Grating Intensity Comparison

The 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 high-resolution separation of the maxima for different wavelengths. Such a multiple-slit is called a diffraction grating.

Grating Intensity

The two aspects of the grating intensity relationship can be illustrated by the diffraction from five slits. The intensity is given by the interference intensity expression modulated by the single slit diffraction envelope for the slits which make up the grating:

Their product gives the total intensity expression: