Physical Chemistry Third Edition

630 14 Classical Mechanics and the Old Quantum Theory

(a)

t 1 : t 2 : t 3 : t 4 :

(b)

A traveling wave:t 1 : t 2 : t 3 : t 4 :

A standing wave:

Figure 14.5 Traveling and Standing Waves.(a) A traveling wave at timest 1 <t 2 <t 3 <

t 4. (b) A standing wave at timest 1 <t 2 <t 3 <t 4.

Scattering centers

Wave trough

Wave crest

Wave front

Wave crests

4 l

3 l

2 l

l

l

Figure 14.6 The Diffraction of a Wave by a Row of Scattering Centers.The direction

of the diffracted wave depends on the wavelength.

interfere to produce a diffracted wave with straight crests, which can travel in a direction

different from that of the incident wave. Light waves and other waves exhibit the same

phenomenon.

Aplane waveis a traveling wave in three dimensions in which a crest of the wave

occupies a plane at right angles to the direction of propagation. Figure 14.6 illustrates

diffractionof a plane wave by a set of equally spaced scattering centers. The broken lines

in Figure 14.6 represent the crests of a plane wave moving from left to right. The arcs

represent the crests of diffracted waves moving outward from the scattering centers.

At a large distance from the scattering centers these crests interfere to produce a plane

wave moving in a different direction from the original direction of propagation. The