PX275 - I1 - diffraction and interference

considering a plane wave approaching an obstructor with an aperture

after passing through the aperture, the wave is curved, which is due to diffraction

now, considering the obstructor has two apertures

the diffracted waves interact at multiple points, which is called interference

if the source is coherent, the interference can occur due to differing path lengths

if the path lengths differ by $n\lambda$, where $n\in \mathbb{Z}$, this gives constructive interference as the waves meet in phase

if the path lengths differ by $n\lambda /2$, this gives destructive interference as the waves meet out of phase