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NN5I · 10-31-2014, 08:51 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.

10-31-2014, 08:51 AM	#3
NN5I Carl, nn5i Join Date: Sep 2011 Location: Tallahassee, FL Posts: 1,441	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. __________________ -- Carl