محاضرة-1-

PROPERTIES OF LIGHT
PROPERTIES OF WAVES:
A wave is a disturbance that transmits energy from one point to another. Any periodic disturbance can be represented by waves; for example, musical sounds, human speech, the swine of a pendulum, a cork bobbing up-and-down in the water, and the violent shock from an earthquake. Light, as it travels through space, also can be described as a wave phenomenon.
Figure 1 illustrates a wave traveling to the right at one instant of time. The displacement is the vertical distance from the equilibrium position at any point along the wave. The amplitude is the maximum displacement of the wave. Wavelength is defined as "the distance over which the wave repeats itself" and is represented by the Greek letter lambda (?).

Fig. 1 Displacement-versus-distance along a
Figure 2 illustrates the same wave as a function of time at a single point. The period is the time over which the wave repeats it self-the time required for one complete cycle of the wave. The frequency of the wave is the number of cycles of the wave in one second and is represented by the Greek letter nu (?).

Fig. 2 Displacement-versus-time at one point on the wave
Frequency and period are related by Equations 1 and 2:
Equation 1

Equation 2

where: T = Period in seconds (sec).
? = Frequency in hertz (Hz).
The unit of frequency is cycles/second, or hertz, and may be expressed symbolically as Hz, sec-1, or /sec. Period is measured in seconds and wavelength in meters. Table 1 lists prefixes used with these base units and the common units formed. The unit micrometer (10-6 m) is a widely-used unit of length and also is called a "micron." The micron is represented by either ?m or ?, with ?m preferred. A common wavelength unit not included in this table is the angstrom (1 Angstrom = 10-10 m).
Prefix Symbol Value Common Use
Wavelength (m) Time (sec) Frequency (Hz)
Tera
Giga
Mega
kilo
milli
micro
nano
pico T
G
M
k
m
?
n
p 1012
109
106
103
10-3
10-6
10-9
10-12 mm
?m or ?
nm ms
?s
ns
ps THz
GHz
MHz
kHz
Since speed is defined as distance traveled divided by elapsed time, the speed of a wave is the distance traveled in one cycle, or the wavelength divided by the time required for one cycle (the period). Equations 3 and 4 are expressions of wave speed.
Equation 3

Equation 4
V = ??
where: T = Period of the wave
? = wavelength
? = Frequency of the wave
V = Speed of the wave
The speed of light waves through a vacuum is 3 x 108 m/sec and is expressed by the symbol c, which can be substituted for V in Equation 4 to give Equations 5, 6, and 7.
Equation 5
c = ??
Equation 6

Equation 7


EXAMPLE A: CALCULATION OF FREQUENCY AND PERIOD
Given: A radio wave has a wavelength of 30 m.
Find: The frequency and period of the wave.
Solution:
EXAMPLE B: CALCULATION OF WAVELENGTH AND PERIOD
Given: Ordinary ac current has a frequency of 60 Hz.
Find: The wavelength and period of the wave.
Solution: