Re: Young's double slit experiment questions

1A) It has to do with constructive and destructive interference of the two
wave fronts from the slits. A single coherent light source is used in the
experiment which produces two in-phase wave fronts on exit from the slits.
It is these two waves that interact causing the characteristic observed
pattern - alternating light and dark bands.

2A) The classical double-slit experiment uses a single coherent light
source that illuminates a plate with two slits cut into it. Therefore, the
two waves exiting the slit will be in-phase. Coherent waves must not only
have the same amplitude, but must also have a constant phase difference (zero).

Goto http://micro.magnet.fsu.edu/primer/java/interference/doubleslit/ for a
visual animation and description of the original experiment.