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Physics
Classical Physics
Poisson Spot Madness: Laser & Coin Lens Experiment
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[QUOTE="Cutter Ketch, post: 6271789, member: 515910"] Let me see if I understand your question. You are looking at the diffraction from the edge of a coin inserted into a wide top hat profile laser beam. Because the edge is curved, the edge diffraction is converging, and on a screen you get a cone of light converging to a tight spot. In analogy to a lens you expected that the tight spot would only occur when the coin was a certain distance from the screen, but you found, to your surprise, that the diffraction converged to a tight spot no matter what distance you placed the coin from the screen and you want to know why. Correct? If I have that right, here is why. Suppose the light diffracted around the edge only at one precise angle. The curved edge would then produce a precise cone of light which would converge to a tight spot at exactly one distance behind the coin. This would be a focus like you expected. The focal length is determined by the curvature of the edge and the angle of diffraction. Now, in actuality the light diffracts around the edge into a whole range of angles, more light at smaller angles, less at larger angles, but a whole spray. For each angle there is a focal length. If you put the screen at a large distance then the bright diffraction of the shallow angles are focused and the spot is closer to the edge (in angle space). The larger angles deeper into the shadow are further and further past focus. You get a triangle of light diverging deeper into the shadow. At a shorter distance, larger diffraction angles are focused. The shallower angles closer to the edge of the shadow are more and more inside their focus. You get a triangle of light diverging as you get closer to the edge. Because the diffraction is brighter at shallower angles the triangle towards the edge is more prominent than the triangle deeper into the shadow. As you move the coin closer and further from the screen you should see the spot move further and closer to the edge of the shadow. [/QUOTE]
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Poisson Spot Madness: Laser & Coin Lens Experiment
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