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General Chemistry Principles, Patterns, and Applications, 2011

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Higher-energy vibrations (overtones) are produced when the string is plucked more strongly; they have wavelengths<br />

given by<br />

Equation 6.17<br />

l = 2Ln<br />

where n has any integral value. Thus the vibrational energy of the string is quantized, <strong>and</strong> only certain<br />

wavelengths <strong>and</strong> frequencies are possible. Notice in Figure 6.18 "St<strong>and</strong>ing Waves on a Vibrating String" that all<br />

overtones have one or more nodes, points where the string does not move. The amplitude of the wave at a node is<br />

zero.<br />

Figure 6.18 St<strong>and</strong>ing Waves on a Vibrating String<br />

Quantized vibrations <strong>and</strong> overtones containing nodes are not restricted to one-dimensional systems, such as strings.<br />

A two-dimensional surface, such as a drumhead, also has quantized vibrations. Similarly, when the ends of a string<br />

are joined to form a circle, the only allowed vibrations are those with wavelength<br />

Equation 6.18<br />

2πr = nλ<br />

where r is the radius of the circle. De Broglie argued that Bohr’s allowed orbits could be understood if the electron<br />

behaved like a st<strong>and</strong>ing circular wave (Figure 6.19 "St<strong>and</strong>ing Circular Wave <strong>and</strong> Destructive Interference"). The<br />

st<strong>and</strong>ing wave could exist only if the circumference of the circle was an integral multiple of the wavelength such that<br />

the propagated waves were all in phase, thereby increasing the net amplitudes <strong>and</strong> causing constructive<br />

Saylor URL: http://www.saylor.org/books<br />

Saylor.org<br />

534

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