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NN5I
10-30-2014, 08:16 AM
We all learned in high school physics that the speed of light (and radio waves) is about 186,300 miles per second. Do radio signals always go this fast?

24-hour rule.

wa8yxm
10-30-2014, 09:32 AM
NO, they do not, Nor does light, else a prism would not work and there would be no such thing as a rainbow.

But to make it easier to understand.. Coax Cable (and other feed lines) have a specification called Velocity Factor.. Coax as I recall is usually 0.66.

The speed of light in metric, very slightly rounded, is 300 Million Meters per second (299.999,I forget how many nines it goes out and ends in a 7 as i recall so it's not much rounded)

in coax it would be 300*0.66 or about 200 meters (Again rounded) per second.

NN5I
10-31-2014, 09:51 AM
YXM is mostly right, but not when he attributes rainbows exclusively to refraction. See below.

We learned that the speed of light (and radio waves) is about 186,300 miles per second. This is true in the vacuum of space; but these waves actually go slower through matter (such as glass or air) than they do in a vacuum. Radio waves inside many kinds of coaxial cable, for example, are slowed to only 2/3 of their speed in empty space. This is sometimes very important. The slowdown ratio, about 66% for most coax, is called the velocity factor, and is essential knowledge when calculating, say, the length of a quarter-wave section of cable. The velocity factor is always equal to, or less than, 1.0.

For light traveling through transparent materials such as glass or air, or radio waves traveling through anything, this slowdown can also cause a change of direction, called refraction, when the wave crosses a boundary at an oblique angle. The index of refraction of a transparent medium is the inverse of the velocity factor, and is always equal to, or greater than, 1.0.

Consider a radio wave traveling along the surface of the Earth. It is in atmosphere, which gets thinner with increasing altitude. The upper part of our wave is in thinner air than is the part near the ground, and will go faster. This causes the wave to bend downward instead of going in a straight line, and the result is that the wave can sometimes follow the curvature of Earth’s surface. That is an example of refraction, and can be useful for radio communication.

Additional note:

Many transparent substances have indices of refraction that vary with the color (frequency) of the light. This dispersion causes light of different colors to be bent through slightly different angles, separating the light into the colors of the rainbow, or into the colors of the spectrum when sunlight is made to pass thrugh a triangular glass prism. It also causes chromatic aberration in camera lenses when the various colors are focused at different distances from the lens.

Lens manufacturers use combinations of different kinds of glass (low-dispersion glass such as "crown glass" or high-dispersion glass of various types), to make lenses essentialy free of chromatic aberration.

It is dispersion, not refraction per se, that produces rainbows and visible spectra.

wa8yxm
10-31-2014, 11:43 AM
Well, we could argue sematics all day but that dispersion is caused because the different frequencies are slowed at different rates. else they would all refract the same.

NN5I
11-01-2014, 09:57 AM
Well, we could argue semantics all day but that dispersion is caused because the different frequencies are slowed at different rates. else they would all refract the same.

My apology, YXM. I meant no offense. I only meant to point out that dispersion and refraction are separate phenomena; neither implies the other. Refraction is almost always accompanied by at least some dispersion, as you imply; but dispersion often occurs without refraction. The dispersion that gives rise to rainbow colors on oil-filmed water, for example, is a result of diffraction, not refraction; and most of the iridescent colors on insect wings also are associated with diffraction and not with refraction.

The rainbow is not yet fully understood, despite centuries of scientific inquiry; fairly recent articles in Scientific American suggest that it may be caused by diffractive dispersion as well as by refractive dispersion.