Dissemination of IT for the Promotion of Materials Science (DoITPoMS)

DoITPoMS Teaching & Learning Packages Diffraction and Imaging Introduction
Diffraction and Imaging AimsBefore you startIntroductionDiffraction patterns 1Diffraction patterns 2Two-dimensional diffractionImage formationInformation obtained from diffraction patternApplications of the theory of optical diffraction and imagingSummaryQuestionsGoing further
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Introduction

The phenomenon of diffraction was first documented in 1665 by the Italian Francesco Maria Grimaldi. The use of lasers has only become common in the last few decades. The laser's ability to produce a narrow beam of coherent monochromatic radiation in the visible light range makes it ideal for use in diffraction experiments: the diffracted light forms a clear pattern that is easily measured.

As light, or any wave, passes a barrier, the waveform is distorted at the boundary edge. If the wave passes through a gap, more obvious distortion can be seen. As the gap width approaches the wavelength of the wave, the distortion becomes even more obvious. This process is known as diffraction. If the diffracted light is projected onto a screen some distance away, then interference between the light waves create a distinctive pattern (the diffraction pattern ) on the screen. The nature of the diffraction pattern depends on the nature of the gap (or mask) which diffracts the original light wave.

Diffraction patterns can be calculated by from a function representing the mask. The symmetry of the pattern can reveal useful information on the symmetry of the mask. For a periodic object, the pattern is equivalent to the reciprocal lattice of the object.

In conventional image formation, a lens focuses the diffracted waves into an image. Since the individual sections (spots) of the diffraction pattern each contain information, by forming an image from only particular parts of the diffraction pattern, the resulting image can be used to enhance particular features. This is used in bright and dark field imaging.

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