Double slit interference: moving waves

[PiotrDab]

As I understand the double slit experiment, an interference is created on the barrier. Usually the drawing is made as the constructive waves hit the wall perfectly in middle. Let's say the wave has a frequency f, the interference should be moving away from the middle at time t+f/4. Why we don't see that move on the screen? Are they standing waves? if yes, where does the resonating come from? (the screen?)

 

[.Scott]

Because the laser source is equidistant to both slits, the slits are being illuminated in phase.

Using the wave model:
Because the midpoint on the screen is also equidistant to both slits, the "peaks" and "troughs" of the waves arrive at the same time from both slits - so the "add up" in amplitude.