Wavelengths of light are in which direction?

Factual Questions
1

I’ve asked this kind of thing before, but I’m trying again.

Textbooks show the wavelength measured along the path of travel, like N-S if it’s traveling north.

So, the light is traveling north toward the screen in a microwave door. I’m told that it can’t fit through the gap in the metal mesh, but that requires viewing the wavelength as being measured E-W. The physics people keep turning the wave 90 degrees, and I admit that it seems to be true, given the effectiveness of the metal screen, and the fact that tv antennas work better if you view the wave as being at 90 degrees to the path of travel from the broadcast point.

I can’t get someone to explain how this works in terms I can grasp. I’ve asked 2 physics majors from actual respectable universities, and the guy leading an AP Physics seminar next door to mine (chemistry). I asked on this board if the wave idea can be viewed as being essentially a sphere, but was told “No.”

I found this phrase, “Basically, because light is a 2-dimensional wave, it will propagate in a helix.” It references this diagram: https://miro.medium.com/max/3000/1*MUBa9GXeZazJT1fzZJNL1Q.jpeg

I’ve been told that the diagrams of the wave being measured along the path of travel are over simplified, but what is the correct view or situation?

2

It’s certainly a wave. But it’s difficult, if not impossible, to model or display it graphically for all all points in space.

I simply think of it as continuous “stuff” being emitted from the antenna.

Let’s say I have a vertical E-field antenna that is transmitting a signal. Let’s also say I have a “point probe” connected to an oscilloscope, and the probe measures the direction and intensity of the electric field at the point. When I stick the point probe 10 feet in front of the antenna, the scope will show the electric field goes up, then down to zero, then down, then up to zero, then up, then down to zero, then down, etc. etc. So the electric field is oscillating at that point in space over time. When I move the probe so that it’s 11 feet in front of the antenna, I get the same thing on the scope, except it’s a little bit “out of sync” vs. the probe at 10 feet in front of the antenna. I can also move the probe up, down, and sideways, and I still get the same waveform on the scope; the electric field goes up, then down to zero, then down, then up to zero, then up, then down to zero, then down, etc. etc. But the waveform will shift in time a little bit as I move it around. The direction for the electric field is always vertical.

The exact same goes for the magnetic field. The only difference is that the magnetic point probe shows the direction of the magnetic field is horizontal:

Let’s say I have a vertical E-field antenna that is transmitting a signal. Let’s also say I have a “point probe” connected to an oscilloscope, and the probe measures the direction and intensity of the magnetic field at the point. When I stick the point probe 10 feet in front of the antenna, the scope will show the magnetic field goes up, then down to zero, then down, then up to zero, then up, then down to zero, then down, etc. etc. So the magnetic field is oscillating at that point in space over time. When I move the probe so that it’s 11 feet in front of the antenna, I get the same thing on the scope, except it’s a little bit “out of sync” vs. the probe at 10 feet in front of the antenna. I can also move the probe up, down, and sideways, and I still get the same waveform on the scope; the magnetic field goes up, then down to zero, then down, then up to zero, then up, then down to zero, then down, etc. etc. But the waveform will shift in time a little bit as I move it around. The direction for the magnetic field is always horizontal.

3

You may not be able to get a satisfactory answer. Hopefully someone will come along with a better post than me.

It sounds to me like there are mathematical ways to account for the behavior of photons, which don’t readily translate into something as simple as “They are waves”. People have designed experiments that “prove” photons are waves. People have designed experiments that “prove” they are particles. Quantum mechanics maybe says photons are kind of both and neither. I am not aware that we have yet come up with a simple, graspable human visualization for what is going on, despite the fact that we can experimentally and mathematically deal with it.

4

You can understand wave motion from a simple mechanical wave.

Suppose you hold a one end of a long rope, and someone else holds the other end. You shake the rope, and a wave propagates along it.

If you look at any point on the rope it moves up and down, but the wave moves along the rope. The rope itself doesn’t move in the direction of the wave. The wavelength is measured along the rope, but the motion is up and down or side to side.

It’s the same with electromagnetic waves, although in that case, you have an electric wave and a magnetic wave moving along simultaneously, the one generating the other.

Now suppose that halfway along the length of the rope is a wooden panel with a hole it that the rope passes through. If the hole is small in relation to the up and down motion of the wave, it will stop the oscillations, and the wave won’t pass through. If the hole is large, it won’t affect the wave motion so much, and most of the motion will pass through to the other side.

5

I just realized I made a small error: when you move the probe from 10 feet to 11 feet in front of the antenna, the amplitude of the “ups and downs” will go down a small amount for the electric field. Same for the magnet field. And as noted, there will be a small “time shift” in the waveform when moving from 10 feet to 11 feet.

6

This is a somewhat problematic analogy, because unlike a rope, with the electromagnetic field there is no physical displacement. When you see a diagram of the field with vectors that change along the path, you aren’t seeing any actual movement. You’re just seeing a representation of the electromagnetic field tensor (which can just be thought of as a pair of 3D vectors, one each for the electric and magnetic fields) at various points in space.

Fields can be difficult to accept due to their seemingly abstract nature, but they’re fundamental to all of modern physics. You can’t understand light or anything else without the existence of the EM field.

7

Going back to your microwave door, it’s not the polarization (East-West vs North-South or horizontal vs vertical, or however you are visualizing it) of the microwave photons that prevents them from going through the holes. You can see the visible light from the light bulb inside because the smaller wavelength of visible light can go through the holes, while the non-visible microwaves have a larger wavelength that can’t fit through the holes, regardless of their orientation.

Rather than a microwave door, play with polarized filters for visible light. The lenses in polarized sunglasses mostly block light that does not match the orientation of the polarization of the lens. If you go to the rack of sunglasses and look through the lenses of a polarized pair of glasses, you can only see the light that has a horizontal orientation for what we are calling “waves” (or maybe a vertical orientation, I don’t what kind of glasses you grabbed), and consequently some of the light is blocked because it has the wrong polarization. If you hold up a second pair of sunglasses in front of the ones you are wearing and look through those lenses too, it won’t be much darker. Any light that gets through the first lens gets through the second lens, too. However, if you slowly rotate the second pair of sunglasses until they are 90 degrees from the orientation of the first pair, you will probably see the image through the stacked lenses go all the way to black, if they are truly polarized lenses. One pair of glasses is now blocking any horizontally polarized light, while the other is blocking any vertically polarized light, so nothing gets through.

8

Here’s what should really bake your noodle: take a third polarizer, angled at 45°, and slip it between your previous two polarizers. Instead of blocking all light, you can now see through them again (dim, but visible). If instead you place the third polarizer in front of or behind the first two, it doesn’t work (it stays black).

9

Oh, that is weird. I gotta go try that sometime! Thanks! :grin:

I would suppose a way of grasping that is each polarization filter forces a new state of polarization so the vertical filter forces light to either “decide” to be sorta vertical, or be blocked. The 45 degree lens then forces the light to “recommit” to a different polarization which isn’t totally opposed to its vertical orientation, and it can “recommit” again when it gets to the horizontally polarized lens. It has a new state after it passes each lens, regardless of the history of how it was previously polarized?

But the jump from vertical polarization to horizontal is too big, so it doesn’t happen?

10

Sure, but I think the issue is a basic understanding of wave motion in general. The concepts of wavelength, amplitude, and frequency are easier to understand in a concrete example of a mechanical wave.

11

But that’s the amplitude, not the wavelength…

12

Due to the difference between photons and rope, amplitude doesn’t mean the same thing. If I oscillate my rope more vigorously, I can make the waves taller along my single rope. If I excite my light source more vigorously, it emits a brighter signal - it generates more photons. Each individual photon has zero amplitude. They don’t waggle up and down like a rope.

13

That’s pretty much it. I’m not aware of a classical explanation, but quantum mechanics explains it clearly. The amount of light that passes is proportional to cosine of the angle between the light and the polarizer. At 0° it all passes; at 90°, none of it does; at 45°, about 70%. But all of the light that passes through is then at the new polarization. So it gets reduced to 70% going through the middle polarizer, and then down to 50% because the second pass is at a 45° angle now, not 90°.

It’s totally different behavior from, say, color filters, which can only ever reduce the amount of light going through, and which don’t depend on the order. It’s a really weird phenomenon that only costs a couple bucks to demonstrate.

14

That’s fair–I didn’t mean to dismiss the example completely. Just pointing out that it is potentially misleading due to the nature of physical waves in a medium vs. waves in a field.

Another difference is that the EM field doesn’t support compressive waves. A rope pulled taut can transmit compression waves along its length (though this is easier to see in a Slinky). The EM field does not support compressive (longitudinal) waves.

15

Surely the rule is that the two fields are at 90° to each other, not that they are vertical or horizontal. Also, this doesn’t really address my point about the diagrams always showing the wavelength being measured at along the point of travel, yet in practical terms we know that the way the length can very much be measured and 90° in the path of travel.

I’ve also thought that a wave must have an amplitude, otherwise it won’t be a wave. It as you say, it has to oscillate from 0 to a point, back through zero into an opposite point, that’s what we mean by a wave. So it has to have a high point and a low point. Half the distance between those is the amplitude. What is the amplitude of a photon? The energy is governed by the wavelength, and I’ve never heard anyone talk about the amplitude of the wave. Is it universal? Does it relate to the wavelength? Does it matter?

16

I made a quick video of the polarizer effect. Minute Physics also has a video, but my video is under a minute while theirs is over 17 minutes :slight_smile: .

17

Diagrams aren’t always a literal depiction of what something “looks” like. Sometimes they are a way of graphing mathematical values or functions. On a guitar, if I pluck a string, it does literally oscillate up and down, from a high point through zero to the low point and back up again. The tighter the string, the faster it vibrates, meaning the higher the frequency. The harder you pluck it, the greater the distance the string fluctuates up and down, meaning the greater the amplitude. What the plucked string does is visually similar to the way it is graphed in a diagram.

However, when you pluck a string and it generates a corresponding sound wave in the air around it, the air is not like the stretched physical string, it is a gaseous medium. The sound wave is a compression wave, not a transverse wave. The oscillation of the wave has no polarization; there is no up-down or side-to-side. It is a fluctuation between higher and lower pressure. The frequency is perceived as pitch or tone, the amplitude is perceived as volume/loudness. Graphing this in a diagram still looks like a sine wave, but the air is not waggling up and down.

When you mention a diagram of a light wave, are you talking about something like this?
Transverse wave - Wikipedia (It goes to a Wikipedia animation if you click it.)
Because of the sort-of-a-wave/sort-of-a-particle quantum mechanical nature of light, there is more than one way of conceptualizing what is going on. To your perception of visible light, wavelength is color and amplitude is brightness. I can find sources that say there is no actual side to side or up and down fluctuation going on in a light wave. It is a graph of the fluctuations in magnitude/strength of the electric and magnetic fields. Yet, polarization of a light “wave” requires the electric field to literally be vibrating perpendicular to its direction of travel. When thinking of light as photons, not waves, you instead use the concepts of spin and superposition of states to explain how polarization works. The different ways of looking at it seem to me to both be right, yet both contradict each other, so people use whichever method(s) best suit their needs at the time.

Light from the sun or most other sources isn’t polarized, you are getting lots of light waves with random orientations. Once that light passes through a polarizer, or reflects off surfaces, you can get waves to start having matching orientations, or polarization.

18

The polarizer effect video is really neat! Thanks!

19

Waves are weird. I was recently watching a video of a tsunami (the one in Japan as it happens) and, contrary to what one might expect, it isn’t a huge wave that comes crashing in, but a surge of pressure that just keeps sending more and more water onto the land.

20

In my writeup I was simply trying to keep thing, well, simple. Yes, the two fields are orthogonal to each other, but you’ll notice I said it was a vertical E-field antenna, hence E is vertical and H is horizontal. There’s a lot more to it, obviously. But I find it best to start of simply.