Waves

The Diffraction Grating

A diffraction grating has thousands of narrow apertures. Consider a typical grating having 500linesmm^-1. The situation can be analysed in the same way as Young’s two slit experiment.

If distance b–c is then waves from a and b will interfere constructively at a point in a direction to the normal.

However, if b–c is then d–e will be 2, f–g will be 3 and so on. Therefore, waves from hundreds of slits will interfere constructively, producing a well defined maximum of the diffraction pattern, called a diffraction image.

Other maxima will occur when b–c = 2, 3 etc.

The width of the apertures is only 1/500mm so the fringe spacing is much wider than in Young’s experiment which means that the angles are not small angles.

Therefore, to find the angular positions of the maxima, we use the equation

where n (= 1, 2, 3 etc) is now called the order of the image of the diffraction pattern.

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