Diffraction, interference, and the wave nature of light.
Young's double-slit, single-slit diffraction, and the diffraction limit.
Bright fringes at dsinθ=mλd\sin\theta = m\lambdadsinθ=mλ, m∈Zm \in \mathbb{Z}m∈Z.
See the wave superposition animation: anim-wave-superposition.