Electromagnetic waves, how do they travel?

Question

Suppose i have a source of electro-magnetic waves. What happens to the environment around that source that makes the wave travel? Do the points around the source start to oscilate and what is the connection of the sinusoidal graphics of say the electric intensity of the field and the matter around the source. Is it correct to asume that at a certain moment each point is on a such sinusoide? I mean do points arounde the source move up an down in a sinusoidal manner, just like water does when you drop a rock? If not, then how on earth and what makes an electro-magnetic wave travel from A to B?

Answer 1

The EM waves (including light) are transverse waves, which means they oscillate at right angles to the direction of propagation. A good transverse wave example is the ripples on a pond example that some other answerer gave.

On the other hand, longitudinal waves, like sound waves, travel in the direction of propagation. Another name for this kind of waves is a compression wave. Earthquakes, for example, are usually a mix of compression and transverse waves.

The height of a transverse wave (A) is called its amplitude. It is a measure of how strong or weak the EM signal is. (For example, a bigger rock will invoke higher, stronger ripples in the pond.)

The vast majority of transverse waves follow a sine pattern defined by A(t) = A sin(wt - phi); where w is the angular frequency of the wave, t = time, and phi = phase angle of the cycling. However, wave generators can create some other shapes as well...a square wave for example.

As the amplitudes of a transverse EM wave rise and fall over time, they form a cycle. One cycle, measured from crest to next crest, for example, has a length L. The velocity of a crest as it moves outward from its source (e.g., a pebble tossed into the pond or a transmitting antenna) is found by v = L/t; where t is the time it takes for the crest to travel one cycle's length.

We all know that EM waves (light radio, TV, and light) travel at the speed of light (c). So v = c = L/t = Lf; where f = 1/t = the frequency of the transverse EM waves (e.g., 560 on your AM dial corresponds to a given f).

So like molecules in the ripples of pond water, the photons (EM particles) are carried at the speed of light by the transverse waves. As c = L/t = Lf, the length and, therefore, the frequency of the transverse wave are determined by the energy (E) of the photons making up that wave.

Which brings us to the photon. In addition to making up and being carried by the transverse waves, each photon has its own intrinsic energy waves. The energy they carry is found by E = hn; where h = Planks constant and n is the discrete frequency of the photon's packet of energy.

It is this energy (not the energy of the transverse waves) that carries the potential for doing environmental damage. In fact, people have been fried from the inside out by very high powered radar antennas because they absorbed the EM radar waves. In the Navy, we always took special precautions to ensure our radars were turned off before sending a maintenance crew aloft for repairs.

Microwave ovens, operating around radar frequencies, send out EM waves that get the food's molecules to vibrating. When molecules vibrate, we call that temperature. Vibrate the molecules fast enough and the food cooks, just like the hapless Navy repairmen cooked when they got too close to an operating radar antenna.

The length of EM waves extends from the so-called Planks length (L = 6 X 10^-34 meters) at one end up through and including the size of the universe itself. [See source.] Visible light hovers at L < !0^-6 meters = 1 micrometer; so that from c = Lf, we have c/L = 3 X 10^9 m/sec/10^-6 m = 3 X 10^15 cycles per sec (aka Herz) is a representative frequency of visible light.

The good news, if EM pollution can have any good news, is that radio and TV waves are relatively long. So their photons do not contain as much energy as their short waved relatives. Thus, TV and radio EM waves are not as likely to cause any adverse environmental impact. However, even their relatively low energies could have an impact over time.

Answer 2

The picture of sinusoids traveling in space is a misnomer for EM waves. They are quite difficult to draw. Picture a stone hitting the water and circles of water traveling out from the center. Now let's assume that the EM wave has its electric field in the same plane as the water. The electric field would be in circles like the water waves but the electric field doesn't go up and down like the water. The electric field is in loops like the water wave and the thing that varies is the intensity and direction of the electric fields in the horizontal direction. The magnetic field forms loops around the electric field and it always a right angles to it. I have never come across a good drawing so that's the best I can do.

Answer 3

Electromagnetic radiation is a bombardment of electrically charged particles, and thus doesn't need a medium to travel through. While traveling in a vacuum such as outer space, it should not be though of like a sound wave, which does require a medium to propagate.

Answer 4

Electromagnetic waves travel as discrete particles called photons. It is a common misconception that electromagnetic waves need a medium to propogate through; they do not. The photon simply moves from some point to another.