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The sun makes very sharp shadows as the photons are essentially parallel. An LED lamp can also make shadows appear very sharp, a florescent lamp, not so much. Your experiment ignores the observational limitations (as pointed out by other answers). It's an important point even in theoretical physics.

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How about a simple test to see if the rings are due to thin film interference? I doubt it, as that implies very high quality manufacturing, with precise rings of very uniform and varying thicknesses, but a test is easy. Try using monochromatic light. Even just the green (or red) 'charging' LED light on a phone/laptop charger should change the pattern if ...

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Judging by the smoothness and regularity of the spectrum, this appears to be simple refraction through the plastic. Thin-film interference would generate a much less regular pattern. In this instance, the plastic disk is acting as a sort of prism. If you want a thin-film interference type of effect, you can look at the disk (from either side, if I recall ...

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We first consider the relation: $$n\delta{\lambda} = d\delta{\theta}\cos{\theta}$$ It's content is that the $n^{th}$ order maximum of a wavelength $\lambda + \delta{\lambda}$ is displaced from the corresponding maximum for a wavelength $\lambda$ by the angle $\delta{\theta}$, related to $\delta{\lambda}$ by the above equation. Now, we can ask the question, ...

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All of these criteria are more or less the same. I'm not aware that there is one "standard" criterion. More importantly, I think, is that these numbers obtain for ideal conditions, but real systems will suffer from various imperfections and aberrations meaning that the actual resolution and spot size will be degraded. Furthermore, specific applications ...

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Perhaps part of the problem is that the statement $2 ( l - l') = n \lambda$ is not correct in general. It applies to the specific situation where both the incident wavefront and the refracted wavefront accumulates a path difference of $l - l'$ between each layer of your crystal. That is true if the diffraction causes a reflection back in the same direction ...

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You will still "see" the object but it will appear blurred in the shape of the so-called Airy diffraction pattern with a size larger than the actual object (dependent on the numerical aperture of the objective lens and the wavelength of the light used to observe it in the microscope). For VIS light and a 100x objective lens with an N.A. of 1.4 this is ...

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