Diffraction is defined as interaction of waves with any solid object, not surfaces, and is not to be confused with refraction. More precisely, diffraction describes the phenomena of interaction of waves at an obstacle, such as an aperture, or an opening, such as a hole or an occurring space between two objects. Hence, diffraction is an essential form of scattering, describing ordered scattering at discrete boundaries. The effect of diffraction can be observed through extended interference patterns or simply by the bending of waves. In the field of microwave remote sensing, diffraction has the practical implication that it limits the spatial resolution of a microwave sensor since it acts on the ability of an imaging system to resolve details. This theoretical limit of resolution is called the diffraction limit. This means, the larger the aperture of the observing system compared to its employed wavelength (dependent on the frequency), the finer the resolution of an imaging system. The diffracted field can be calculated with analytical models, such as the Fraunhofer diffraction approximation in case of far field conditions, where the object is far away and the incident waves are assumed to be plane waves, or the Fresnel diffraction approximation in case of near field conditions, where the waves are spherical.
One simple example of diffraction is the diffraction of sound, for example the possibility to hear sounds around corners.