Page 34 - PRINCIPLES OF QUANTUM MECHANICS as Applied to Chemistry and Chemical Physics

1.6 Young's double-slit experiment 25

B and vice versa. To answer this question, Young's experiment is repeated with
both slits open and with only one photon at a time emitted by S. The elapsed
time between each emission is long enough to rule out any interactions among
the photons. While it might be expected that, under these circumstances, the
pattern in Figure 1.9(b) would be obtained, in fact the interference fringes of
Figure 1.9(c) are observed. Thus, the same result is obtained regardless of the
intensity of the light beam, even in the limit of diminishing intensity.
If the detection screen D is constructed so that the locations of individual
photon impacts can be observed (with an array of scintillation counters, for
example), then two features become apparent. The ®rst is that only whole
photons are detected; each photon strikes the screen D at only one location.
The second is that the interference pattern is slowly built up as the cumulative
effect of very many individual photon impacts. The behavior of any particular
photon is unpredictable; it strikes the screen at a random location. The density
of the impacts at each point on the screen D gives the interference fringes.
Looking at it the other way around, the interference pattern is the probability
distribution of the location of the photon impacts.
If only slit A is open half of the time and only slit B the other half of the
time, then the interference fringes are not observed and the diffraction pattern
of Figure 1.9(b) is obtained. The photons passing through slit A one at a time
form in a statistical manner the pattern labeled I A in Figure 1.9(a), while those
passing through slit B yield the pattern I B . If both slits A and B are left open,
but a detector is placed at slit A so that we know for certain whether each given
photon passes through slit A or through slit B, then the interference pattern is
again not observed; only the pattern of Figure 1.9(b) is obtained. The act of
ascertaining through which slit the photon passes has the same effect as closing
the other slit.
The several variations on Young's experiment cannot be explained exclu-
sively by a wave concept of light nor by a particle concept. Both wave and
particle behavior are needed for a complete description. When the photon is
allowed to pass undetected through the slits, it displays wave behavior and an
interference pattern is observed. Typical of particle behavior, each photon
strikes the detection screen D at a speci®c location. However, the location is
different for each photon and the resulting pattern for many photons is in
accord with a probability distribution. When the photon is observed or
constrained to pass through a speci®c slit, whether the other slit is open or
closed, the behavior is more like that of a particle and the interference fringes
are not observed. It should be noted, however, that the curve I A in Figure
1.9(a) is the diffraction pattern for a wave passing through a slit of width
comparable to the wavelength of the wave. Thus, even with only one slit open