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Diffraction grating under a microscope

And how to make the grating visible

Background

While building the spectroscope i noticed when handling the diffraction grating that any fingerprints on it were really visible and couldn’t be removed anymore. I suspected that this was because some of the substances on my fingers got into the ‘grating’ and was distorting the refraction of light. As I recently bought a microscope and had lots of grating left, I decided to look in detail at the sheet and see if i could discern the slits of one thousandth the size of a millimeter, and if there was any noticeable difference between the dirty and clean grating. In direct light nothing was observable. I decided to cover up part of the light at the bottom, which resulted in a clear difference! With the light hitting the grating at a bit of an angle but below the refraction index, the dirty grating unveiled some straight lines, while the clean one had a clean shine across its’ entirety. Covering a larger part of the microscope light meant a larger angle of incidence, and here the difference is observable even more beautifully!

Clean diffraction grating

Spectrum of computer screen

Clean diffraction grating in direct light 400x magnification

Spectrum of computer screen

Clean diffraction grating in light below refraction index 400x magnification

Spectrum of computer screen

Clean diffraction grating in light above refraction index 400x magnification

Dirty diffraction grating

Spectrum of computer screen

Dirty diffraction grating in direct light 400x magnification

Spectrum of computer screen

Dirty diffraction grating in light below refraction index 400x magnification

Spectrum of computer screen

Dirty diffraction grating in light above refraction index 400x magnification

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