I know that light has a wavelength and a frequency, and I know that the wavelength and frequency of light are related since the speed of light is always 299,792,458 meters a second, but I have no idea what the wavelength and frequency of light represent or mean, really. Does light really travel in an up and down fashion on its way to wherever it’s going? Does this mean that in certain situations it can go around things? Is a light wave two-dimensional or does it enter three dimensions in some way?
Crude illustration follows:
VVVV.VVVV
Consider the V’s as a light wave (by making the tops and bottoms of the V’s rounder). It seems that if the light is traveling up and down along the V’s that it could miss an object represented by the period. And is the lightwave embedded into the plane of the V’s or does some part of it stick “out” from the screen? Also, is the wave “height,” that is, the height of the V’s in my crude illustration constant, or does that change with wavelength or frequency?
I have tried to read up on this on the Wikipedia and elsewhere, but I don’t really understand what’s going on. Apparently, there’s a part of a lightwave that’s perpendicular to its direction of travel, but I can’t wrap my head around what that refers to. And finally, does any of this have to do with photons, and if so, how?
Any explanations you would have, I would greatly appreciate, but keep in mind that I’m a moral philosopher, so my knowledge of even basic physics is not all that good.
The position of the light isn’t waving. It’s not like a water wave, where something is actually going up and down. What’s waving is the magnitude of electric and magnetic fields.
So, in this sense it’s more like a sound wave, where the up and down represents relative pressure due to compression and rarefaction, except that it’s not air (or whatever), but electric and magnetic stuff? But why then is the wavelength in meters? How does a distance relate to the magnetic and electric stuff?
What you need to wrap your head around is that light is made of “particles”; point-like packets of energy (photons) that are measured in a certain quanta. The value of that quanta is determined by the frequency (or wavelength, if you like). However, the interaction of light with other objects and media, in a classical optics sense, is best described mathematically as a wave, and for a long time it was assumed by all physicists that light was the propagation of electromagnetic energy in a field of lumeniferous aether that permeated everything. However, a combination of discoveries (the ultraviolet catastrophe, the photoelectric effect, and an inability to determine movement against the presumably privileged frame of the aether) indicated both that light is quantized and that it is self-propagating; like a thrown baseball, it doesn’t need any medium in which to be conveyed.
In an attempt to combine the apparently paradoxical nature of this supposed duality, some people imagine light as being a point particle that is bouncing up and down like a beach ball in the waves. It would be better (though still not strictly accurate) to imagine a photon as being a blob of liquid that is rhythmically flexing and contracting as it flies through space, both a “particle” and a (self-contained) “wave”.
Polarization of light doesn’t describe the physical position or orientation of the photon itself, but rather (in a classical sense) the orientation of the electric field and magnetic field as expressed by the Jones vector, or (in a quantum mechanics sense) the spin state of the photon. It is a mistake to try to visualize this as a mechanical interaction or motion of the photon; it is just mathematically how photons work.
Ignore the quantum stuff for later, you need to understand light at the classical level first. The way it gets quantized into photons is a much harder to understand, and still fraught with mathematical difficulties.
James Clerk Maxwell finally figured out what light was in the 19th century in one of the triumphs of physics. With quantum refinements, his theory is still the basis for our understanding of quantum electrodynamics.
Maxwell discovered that light is nothing more than a self propagating collection of electric and magnetic fields. The electric field is perpendicular to the direction of propagation and the magnetic field is perpendicular to that. Think of three perpendicular directions in space, like North, East, and Up, for example. Imagine the light is propagating from West to East, the magnetic field points Up or Down, the electric field points North or South. If you exanube the amplitude of the magnetic field as the wave travels toward the East, it starts at zero, gets larger pointing up, reaches a peak, then gets smaller, reverses direction, etc. If you plot up the amplitude vs. East/West position, you get a sine wave. The distance between the peaks or troughs is called the wavelength. You can do the same thing with the electric field and find that the electric field is at a peak or trough when the magnetic field is zero (and vice versa). Thus, the electric field plotted versus East/West distance is also a sine wave, with the same wavelength. If you sit at one spot and measure the number of peaks of magnetic (or electric) field that go by every second, that is the frequency of the wave.
Maxwell’s theory predicted all of this from his set of equations. Whenever the electric field is changing in time, it creates a perpendicular magnetic field. Whenever the magnetic field is changing in time, it creates a perpendicular electric field. Thus, these fields keep oscillating back and forth, propagating at a velocity Maxwell calculated from measurements of electric and magnetic forces. When he plugged in the numbers, Voila!, out came the speed of light.
I’ve described the simplest case, i.e. linearly polarized light. There are two linear polarizations. In my example these would correspond to the electric field being North/South with the magnetic field Up/Down or vice versa. By combining different amounts of these two polarizations, you can create a linear polarized beam with the electric field pointed in any arbitrary direction in the Up/Down/North/South plane, with the magnetic field still perpendicular to it.
A directly related question: I’ve always just accepted it when people tell me that microwaves can’t get through the screen in the door because the holes are smaller than the wavelength. First, why would the wavelength matter at all? Second, how do we reconcile the wave/particle duality to this situation?
(PS In high school I did the slit experiment but now I understand that they have done it one photon at a time and still get the interference pattern. Maybe we can talk about that after we clear up more basic questions.)
The light isn’t traveling along the wave, it is the wave. If you looked at that wave an instant later, the locations of the crests and troughs would all be moved over a little. So no, you couldn’t miss an object that way with a light wave, nor with any other kind of wave. Light will, however, interact differently with objects on a scale comparable to the wavelength, with effects like bending around such an object to some degree, and reflecting in multiple directions at once off a surface.
The answer has been covered pretty well. I just want to note that Stranger’s explanation, while (as usual) correct, is very misplaced here. Bringing photons into a discussion better handled with classical electromagnetic wave theory is just going to cause needles confusion – no one had raised the whole wave-particle duality yet, and this is just dragging it in for no obvious purpose.
And it’s really pointless and confusing to toss the Jones Vector at someone who’s already having trouble with visualizing EM waves.
Next up, to really confuse people — Mueller Matrices!
Maybe I misinterpreted your question but I don’t think some of replies directly address your confusion. Actually, I think Stranger On A Train explains this but it’s somewhat buried in his comments.
The VVVV crests and troughs is not a picture of what the light wave actually is. It’s a mathematical representation of it’s behavior. It’s a mathematical plot.
Suppose I plot on a graph the beating of a human heart. The x-axis represents time. The y-axis represents electrical impulse. After a minute, you’d see a graph wavy crests on it (like a EKG). Now, if I asked you, “So does that mean the heart moves up and down in a wave pattern when it beats in your chest cavity? Or does it mean the heart muscle is the shape of VVVVV?” … how would you answer that?
Actually, I don’t believe so. This is all a thought experiment, because you can’t really, but If you could watch a 3 million km photon as it passed you, it would take about 10 seconds from front to back. But if the front of the photon was at the crest as it started past you, you would see it waving perpendicular to the motion, and away from you. And you would see the back point of the wave waving as it approached. But, that crest in front of you would remain motionless for the entire duration of it’s existence in front of you, until the end of the photon comes past and it ceases to occupy that point. Photons as described by relativity are timeless.
You actually want the holes to be less than ~lamda/4. And for minimal leakage lamda/10 is a good target.
A highly conductive piece of metal plate literally shorts out the electric field portion of the wave. This causes a high local current flow in the metal, which causes a strong magnetic field, which cancels the original field, and even causes a copy of the original wave to be emitted from the surface in the opposite direction. It all happens at once, and all interacts, but those are the basic cause and effects.
The paragraph above is a detailed description that is usually shortened to the word “reflection”. It also explains why good electrical conductors tend to be very shiny. Note that there are some other ways to produce reflection that do not depend on bulk electrical conductivity of the reflector.
Now if the metal has holes in it, then the current has to flow around those holes. This causes a phase shift, so the induced current no longer completely cancels the incoming wave…some of it passes through the screen. Low frequency (long wavelength) fields can stand significant delays before the phase shift becomes large enough to reduce the reflection.
ETA: I’m just a dumb EE with some optics background. I’ll let the physicists address the particle part of your question.