Shining light on a screen
produces basically uniform illumination.
Shining light which has
passed thru a pair of slits onto a screen produces an
interference pattern.
If we turn down the
intensity of the light so that only one quantum is passing
from source to screen at a time, we still
get the pattern!
Which slit did the quantum
go through?
Closing either slit
destroys the pattern!
How does the quantum know
about both slits?