from nearby points may continue for such a long time that they interfere with
the returning echoes from targets.
1-76. Any disturbance, man-made or natural, that causes an undesirable
response or the degradation of a wave is referred to as interference. Two
sound waves moving simultaneously through the same medium will advance
independently, each producing a disturbance as if the other were not present.
If the two waves have the same frequency--are in phase with each other and
are moving in the same direction--they are additive and are said to interfere
constructively. If the two waves have the same frequency and are moving in
the same direction, but out of phase with each other, they are subtractive and
are said to interfere destructively. If these two subtractive waves have equal
amplitudes, the waves cancel each other. This addition or subtraction of
1-77. At some time during your life you probably observed someone putting
his or her head into an empty barrel or other cavity and making noises
varying in pitch. When that person's voice reached a certain pitch, the tone
produced seemed much louder than the others. The reason for this
phenomenon is that at that a certain pitch the frequency of vibrations of the
voice matched the resonant (or natural) frequency of the cavity. The resonant
frequency of a cavity is the frequency at which the cavity body will begin to
vibrate and create sound waves. When the resonant frequency of the cavity
was reached, the sound of the voice was reinforced by the sound waves
created by the cavity, resulting in a louder tone. This phenomenon occurs
whenever the frequency of vibrations is the same as the natural frequency of
1-78. The most complex sound wave that can be produced is noise. Noise has
no tonal quality. It distracts and distorts the sound quality that was intended
to be heard. Noise is generally an unwanted disturbance caused by spurious
waves originating from man-made or natural sources, such as a jet breaking
the sound barrier, or thunder.
1-79. The concepts used to describe the behavior of waves--including
of light and light waves described below. The relationship between light and
light waves (rays) is the same as for sound and sound waves.
1-80. Light is a form of electromagnetic energy. It can be produced by various
means (e.g., mechanical, electrical, and chemical). We can see objects because
the light waves (rays) they give off or reflect reach our eyes.
1-81. The exact nature of light is not fully understood, although scientists
have been studying the subject for many centuries. Some experiments seem
to show that light is composed of tiny particles, and some suggest that it is
made up of waves.