 Defraction refers to various phenomena that occur when a wave encounters an obstacle or a slit. It is defined as the bending of waves around the corners of an obstacle or aperture into the region of geometrical shadow of the obstacle. In classical physics, the defraction phenomenon is described as the interference of waves according to the Huygens-Franell principle that treats each point in the wave front as a collection of individual spherical waveflits.These characteristic behaviors are exhibited when a wave encounters an obstacle or a slit that is comparable in size to its wavelength. Similar effects occur when a light wave travels through a medium with a varying refractive index, or when the sound wave travels through a medium with varying acoustic impedance. Defraction has an impact on the acoustic space. Defraction occurs with all waves, including sound waves, water waves, and electromagnetic waves such as visible light, x-rays and radio waves. Since physical objects have wave-like properties at the atomic level, diffraction also occurs with matter and can be studied according to the principles of quantum mechanics. The Italian scientist Francesco Maria Grigaldi coined the word diffraction and was the first to record accurate observations of the phenomenon in 1660. While diffraction occurs whenever propagating waves encounter such changes, its effect start generally most pronounced for waves whose wavelength is roughly comparable to the dimensions of the diffracting object or slit. If the obstructing object provides multiple, closely spaced openings, a complex pattern of varying intensity can result. This is due to the addition, or interference, of different parts of the wave that travel to the observer by different paths, where different path lengths result in different phases see diffraction grading and waves superposition. The formalism of diffraction can also describe the wave in which waves of finite extent propagate in free space. For example, the expanding profile of a laser beam, the beam shape of a radar antenna and the field of view of an ultrasonic transducer can all be analyzed using diffraction equations.