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Dan Forth

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hoping someone can help me with this problem. If I block plane monochromatic light (wavelength lambda) with a circular disc of radius r, and then look at the diffraction pattern on a screen placed a distance x behind the disc, I get a bright spot in the centre of the shadow. All well and good. What I want to know is how do the intensity and radius of that central spot vary as a function of x. Specifically I want to know how small I need to make x in order to effectively eliminate the bright spot. Clearly at x=0 there is no bright spot - as x increases I don't know whether the spot appears with increasing radius or increasing intensity (presumably both) but I need to be able to put some numbers into find at what point the spot becomes intolerably bright/large. For anyone interested the reasoning behind my problem is a botched photolith job that I'm trying to do an autopsy on and avoid repeating the same mistakes!

Thanks very much for any help anyone can give,

Dan