Interference
- Spacing of "fringes" in interference pattern from 2 slits
- Single slit diffraction and the effect on double slit diffraction
Two slit interference pattern
We have seen that you can qualitatively find the interference pattern from 2 slits by drawing the peaks and troughs from each slit, and then seeing where there is constructive or destructive interference.
The criteria for constructive interference at any point is that crests from both sources must arrive at the same time. We could measure the distance to slit 1 (call it $d_1$) and the distance to slit 2 (call it $d_2$).
The criteria for constructive interference is that the path difference ("pd") be an integer number of wavelengths: $$\text{pd}=|d_2-d_1| = n \lambda.$$ where $n$ is an integer, one of 0, 1, 2, 3, ...
Working this out for the case that the slits are much closer to each other than the distance to a "screen" on which we're observing the diffraction:
