5 The Incredible Quantum Mechanics
93
Fig. 5.4 Light waves passing through two slits and impinging on an optical screen,
or a photographic plate
Fig. 5.5 Interference pattern from light (coherent) passing through two slits
and falling on a photographic plate. Image, public domain, courtesy of Pieter
Kuiper (Image adapted from https://commons.wikimedia.org/wiki/File:SodiumD_four_
double_slits.jpg (accessed 2020/05/23))
other, be diffracted in the manner we discussed in the last Section, and then
impinge on the photographic plate. We would expect over time to see a buildup of the diffraction pattern from one slit (Fig. 5.3) superimposed on that of
the other, but not the fine interference structure shown in Fig. 5.5. However,
the observed interference pattern remains exactly the same as in Fig. 5.5. It
just takes longer to build up because of the weak beam intensity.
It is as though the photon is not really specified in location until it has
impinged on the photographic plate, and its wave function has collapsed.
Before then it could be anywhere, and it could be passing through either slit,
or even through both slits simultaneously, such is its ghostly nature.
93
Fig. 5.4 Light waves passing through two slits and impinging on an optical screen,
or a photographic plate
Fig. 5.5 Interference pattern from light (coherent) passing through two slits
and falling on a photographic plate. Image, public domain, courtesy of Pieter
Kuiper (Image adapted from https://commons.wikimedia.org/wiki/File:SodiumD_four_
double_slits.jpg (accessed 2020/05/23))
other, be diffracted in the manner we discussed in the last Section, and then
impinge on the photographic plate. We would expect over time to see a buildup of the diffraction pattern from one slit (Fig. 5.3) superimposed on that of
the other, but not the fine interference structure shown in Fig. 5.5. However,
the observed interference pattern remains exactly the same as in Fig. 5.5. It
just takes longer to build up because of the weak beam intensity.
It is as though the photon is not really specified in location until it has
impinged on the photographic plate, and its wave function has collapsed.
Before then it could be anywhere, and it could be passing through either slit,
or even through both slits simultaneously, such is its ghostly nature.