When a wave passes through an opening or around an obstacle whose size is comparable to its wavelength, it spreads into regions that simple ray optics would predict as sharply shadowed. This wave behavior is diffraction.

Single-slit diffraction

For a slit of width $a$ illuminated by monochromatic light of wavelength $\lambda$, destructive interference produces minima approximately at

$$a\sin\theta=m\lambda,\qquad m=1,2,3,\ldots$$

The central bright region is wider than the neighboring bright fringes.

Why a single opening interferes with itself

Different parts of the wavefront across the opening act as contributions with different path lengths to a distant observation point. Their superposition produces the diffraction pattern.

Dependence on wavelength and aperture

Diffraction becomes more pronounced when the wavelength is large compared with the opening. A very wide aperture compared with $\lambda$ produces a narrow diffraction pattern and ray optics becomes a better approximation.

Resolution

Because every finite aperture diffracts light, optical systems cannot focus a point source to an arbitrarily small point. Diffraction therefore places a fundamental wave limit on angular and spatial resolution.