Introduction

Of the three phenomena that result from the wavelike nature of light – polarization, interference, and diffraction – the third is the most puzzling. It does not render itself to intuitive explanation, since intuition suggests that light propagates in straight lines. Diffraction, however, allows for light under certain conditions to travel “around corners.” Because of this effect, light may be detected at points that could not be reached by straight rays. This effect also prevents indefinite propagation of collimated beams; invariably, after a certain distance, collimated beams appear to diverge. Similarly, when a focusing lens designed using considerations of geometrical optics is employed to focus radiation, the spot size at the focus cannot be reduced below a defined limit. In these examples, diffraction is seen to pose limitations on the application range of many optical devices. Thus, imaging resolution is reduced by the diffraction limits of lenses, power delivery by collimated laser beams is limited by their divergence, and the application of masks for processing semiconductor chips with photolithographic techniques is limited by diffraction induced by the minute pattern of the masks.

However, there exist numerous applications where diffraction presents an advantage. One example is the diffraction grating used for spectral separation of radiation (see Section 7.3). Another example is the advent of Fourier optics. This relatively new technology is based on the diffraction-limited imaging properties of lenses.