Diffraction by an aperture

If a long narrow slit is illuminated by monochromatic light the intensity pattern observed far from the slit (the Fraunhofer diffraction pattern) is given by the expression  

When the slit is shortened to form a rectangular aperture the diffraction maxima will take the form of small rectangular spots running in two perpendicular directions. White light will produce coloured spots via the same mechanism as described above. 

The form of the diffraction pattern produced by a circular aperture consists of a series of bright and dark circles concentric with the original aperture. The spacing of the maxima and minima is given by  

Just as with the slit, the dependence of the diffraction angle on wavelength means that a circular aperture illuminated with white light will produce a set of coloured rings, rather like miniature circular rainbows. The formula indicates that each ring will have a violet inner edge and a red outer edge. 

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Figure 4.7 (a) Diffraction by an opaque planar obstacle, (b) Diffraction by an aperture with the same shape as the obstacle. 

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