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.
![](https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjQ7CvhG563mKF3bQalzUijcCd8QBRpsORLtlt9aWcw-4LOiw9fqkoyzbIOki8T8gBqScxqnv-oVuEkunQmzU2vWuMbMiizIJdBgYET5uKjAaePqQVtNsH-m8Gg4LGCmDvwkedLvzHvizo/s400/Lasers+DiffractionSlit.jpg)
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.
![](https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhQ7Hs6TyGmwbhTDapob-b0Aww0dTFheh7n4Eh0907sAxROpwsAlLuBMIMdLXPzHzoVf47FoZu37d35n43R9vgbQ_xK5lkIcZpO8rYYnAmkh81hJYdJfhc1QgTGG971aZswKOaerd3dRI0/s400/Diffraction.jpg)
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.
![](https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjSH4U19C7Lrbo82SVC-5HBZrU70K2YSNVQTxJUwWqlVids3W7d4eISRM-X-bkA0kXQipa2E8udRACAzI9S9KLco3v9Pcp02xJOWWaYKyx_lwNDSNMpzYRVA-6kEA8smQ4BKkZl4e12Tjc/s400/Aperture.jpg)
This is the
aperture I hacked together from a piece of 1mm aluminium and a drawing pin. The ruler markings are 1mm apart.
![](https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjtf8Rxt5oIA7gqMNu3_P4InIQU8jTMJGy_JYnpqKZrz6vxKI0k1a5vmT-ijfXKvP0A7kA0HdLYDqHjLtoWf1nz212WVhdYDnS5YOIYgPl6INlwRoZtOA23XenN_urS2lhfu4F1-UphZ1w/s400/ApateureDiffraction.jpg)
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.
![](https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhnAzP1G_Jy0gR4urQ99XzOScGxRlBcN_fsWihCq-mACQD0CPr53kT7mqsJb-gsEk8y3ee3uwjshhmn2oo3Li1i32c2EVEGTTObhoZ0WhX5QkOMLTMw4_FVr5uMs7yAkVmiHWhsf7LXE1E/s400/FFTofApateurDiffraction.jpg)
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:
ImageJImage management:
Paint.net
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