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DK2ZA > INFO 18.08.02 23:16l 75 Lines 2909 Bytes #999 (30) @ WW
BID : I8CDB0MRW02X
Read: DB0FHN GUEST DK2ZA
Subj: dispersion, diffraction etc.
Path: DB0FHN<DB0ZWI<DB0CHZ<DB0ERF<DB0SON<DB0MRW
Sent: 020818/2354z @:DB0MRW.#BAY.DEU.EU [Wunsiedel, JO60AB] Bcm1.46 LT:030
From: DK2ZA @ DB0MRW.#BAY.DEU.EU (Helmut)
To: INFO @ WW
X-Info: Sent with login password
Hello Warren and all,
the meanings of the words divergence, diffraction, refraction and
dispersion can easily be mixed up.
divergence:
The rays of a beam of light (coming out of a flashlight or a laser)
are not exactly parallel, so that the beam widens (diverges) as it
goes away from its source. The light becomes dimmer and dimmer with
distance. Does it finally disappear? If it hits obstacles like
interstellar gas, it will. Or else we have to look at it as a rain
of photons which can be thinned out indefinitely.
diffraction:
You cannot see somebody standing behind a big tree, but you can hear him.
The soundwaves are bent around the obstacle because their length is
comparable to the thickness of the tree. The light waves are also bent
(diffracted) around ostacles, but owing to their small wavelength this
is not so easy to demonstrate. Use a laser pointer and observe the
shadow behind a razor blade that blocks half of the light beam.
refraction:
Light in glass or in water is slower than in air. This has the consequence
that a beam of light that strikes the surface of a cup of water not (!)
vertically is broken (refracted) as it enters the water. Its way in the
water is also a straight line but it is steeper. In glass lenses this
effect is used to make pictures.
The refractive index n is a number that shows how large this effect is
in different transparent media. It is always greater than or equal to 1.
The speed of light in the medium is v = c/n where c is the speed of
light in a vacuum. For a glass with n = 1.5 the speed of light in it
is c/1.5 = 0.667*c = two thirds of c.
The refractive index for light in a vacuum is 1. In air it is 1.000272
at 20 degrees Celsius and 1013 mbar air pressure for light with a
wavelength of 589 nm. This value becomes smaller as the air gets thinner
with height. So light waves and radio waves are refracted downward little
by little when they left the surface of the earth under a very small angle.
dispersion:
For most transparent media (notable exception: vacuum!) the speed of light
(and consequently the refractive index) changes with its wavelength. This
is called dispersion. Almost always n becomes smaller as the wavelength
increases. This is used to produce colorful spectra with a glass prism.
Now I hand the keyboard over to Albert E.:
The velocity of light in a vacuum (!!) is the same for all observers
if the law of inertia holds for them.
(law of inertia: any material object stays at rest or moves in a straight
line at constant velocity if no force acts upon it.)
The velocity of light in a transparent medium is always smaller than in
a vacuum and depends on the medium and on the wavelength.
vy 73 to all amateur scientists de Helmut, DK2ZA
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