Grating Intensity ComparisonThe grating intensity expression gives a peak intensity which is proportional to the square of the number of slits illuminated. Increasing the number of slits not only makes the diffraction maximum sharper, but also much more intense. If a 1 mm diameter laser beam strikes a 600 line/mm grating, then it covers 600 slits and the resulting line intensity is 90,000 x that of a double slit. As the intensity increases, the diffraction maximum becomes narrower as well as more intense. When you have 600 slits, the maxima are very sharp and bright and permit highresolution separation of the maxima for different wavelengths. Such a multipleslit is called a diffraction grating. 
Index Diffraction concepts Fraunhofer diffraction  

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Grating Intensity
Their product gives the total intensity expression: 
Index Diffraction concepts Grating concepts References: Hecht, 2nd Ed. Sec 10.2 MeyerArendt, 2nd Ed. Ch 2.4  

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