First, for some background, look over the this chart, which shows the full range of electromagnetic waves.
Click to enlarge.
This chart treats all EMR as waves, but they are quanta, so really they are neither waves nor particles, but they can be detected or measured as either.
The six horizontal rows of information in this chart:
1) wavelengths of electromagnetic radiation in meters, m;
2) various familiar objects, each of whose actual size is in the same range as the radiation;
3) common names of the various forms of radiation, with arrows showing their range of wavelengths;
4) devices or processes that produce EMR in that range;
5) frequencies of the corresponding EMR; and
6) energies of the corresponding EMR.
So looking vertically down the chart at a particular wavelength, you see objects of that size, the common name and range of that EMR, how that EMR is produced, and its corresponding frequency and energy. Wavelengths, energy, and frequency are related to each other thus:
where
E = energy, h = Planck's constant, ν = frequency,
So shorter wavelengths of EMR have higher frequency and energy; longer wavelengths have lower frequency and energy.
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Now at last, some insights into How Scientists Know about EMR,
Watch These Videos
James Clerk Maxwell, building on the experimental and conceptual work of several scientists, especially that of Faraday, develops a mathematical description of electromagnetic waves and fields, and ultimately shows that 1) light is electromagnetic radiation, and 2) there should be a lot of EMR that we cannot see.
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Heinrich Hertz showed the existence of radio waves, confirming Maxwell's notion of EMR outside the visible range:
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Fields are very strange in comparison with objects in our daily experience. For that reason, I have included some extra materials to help you imagine electrical and magnetic fields.
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What is a gradient? What is a field?
What is a gradient? What is a field?
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Easy ways to see the effects of electrical fields.
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The remaining videos have helped me get a better understanding of EMR, but they focus more on what scientists know than on how they know.
In the next video, Nick Lucid (you think that's his real name?) helps you to visualize the physical meaning of the Maxwell equations.
If you like Nick Lucid's explanations, HERE is a link to all of his videos on electricity and magnetism, in a sensible order (currently, 18 of them). They make a great little introductory course on electromagnetism.
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How do antennas send and receive electromagnetic waves?
Here are two animations that show antennas and EM waves in action. Click on either to read a fuller description of what you are seeing.
Antenna Sending
Alternating current in the antenna (the two bent metal bars in the center) generates an electromagnetic wave moving outward (moving black lines). The alternating current is produced by a radio transmitter, not shown, connected to the parts of the antenna that point out perpendicular to the screen. The magnetic component of the electromagnetic wave is not shown. Click figure for fuller description and for figure credits.
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Antenna Receiving
The incoming electrical component (green) of an electromagnetic wave (such as a radio wave) pushes electrons (black arrows) back and forth in the antenna (the two bent metal bars), generating an alternating current that travels to a receiver (R). The magnetic component of the incoming electromagnetic wave is not shown. Click figure for fuller description and for figure credits.
Notice 1) the reciprocal nature of antenna transmission and reception (both diagrams could be used to illustrate both processes), and 2) the lack of relevance of the magnetic component (not shown) of electromagnetic radiation to either process. The magnetic component does actually play a role: it sustains the electrical component as the electromagnetic wave travels through space.
Click HERE to read all about antennas at Wikipedia.
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The Meaning of the Maxwell Equations
Beyond this point are a bunch of videos, perhaps still in no particular order, on what the Maxwell equations mean (their physical meaning). I am saving them here because I am trying to understand more about them, and I don't want to lose them.
For the mathematically very literate reader, HERE is a very technical Wikipedia article about what the Maxwell equations mean.
This video, also for the mathematically literate, describes the physical meaning of the Maxwell equations. As in all Crash-Course videos, the narrator talks fast and the figures come and go fast; but remember: it's a video; you can pause it and look at a figure, you can rewind (5 seconds at a time with your left cursor) and re-listen.
This one a good attempt to help you understand the mathematics and physics of one of the Maxwell equations.
Here he is again, talking about another of the Maxwell equations. Above he used the differential form of the equation; below, he uses the integral forms, and he explains the difference. The Wikidpedia entry mentioned above (HERE) shows all four equations in both forms.
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The speed of light is greatest in the vacuum of space. All matter slows light down a little.
You can see the effect of slower light by watching this video with both eyes open, but with one eye looking through a lens, glass, or plastic sheet. Just relax and gaze at the middle of the image. Describe to yourself what you are seeing before you read on.
For most people, the presence of the lens in front of one eye changes the "snow" from flat to three-dimensional, with the illusion that speckles that appear to be closer to you and moving horizontally either left to right or vice versa, and speckles that appear to be farther from you and moving in the opposite direction. If you move the lens to the other eye, the direction of this apparent rotation is reversed.
I am looking for a good explanation of this effect, but I read long ago that it is caused by the reduction in speed of light through the lens, and that your eyes are seeing the same thing, but the covered eye is seeing it slightly later. Amazing to me that such a slight delay (time it takes for light to travel through a very thin lens or film) would give a detectable effect. But again, I am looking for an authoritative explanation of this effect, and this explanation is NOT authoritative. To me, the effect, whatever the explanation, is startling.
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In the next video, Nick Lucid (you think that's his real name?) helps you to visualize the physical meaning of the Maxwell equations.
If you like Nick Lucid's explanations, HERE is a link to all of his videos on electricity and magnetism, in a sensible order (currently, 18 of them). They make a great little introductory course on electromagnetism.
••••••
How do antennas send and receive electromagnetic waves?
Here are two animations that show antennas and EM waves in action. Click on either to read a fuller description of what you are seeing.
Antenna Sending
Alternating current in the antenna (the two bent metal bars in the center) generates an electromagnetic wave moving outward (moving black lines). The alternating current is produced by a radio transmitter, not shown, connected to the parts of the antenna that point out perpendicular to the screen. The magnetic component of the electromagnetic wave is not shown. Click figure for fuller description and for figure credits.
_____
Antenna Receiving
The incoming electrical component (green) of an electromagnetic wave (such as a radio wave) pushes electrons (black arrows) back and forth in the antenna (the two bent metal bars), generating an alternating current that travels to a receiver (R). The magnetic component of the incoming electromagnetic wave is not shown. Click figure for fuller description and for figure credits.
Notice 1) the reciprocal nature of antenna transmission and reception (both diagrams could be used to illustrate both processes), and 2) the lack of relevance of the magnetic component (not shown) of electromagnetic radiation to either process. The magnetic component does actually play a role: it sustains the electrical component as the electromagnetic wave travels through space.
Click HERE to read all about antennas at Wikipedia.
The Meaning of the Maxwell Equations
Beyond this point are a bunch of videos, perhaps still in no particular order, on what the Maxwell equations mean (their physical meaning). I am saving them here because I am trying to understand more about them, and I don't want to lose them.
For the mathematically very literate reader, HERE is a very technical Wikipedia article about what the Maxwell equations mean.
This video, also for the mathematically literate, describes the physical meaning of the Maxwell equations. As in all Crash-Course videos, the narrator talks fast and the figures come and go fast; but remember: it's a video; you can pause it and look at a figure, you can rewind (5 seconds at a time with your left cursor) and re-listen.
This one a good attempt to help you understand the mathematics and physics of one of the Maxwell equations.
Here he is again, talking about another of the Maxwell equations. Above he used the differential form of the equation; below, he uses the integral forms, and he explains the difference. The Wikidpedia entry mentioned above (HERE) shows all four equations in both forms.
••••••
The speed of light is greatest in the vacuum of space. All matter slows light down a little.
You can see the effect of slower light by watching this video with both eyes open, but with one eye looking through a lens, glass, or plastic sheet. Just relax and gaze at the middle of the image. Describe to yourself what you are seeing before you read on.
For most people, the presence of the lens in front of one eye changes the "snow" from flat to three-dimensional, with the illusion that speckles that appear to be closer to you and moving horizontally either left to right or vice versa, and speckles that appear to be farther from you and moving in the opposite direction. If you move the lens to the other eye, the direction of this apparent rotation is reversed.
I am looking for a good explanation of this effect, but I read long ago that it is caused by the reduction in speed of light through the lens, and that your eyes are seeing the same thing, but the covered eye is seeing it slightly later. Amazing to me that such a slight delay (time it takes for light to travel through a very thin lens or film) would give a detectable effect. But again, I am looking for an authoritative explanation of this effect, and this explanation is NOT authoritative. To me, the effect, whatever the explanation, is startling.
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