 # Multiple Slit Diffraction Pattern

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In this Demonstration we visualize the diffraction pattern of equally spaced slits of equal width, also known as a diffraction grating. It can be shown that the diffraction pattern is equivalent to the diffraction pattern for delta function slits modulated by the diffraction pattern of a single slit of finite width. The latter thus acts as an envelope, shown by the thick dashed line. Special cases of this system include the single ( ) and double ( ) slits, which appear in introductory physics courses. The horizontal scale is arbitrary and the vertical scale normalized to the peak intensity.

Contributed by: Peter Falloon (March 2011)
Open content licensed under CC BY-NC-SA

## Snapshots   ## Details

For an array of slits of width and equal spacing , the intensity of the diffracted light on a screen may be neatly expressed as ,

where is the peak intensity and is the Chebyshev polynomial of the second kind, which appears when we simplify the ratio .

The parameter is a normalized wavenumber. It is related to the actual wavenumber , the perpendicular distance from the diffraction grating to the screen on which the pattern is observed, and the distance from the center of the pattern , by .

The formula for the intensity is valid within the Fraunhofer diffraction regime, for which . In this case, the diffraction pattern is equivalent to the Fourier transform of the diffraction grating. This explains why, since an array of finite-width slits is equivalent to the convolution of an array of delta function slits with a single slit, the resulting diffraction pattern is the product of the two corresponding diffraction patterns.

Snapshot 1: for a single, infinitely narrow slit, the diffraction pattern is constant; this is essentially because the Fourier transform of the delta function is constant

Snapshot 2: for multiple infinitely narrow slits, there is an infinitely repeating pattern of peaks and troughs, corresponding to constructive/destructive interference between paths from different slits

Snapshot 3: for a single slit of finite width, the diffraction pattern has the well-known form of a sinc function

Snapshot 4: for multiple slits of finite width, the diffraction is a pattern of peaks and troughs modulated by the sinc function pattern arising from the finite width of each slit

## Permanent Citation

Peter Falloon

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