# 19.2: Diffraction Patterns


Laser diffraction experiments can be conducted using an optical bench, as shown below. Light from the laser (of wavelength λ) is diffracted by a mask (usually a small aperture or grating) and projected onto the screen, located at a large distance away, such that Fraunhofer geometry applies. The light on the screen is known as the diffraction pattern.

The form of the diffraction pattern from a single slit mask, of width w, involves the mathematical “sinc function”, where

$\operatorname{sinc}(z)=\frac{\sin (z)}{z}$

The observable pattern projected onto the screen (a distance L away) has an intensity pattern as follows, where x is the distance from the straight-through position:

$I(x)=I_{o} \operatorname{sinc}^{2}\left(\frac{\pi x w}{\lambda L}\right)$

Note that sinc(0) = 1.

Diffraction patterns can be calculated mathematically. The operation that directly predicts the amplitude of the diffraction pattern from the mask is known as a Fourier Transform (provided the conditions for Fraunhofer Diffraction are satisfied). The derivation of some simple patterns can be found here.

 (a) s = 12 μm (b) s = 3 μm

A diffraction grating is effectively a multitude of equally-spaced slits. The diffraction pattern from a complex mask such as a grating can be constructed from simplier patterns via the convolution theorem. The observed diffraction pattern is composed of repeated "sinc-squared" functions. Their positions from the central spot are determined by s (the spacing between slits) and their relative intensity is dependent on w (the width of individual slits).

Slit spacing s and slit width w

Diffraction patterns from gratings (a) and (b).

By considering diffraction from a grating, the reciprocal nature of the pattern can be derived. This relationship can be seen in the diffraction patterns of the slits: small features of the diffracting object give wide spacings in the diffraction pattern

$s=\frac{\lambda L}{x}$

Diffraction patterns from slits of different widths.

More complicated masks, for example a periodic row of apertures, will show more intricate diffraction patterns, but still follow the same basic inverse relationship.

Mask consisting of periodic row of apertures

Diffraction pattern for periodic row of apertures

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