TC 9-64 _________________________________________________________________________
Figure 1-11. Analogy of Reflection
Diffraction
1-43. Diffraction is the bending of the wave path when the waves meet an
obstruction. The amount of diffraction depends on the wavelength of the
wave. Higher frequency waves are rarely diffracted in the normal world that
surrounds us. Because light waves are high frequency waves, you will rarely
see light diffracted. You can, however, observe diffraction in sound waves by
listening to music. Suppose you are outdoors listening to a band. If you step
behind a solid obstruction, such as a brick wall, you will hear mostly low
notes. This is because the higher notes, having short wave lengths, undergo
little or no diffraction and pass by or over the wall without wrapping around
the wall and reaching your ears. The low notes, having longer wavelengths,
wrap around the wall and reach your ears. This leads to the general
statement that lower frequency waves tend to diffract more than higher
frequency waves. Broadcast band (AM band) radio waves (lower frequency
waves) often travel over a mountain to the opposite side from their source
because of diffraction, while higher frequency TV and FM signals from the
same source tend to be stopped by the mountain.
Doppler Effect
1-44. The last, but equally important, characteristic of a wave that we
discuss is the Doppler effect. The Doppler effect is the apparent change in
frequency or pitch when a sound source moves either toward or away from
the listener, or when the listener moves either toward or away from the
sound source. This principle, discovered by the Austrian physicist Christian
Doppler, applies to all wave motion.
1-45. The apparent change in frequency between the source of a wave and
the receiver of the wave is because of relative motion between the source and
the receiver. To understand the Doppler effect, first assume that the
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