Wavelengths of light are in which direction?

Factual Questions
21

It definitely is a wave - but it’s a wave with such an uncommonly long wavelength that it’s hard to perceive it as a wave. Spend some time on a beach, and you become accustomed to wind-generated waves with common wavelengths and periods: the waves kids play in will be maybe 50 feet and ten seconds apart, and the big waves surfers ride will be a few hundred feet and maybe 30 seconds apart. tsunamis out in the open ocean have a wavelength of several hundred miles and a period of somewhere between ten minutes and two hours. The wavelength decreases as it moves into shallow water (which also increases its height), but the period doesn’t change. So the “surge” you see is one long-duration wave crashing onto the shore. Wait around, and you’re likely to see one or more successors arriving within minutes or hours. Watch a surveillance video of an area that’s impacted by a tsunami, but speed it up 30X, and you’ll start to comprehend the rhythmic sloshing action.

As to “wave” vs. “surge”, you can get either one, depending on the shape of the sea floor and shoreline in the area.

22

Okay, let’s look at this again.

As has already been pointed out, the electric and magnetic fields do not move in location from side to side. The diagrams show the strength of the fields, which is directional. The directions of the electric and the magnetic field strengths are perpendicular to the direction of motion of the wave, and to each other, but they are on the path of the wave, not moving from side to side next to it.

The wavelength is measured along the direction of propagation.

  • The question is, how and why does wavelength affect the blocking of EM waves by a Faraday cage (metal mesh)? Why will a mesh block EM waves with a long wavelength, but not a short wavelength?

Firstly, wavelength and frequency are not independent. They are inversely related by

\large\lambda = \frac{c}{f}

where \lambda = wavelength, c = the speed of light, and f = frequency

So the longer the wavelength the lower the frequency, and the shorter the wavelength the higher the frequency.

So a mesh will block low frequency EM waves, like microwaves and radio, but not higher frequency EM waves, like visible and ultraviolet light.

The way a Faraday cage works is by blocking electric fields. Counter fields are induced in the metal that cancel it out. If the electric component of an EM wave is blocked then the magnetic component, which is not independent, will also be blocked.

The relationship between the size of the blocking effect and the size of the holes is extremely complicated. It depends on the size and shape of the holes, the thickness and shape of the metal (flat or round wires), and the type of metal, as well as the nature of the wave.

But, as a rough rule of thumb, it’s said that the size of the holes should smaller than the wavelength in order to block it. This is not an absolute rule, it’s a very rough guide to the relationship. In practice engineers will tend to make the holes a lot smaller than the wavelength if they want to block most of the EM waves.

The reason that EM waves get through the mesh at all is that above certain frequencies the mesh acts as a waveguide, and allows the EM wave to propagate.

However, waveguides have a cutoff frequency, due to modes of standing waves. Below a certain frequency (i.e. above a certain wavelength) the mesh will stop acting as a waveguide, and the EM waves will be blocked.

That is the reason why a mesh will block will block longer wavelengths (lower frequencies), but not shorter ones.

23

… and the amplitude (signal strength) or the height and depth of the peaks and valleys in the diagram of the wave has nothing to do with whether the wave goes through the holes. The orientation of the EM field, whether vertical to the observer, or horizontal, or at any other angle in between also does not affect whether the wave goes through the holes.

Are there materials or properties that affect or interact with radio frequency waves in a manner similar to polarizers for visible light? When a vertical antenna generates a wave with a vertical EM orientation, what, if anything, can cause the wave to reorient to some other angle, like a random 23 degrees off the vertical? (Passing through a crystal lattice or magnetic field are candidates in my mind. I should probably just go Google it! :grin:)

24

The wavelength is, in fact, measured in the direction that the light is traveling in. That part is accurate. The oversimplification comes when you say that long wavelengths “don’t fit through” small holes. That’s not what’s actually happening. What actually happens is that the waves trying to pass through the holes all interfere with each other, in such a way that the reflecting waves are reinforced, and the transmitting waves are almost entire canceled out. This does depend on the relative sizes of the holes and of the wavelength, but it’s a lot more complicated than “you can’t park a Buick in a broom closet”.

25

Of course. All EM radiation (visible, radio, X-ray, etc) can be polarized. There’s nothing particularly unique about polarization of visible light except that it’s easier for us to observe without lots of extra equipment.

ETA: Yes, magnetic fields would work if set up properly.

26

There’s a similar oversimplification for polarizers. The common analogy is that the polarizer is like a bunch of parallel slats, and that a waving rope can move in one direction but is blocked in the perpendicular direction. But in fact this is exactly backwards! Polarizers are a bunch of parallel, conductive “wires”. But the waves that pass are perpendicular to the wires. The light isn’t somehow squeezing through the slots. It’s… more complicated than that.

27

I think confusions of this sort often originate in the mistaken impression that there is something about light that physically resembles a squiggly line in shape. The squiggly lines are just in graphs, as GreenWyvern was alluding to. The extensive direction on the graph can correspond to time or to physical distance, but the dimension along which it just goes back and forth doesn’t represent a mechanical dimension, it’s just a way to represent values such as electric field intensity or magnetic field intensity.

Drawing a graph of the Dow Jones Industrial Average, and making the height of the graph fit through a hole, says nothing about the Dow. The graph isn’t a mechanical diagram. It might as well have been a list of numbers.

28

Well, the fields that make up the light wave do have a direction, so you can refer to that as “a mechanical dimension”, in that sense. But the amplitude isn’t a length, so there still isn’t a “squiggly shape” per se.

Now that I think of it, of course you’re absolutely right. Why have I never realized that before?

Oh, and while there is a difference in polarizers at different frequencies, it’s that it’s much easier to polarize long wavelengths like radio waves, than it is for short wavelengths like x-rays or gamma rays. For radio waves, a polarizer could, literally, be a picket fence (as long as the pickets were metal or some other conductor, and the fence was fairly tall and wide). On the other hand, you can polarize x-rays or gamma rays, but it usually takes some fairly extreme physical process.

29

Rule of thumb, hole size in a metal sheet or width of slits in your “diffraction grating” should be the same size as the wavelength you’re manipulating, to within an order of magnitude or so.

30

Absolutely right. Wire Grid polarizers, used in the infrared, literally are a set of parallel wires string like a harp. You can get away with that because the infrared wavelengths are so long.

The genius of Edwin Land;s Polaroid sheets is that it mimics this construction. He mixed crystals of iodoquinine sulphate in nitrocellulose, cast it into a sheet, then stretched it in one direction, aligning the crystals. Light with polarization perpendicular to the direction of the crystals gets transmitted.

As for the “size” of photons – that’s a wonderfully complex question. Whenever I see people drawing pictures of what are supposed to be photons they invariably draw a hard, well-defined sphere for some reason. I submit that a better mental picture would be a very hairy sphere, with an undefined edge that, like “s” atomic orbitals, gradually tapers off. The reason is that, while photons can “fit” through very small apertures, they can “sense” the width of much larger apertures. The diffraction pattern resulting from light passing through a slit will have a characteristic shape whose width is inversely proportional to the width of the slit. This will be true even if the light source is so low in intensity that you can be certain that individual photons are passing through at any time, and you let the pattern build up over a long exposure.

31

I had the good fortune to be given a small roll from Land’s first batch, by a mutual friend (though both are long gone).

32

You lucky son of a photon.

33

Like you, early researchers wondered. In particular, Tesla didn’t believe the math, and thought that Electro/Magnetic radiation must propagate with a Longitudinal, non-Hertzian, superluminal wave, the way sound does.

‘Non-Hertzian’ because Hertz demonstrated transverse waves, like the ripples on water, which can be blocked if the holes in the screen are too small for the N/S or E/W vibration.

The scientific unit for the frequency of light is Hertz, not Tesla, which tells you all you need to know about the ‘correct’ view of light waves :slight_smile:

The pictures in a text book are just pictures.

34

To be honest, I’ve never been all that clear on just what Tesla got right. I’m sure there must be something, but it’s lost in the noise.

35

Tesla was a great inventor but a bad theorist. Among other things, he invented the AC induction motor and polyphase AC. The inventions obviously work extremely well, but it’s not clear he ever had much understanding of why. Sometimes even a bad theory can be effective in a limited space of application. But eventually Tesla tried to apply his ideas beyond their limits and his inventions based on them became impractical or impossible.

36

This is perhaps a slight hijack, but on the other hand may get at a deeper truth of it.

Theories can be wrong, and we can know for certain they are wrong, and they can still propel us forward. I think the wave particle duality illustrates this perfectly. In a nutshell it is easy to prove that light operates as waves and not as particles, and it is also easy to prove the opposite. The wave theory does a great job of explaining a variety of behaviors. The particle theory does a great job of explaining another variety of behaviors. It’s obvious that both theories are useful, and also obvious that neither one is correct. I suspect there are fancier theories that combine the two such that there’s nothing known to be wrong about the combined theory, but I’m not sure about that, and I’ve never heard of anybody using them – I’ve only ever used and heard of others using the wave or the particle theory. In fact, to prove a point, you could analyze a double slit experiment where you measure brightness with a photodiode tube, and actually use each of these in rapid succession.

Moreover, I think I understand what most (all?) of us do: we know as fact that both wave and particle theories are wrong, but we apply them productively, and while we use them we actually believe in them. I think it’s like watching the sleigh bell scene in Polar Express: you can’t really get the scene unless you believe in Santa Claus for a few seconds, and so you do. I think if there were a neurological test that could detect belief, it’d light up while a person applies either wave or particle theory, and then wink out again as the person stands back and contemplates the result.

37

That would be quantum mechanics.

38

I tried to explain this a decade ago on this very same forum - maybe the time is right, now.

Light has no direction, because it is a wave. A mechanical wave (like those you see on water) is a circle - light is a sphere. An you cannot measure time, but frequency that pushes the wavefront at the speed of light. Now, push is the correct verb as you need multiple jumps of electrons to get that wavefront going (Bohr model - no photons here).
Depending on the frequency, waves either collide with your obstacle or go through it.

39

I think you are trying to say that light coming from a point has no direction, since it is a spherical wave (so it goes in all directions equally and simultaneously). But you should consider that not all waves are spherical waves, and that consequently not all waves 'have no direction". An infinite plane wave is certainly a wave, and has a definite direction. A spherical wave blocked on one side by a barrier will , on average, have more motion in one direction than the other, and can be regarded as mostly in a particular direction. A Gaussian wave front (as from a single mode laser) has a definite form that expands as it moves forward in one direction, and so on.

What happens when light meets an obstacle is interesting. It is generally some combination of being absorbed, being reflected, and being transmitted, the amounts being determined by the physical properties of the material and the frequency or wavelength of the light. But the general case is that some fraction of the wave goes into each possibility.

40

So what happens to the energy from a single jump of an electron according to this “let’s freeze science at an arbitrary point in the past”-description of light?