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Diffraction When light passes an edge, it will deviate from rectilinear propagation. This phenomenon (which is a natural consequence of the wave nature of light) is known as diffraction and plays an important role in optics. The term diffraction has been conveniently defined by Sommerfeld as ‘any deviation from rectilinear paths which cannot be interpreted as reflection or refraction’. A rigorous theory of diffraction is quite complicated. Here we develop expressions for the diffracted field based on Huygens’ principle of secondary spherical wavelets. It is a very fortunate coincidence that the aperture and the diffracted far field are connected by a. | Optical Metrology. Kjell J. Gasvik Copyright 2002 John Wiley Sons Ltd. ISBN 0-470-84300-4 4 Diffraction INTRODUCTION When light passes an edge it will deviate from rectilinear propagation. This phenomenon which is a natural consequence of the wave nature of light is known as diffraction and plays an important role in optics. The term diffraction has been conveniently defined by Sommerfeld as any deviation from rectilinear paths which cannot be interpreted as reflection or refraction . A rigorous theory of diffraction is quite complicated. Here we develop expressions for the diffracted field based on Huygens principle of secondary spherical wavelets. It is a very fortunate coincidence that the aperture and the diffracted far field are connected by a Fourier transform relationship. Because of that optics and electrical engineering have for a long time shared a common source of mathematical theory. As a consequence of diffraction a point source cannot be imaged as a point. An imaging system without aberrations is therefore said to be diffraction limited. DIFFRACTION FROM A SINGLE SLIT Figure shows a plane wave which is partly blocked by a screen Si before falling onto a screen S2. According to geometrical optics a sharp edge is formed by the shadow at point A. By closer inspection however one finds that this is not strictly correct. The light distribution is not sharply bounded but forms a pattern in a small region around A. This must be due to a bending of the light around the edge of S1. This bending is called diffraction and the light pattern seen on S2 as a result of interference between the bent light waves is called a diffraction pattern. Another example of this phenomenon can be observed by sending light through a small hole. If this hole is made small enough the light will not propagate as a narrow beam but as a spherical wave from the centre of the hole see Figure . This is evidence of Huygens principle which says that every point on a .