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Fraunhofer diffraction
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====Semi-quantitative analysis of single-slit diffraction==== [[Image:single slit diagram.svg|100px|right|thumb|Geometry of single-slit diffraction]] We can find the angle at which a first minimum is obtained in the diffracted light by the following reasoning. Consider the light diffracted at an angle {{math|θ}} where the distance {{math|''CD''}} is equal to the wavelength of the illuminating light. The width of the slit is the distance {{math|''AC''}}. The component of the wavelet emitted from the point A which is travelling in the {{math|θ}} direction is in [[phase (waves)#phase shift|anti-phase]] with the wave from the point {{math|''B''}} at middle of the slit, so that the net contribution at the angle {{math|θ}} from these two waves is zero. The same applies to the points just below {{math|''A''}} and {{math|''B''}}, and so on. Therefore, the amplitude of the total wave travelling in the direction {{math|θ}} is zero. We have: <math display="block">\theta_\text{min} \approx \frac {CD} {AC} = \frac{\lambda}{W}.</math> The angle subtended by the first minima on either side of the centre is then, as above: <math display="block">\alpha = 2 \theta_\text{min} = \frac{2\lambda}{W}.</math> There is no such simple argument to enable us to find the maxima of the diffraction pattern.
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