diffraction

Diffraction is an effect on waves that occurs as they encounter an obstacle or opening, i.e., when traveling waves span a limited cross-section. One of diffraction's effects is that some of each wave bends around the edge of the obstacle or opening, proceeding in directions other than the waves' direction while approaching. A result is (an effect like) waves crossing each other with resulting interference. This affects light waves, affecting the resolving ability of telescopes, such that the larger its aperture, the better the angular resolution. A telescope is said to be diffraction limited if all its other optical properties are good enough that the telescope aperture's diffraction is its main limitation in resolving images. Diffraction also affects particles as per quantum mechanics' particle/wave duality. Much effort has gone into characterizing diffraction's effects, theories and models including:

• Huygens-Fresnel principle
• Kirchhoff diffraction formula
• Fresnel diffraction
• Fraunhofer diffraction
• geometrical theory of diffraction
• uniform theory of diffraction

Refraction, on the other hand, is the change in direction of a light ray as it passes between two materials with different densities and is the mechanism of lenses.