What distribution do we see now? Actually we will seeP 1 +P 2 if we can tell which slit the electron
went through. Ourobservation of the electron as it passes through the slit haschanged
the resulting intensity distribution. If we turn the light off, we go back to measuringP 12.
Can you explain why the light causes the diffraction pattern to disappear?
Is it the mere observation? Does the light change the phase of the electron?
There are many examples of an observer changing the result of a Quantum experiment. Indeed, it
is held that when a state is observed, its wave function collapses intothe state “seen”. In this case,
all we had to do is turn on the light. We didn’t have to look.
Finally, we will do a two slitdiffraction experiment with bullets. We must make slits big
enough for the bullets to pass through.