________________________________________________________________Wave Propagation
1-33. Let us work one more problem, this time using the metric system.
Suppose the wavelength is 0.4 meters and the frequency is 12 kHz. What is
the velocity? Use the following formula:
velocity = wavelength x frequency (v = λf)
Given:
λ = 0.4 meters
f = 12 kHz
Solution:
v= λxf
v = 0.4 meters x 12,000 Hz
v = 4,800 meters per second
1-34. Other important characteristics of wave motion are reflection,
refraction, diffraction, and the Doppler effect. Big words, but the concept of
each is easy to see. For ease of understanding, we explain the first two
characteristics using light waves, and the last two characteristics using
sound waves. You should keep in mind that all waves react in a similar
manner.
1-35. Within mediums, such as air, solids, or gases, a wave travels in a
straight line. When the wave leaves the boundary of one medium and enters
the boundary of a different medium, the wave changes direction. For our
purposes, a boundary is an imaginary line that separates one medium from
another.
1-36. When a wave passes through one medium and encounters a medium
having different characteristics, three things can occur--
Some of the energy can be reflected back into the initial medium.
Some of the energy can be transmitted into the second medium where
it may continue at a different velocity.
Some of the energy can be absorbed by the medium. In some cases, all
three processes (reflection, transmission, and absorption) may occur
to some degree.
Reflection
1-37. Reflection waves are simply waves that are neither transmitted nor
absorbed, but are reflected from the surface of the medium they encounter. If
a wave is directed against a reflecting surface, such as a mirror, it will reflect
or "bounce" from the mirror. Refer to figure 1-9. A wave directed toward the
surface of the mirror is called the incident wave. When the wave bounces off
of the mirror, it becomes known as the reflected wave. An imaginary line
perpendicular to the mirror at the point at which the incident wave strikes
the mirror's surface is called the normal, or perpendicular. The angle between
the incident wave and the normal is called the angle of incidence. The angle
between the reflected wave and the normal is called the angle of reflection. If
the reflecting surface is smooth and polished, the angle between the incident
ray and the normal will be the same as the angle between the reflected ray
and the normal. This conforms to the law of reflection, which states: The
angle of incidence is equal to the angle of reflection.
1-11
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