Sunday, 12 July 2009

Laser Diffraction

My most important point: lasers are awesome! There is a surprising amount of DIY geeky physics that you can do with a laser pointer and some bodged together diffraction slits and holes.Using two razor blades held a few tenths of a millimetre apart and a red and green laser pointer you can get some pretty good diffraction patterns.
It even behaves as expected! The increase in wavelength gives a corresponding increase in fringe spacing.

Laser diffraction patterns are a perfect test ground for fourier transform analysis, the fourier transform of the diffraction pattern gives the shape of the diffraction slit used.
This is the aperture I hacked together from a piece of 1mm aluminium and a drawing pin. The ruler markings are 1mm apart.
The diffraction pattern encodes the information about the aperture shape in the positioning of the fringes. The roughly circular diffraction pattern shows that the aperture is approximately circular.

Calculating a 2D fourier transform of the diffraction pattern gives the above image, an accurate reconstruction of the shape of an aperture only a few tenths of a millimetre across.

Software used:
Fourier transforms: ImageJ
Image management: Paint.net

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