Page 33 - PRINCIPLES OF QUANTUM MECHANICS as Applied to Chemistry and Chemical Physics

24 The wave function

A
I A I 1 I B
A A A

B I B B B

(a) (b) (c)
Figure 1.9 (a) Intensity distributions I A from slit A alone and I B from slit B alone. (b)
The sum of the intensity distributions I A and I B . (c) The intensity interference pattern
when slits A and B are open simultaneously.

experiment, an interference pattern as shown in Figure 1.9(c) is observed. The
intensity pattern in this case is not the sum I A ‡ I B , but rather an alternating
series of bright and dark interference fringes with a bright maximum midway
between points A and B. The spacing of the fringes depends on the distance
between the two slits.
The wave theory for light provides a satisfactory explanation for these
observations. It was, indeed, this very experiment conducted by T. Young
(1802) that, in the nineteenth century, led to the replacement of Newton's
particle theory of light by a wave theory.
The wave interpretation of the interference pattern observed in Young's
experiment is inconsistent with the particle or photon concept of light as
required by Einstein's explanation of the photoelectric effect. If the monochro-
matic beam of light consists of a stream of individual photons, then each
photon presumably must pass through either slit A or slit B. To test this
assertion, detectors are placed directly behind slits A and B and both slits are
opened. The light beam used is of such low intensity that only one photon at a
time is emitted by S. In this situation each photon is recorded by either one
detector or the other, never by both at once. Half of the photons are observed to
pass through slit A, half through slit B in random order. This result is consistent
with particle behavior.
How then is a photon passing through only one slit in¯uenced by the other
slit to produce an interference pattern? A possible explanation is that somehow
photons passing through slit A interact with other photons passing through slit