in terms of intensity and color? I assume detector A and B will "see" different things since the waves traveling to detector A are traveling similar to how seismic P-waves travel from one point to another and detector B will see normal blue light?

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15. Sep 30, 2015

### Staff: Mentor

The frequency would be the same, but the intensity would not.