1-20. Refer to wave 1 in figure 1-7. Notice how similar it is to the sine wave

you have already studied. All transverse waves appear as sine waves when

viewed from the side. In figure 1-7, wave 1 has four complete wave cycles.

Points ABCDE comprise one complete cycle having a maximum value above

and a maximum value below the reference line. The portion above the

reference line (between points A and C) is called a positive alternation and

the portion below the reference line (between points C and E) is known as a

negative alternation. The combination of one complete positive and one

complete negative alternation represents one cycle of the wave. At point E,

the wave begins to repeat itself with a second cycle completed at point I, a

third at point M, and so forth. The peak of the positive alternation (maximum

value above the line) is sometimes referred to as the top or crest, and the

peak of the negative alternation (maximum value below the line) is

sometimes called the bottom or trough, as depicted in the figure. Therefore,

one cycle has one crest and one trough.

1-21. A wavelength is the distance in space occupied by one cycle of a radio

wave at any given instant. If the wave could be frozen in place and measured,

the wavelength would be the distance from the leading edge of one cycle to

the corresponding point on the next cycle. Wavelengths vary from a few

hundredths of an inch at extremely high frequencies to many miles at

extremely low frequencies; however, common practice is to express

wavelengths in meters. In figure 1-7 (wave 1), the distance between A and E,

or B and F, etc., is one wavelength. The Greek letter lambda (λ) is used to

signify wavelength. Why lambda and not "l" or "L"? This is because "L" is