![]() On a certain direction where electromagnetic wave fields are projected (or considering a situation where two waves have the same polarization), two waves of equal (projected) amplitude which are in phase (same phase) give the amplitude of the resultant wave sum as double the individual wave amplitudes, while two waves of equal amplitude which are in opposite phases give the zero amplitude of the resultant wave as they cancel out each other. When two light waves as electromagnetic fields are added together ( vector sum), the amplitude of the wave sum depends on the amplitudes, the phases, and even the polarizations of individual waves. It is generally not straightforward to calculate the wave amplitude given by the sum of the secondary wavelets (The wave sum is also a wave.), each of which has its own amplitude, phase, and oscillation direction ( polarization), since this involves addition of many waves of varying amplitude, phase, and polarization. ![]() These effects can be modelled using the Huygens–Fresnel principle Huygens postulated that every point on a wavefront acts as a source of spherical secondary wavelets and the sum of these secondary wavelets determines the form of the proceeding wave at any subsequent time, while Fresnel developed an equation using the Huygens wavelets together with the principle of superposition of waves, which models these diffraction effects quite well. When a beam of light is partly blocked by an obstacle, some of the light is scattered around the object, light and dark bands are often seen at the edge of the shadow – this effect is known as diffraction. Main article: Fraunhofer diffraction equation Example of far field (Fraunhofer) diffraction for a few aperture shapes.
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