Page 338 - NS-2 Textbook

PHYSICAL SCIENCE 333

• The frequency, abbreviated as f, is the 11lUllber of the German pioneer in electromagnetic radiation, Hein-
cycles repeated dming 1 second of tinle. If the time rich Hertz (1857-1894). One hertz is defined as one cycle
frame shown in the figure were 1 second long, for per second. Frequency is expressed in terms of numbers
example, it could be said that the frequency of the of thousands (kilo), millions (mega), or billions (giga) of
V\Tave shown is 3 cycles per second. hertz. For example, 10,000 cycles per second is expressed
• The period, abbreviated by the Greek letter 1: (tau) as 10 kilohertz, abbreviated 10 kHz; 2.5 million cycles per
is the tinle required to complete one cycle of the second would be 2.5 megahertz, or 2.5 MHz.
wave. In the example above, the period would be The behavior of an electromagnetic 'wave is depen-
'h second. Period and frequency are l-elated by the dent upon its frequency and corresponding wavelength.
formula 1: = l/f. For descriptive purposes, electromagnetic frequencies
can be arranged in ascending order to form a Ufrequency
In the vacuum of space, an electronlagnetic ",vave is the- U
spectrum diagram. Electronlagnetic waves are classi-
orized to travel at a velocity appl"oachlng the speed of fied as audible-frequency waves (20-20,000 Hz) at the
8
light, or 3 X 10 meters pel" second (186,000 miles per sec-
Imver end of the SreCh'lun, radio 'waves from about 5 kHz
ond). Frequency and wavelength of an electromagnetic to 30 GHz, and visible light and various kinds of rays at
8
wave are related by the formula A = 3 X 10 / f, where A is
the upper end of the spectrum.
the wavelength in meters and f the frequency in cycles
TIlough electromagnetic wave h·equencies withln
per second. Thus, evelY specific electromagnetic fre-
the range of 20 to 20,000 Hz are called the audible fre-
quency is radiated at a specific wavelength.
quencies, it must be remembered that to be heard such
In recent years, the term hertz, abbreviated Hz, has
,vaves must be transformed into mechanical SOlllld
corne to be used in place of cycles per second, in honor of
'waves tlu'ough devices called speakers.
PROPAGATION EFFECTS
Wavelength,
Frequency, Hz nanoDlelers
The meditllll through 'which radio 'waves travel vvill af-
fect their path. Weather or atmospheric conditions can
10 23
10· 5 cause variation from the straight path that the 'waves
10 22 might otherwise take in a vacuum. TIl€Se variations are
10- 4
IO~I I X ul called refraction, reflection, diffraction, and trapping.
Gamnla-
10 20 rays 10- • Refractio11 (bending) occms when there is a change in
10- 2 the density or the atnlosphere in 'which the wave is trav-
10 19
10- 1 I A eling. Take for example a radio wave transiting the at-
lOlA
X-rays I mosphere. Because the atmosphere gradually decreases
10 17 10 in density with altitude, the wave is refracted, or bent
16
10 15 1 Ulu-d. violet r 10 2 downward. This increases the horizontal distance the
10 3 \vave vdll have to travel to get out of the atmosphere.
H 10 lJLIn Refraction also causes bending of radio waves over
IO 4
10 the horizon. Lo\v-frequency waves are bent more readily
13
10
J Infrared 10 5 than high-frequency 'waves, so low frequency,vaves are
10 12 used for long-range radars and long-distance radio com-
10 6
10" 7 munication.
1010 10 lcm Radio 'waves are reflected frOln the ionosphere, ,vhich
10 8 is generally from 30 to 250 miles above Earth. The dis-
9
10 Short radio waves 9
lOB 10 1m tance between the transmitter and the point where the re-
1010 flected sky V\rave returns to a ground receiver is called
10 7 the skip distallce.
I MHz 10 6 10" Diffractioll causes spreading of radio waves behlnd
10 12 I km obstructions. It results from the generation of secondary
5
10 10"
10 4 ,"vaves by the prin1ary lvave.
Long rd.dio waves 10 14 Trappi11g occurs 'when a telnperature inversion in the
1 kHz 10' 15 atmosphere traps cold air close to Earth's smface. Under
10
2
10 those circumstances radio signals may be reflected from
10 16
10 the warmer air above back to Earth a number of times.
This will increase the range of the transmitted signal.
The electromagnetic spectrum. TIlis trapped cold air is called a dllct.