Part of the Macmillan Work Out Series book series (MCWO)
Diffraction can be used to describe the departure of radiation from its rectilinear path, such as may occur when it meets a slit or a circular aperture. The important parameter to be determined is the ratio wavelength λ: characteristic aperture size a; for a slit, a would be its width and for a circular aperture it would be the diameter. If λ/a is ⪡ 1, diffraction can be ignored and the transmitted wavefront propagates in the same direction as the incident wavefront. However, if λ/a is ⪢ 1 the energy of the incident wave is uniformly distributed in all directions (see Fig. 4.1).
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© J. Beynon 1988