Diffraction
The key idea
Diffraction is the spreading and restructuring of a wave when it encounters an aperture or obstacle whose dimensions are relevant to its wavelength.
Why it matters
It is not a defect of waves: it is one of their signatures, and it ultimately limits the resolving power of optical instruments.
A picture to remember
It is not a defect of waves: it is one of their signatures, and it ultimately limits the resolving power of optical instruments.