Huygen's Theory to expalin fringes in double slit experiment

[jmorris]

Each slit diffracts light
Wavelets are emitted by each slit.
The two slits emit coherent sources of light (constant phase difference, same frequency).
The waves from each slit overlap.
If they arrive at the screen in phase, they constructively interfere and produce a bright fringe.
If they arrive at the screen out of phase by Pi radians or 180 degrees, they destructively interfere and a dark fringe is produced.
The path difference to a bright fringe is a whole number of wavelengths.
The path difference to a dark fringe is a whole number of wavelengths +1/2.