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In this comment on one of the answers to an earlier question of mine, @CarloFelicione points out how even the tiniest of imperfections can make a stealth aircraft much more visible to radar:

A screw not properly seated on a maintenance panel can make the plane show up like a barn door on a radar. Not the size, it’s the reflectivity that matters

How is it possible for a single loose screw to increase a stealth aircraft's radar cross-section that dramatically? Why does it show up like a barn door, rather than like, well, a screw?

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    $\begingroup$ It's not the screw but the edge of the panel which causes the RCS increase. $\endgroup$ – Peter Kämpf May 13 '18 at 17:03
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    $\begingroup$ I'm pretty sure the original source for this piece of trivia is "Skunk Works: A Personal Memoir of My Years of Lockheed" by Ben R. Rich. A worthy read if you're interested in aviation history; light on the technical side but enlightening on the management behind Have Blue. The screw comment is about a single test where the prototype had a weirdly large RCS due to some screws not being seated properly. $\endgroup$ – AEhere supports Monica May 14 '18 at 8:17
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In RCS, size does not matter once you move beyond the scale of the wavelength of the radar. Therefore, the RCS of a panel with an edge length of 10 cm is (theoretically at least) the same as one of a panel with 1 m edges. The screw must be so poorly placed that it will cause a larger panel to stick out so it has no electrical contact, with the edge pointing in the right direction, then this massive increase in RCS is indeed possible.

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    $\begingroup$ It is the electrical discontinuity that does the damage, two metal sheets in firm contact look like one surface, two sheets with a slot because they are NOT in firm contact looks like an RF antenna at a frequency where the slot length is a multiple of a 1/4 wavelength. Search term "Slot radiator". $\endgroup$ – Dan Mills May 13 '18 at 21:43
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    $\begingroup$ Hmm, why does this work differently from visible wavelengths? If I shine light on an object and measure how much I get back, larger objects will generally reflect more light, a relation that continues far beyond the wavelength of my light. Are microwaves different somehow? $\endgroup$ – Henning Makholm May 14 '18 at 9:22
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    $\begingroup$ @HenningMakholm I think the point is that much of low radar cross-section is about reflecting the incoming energy away from its source. If part of your plane is acting as an antenna, it's radiating in all directions $\endgroup$ – David Richerby May 14 '18 at 12:20
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    $\begingroup$ @HenningMakholm That isn't really true when you start dealing with objects 100 nanometers across or smaller, where they don't really "reflect" light so much as "scatter" it. The physical characteristics of such small particles can make a large difference in how they scatter light. $\endgroup$ – Skyler May 14 '18 at 13:49
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    $\begingroup$ @HenningMakholm If you shine a light on a slit a few hundred nanometers wide you will indeed a get an attenuated return. Imagine you've got a flashlight shining on a white countertop and you are looking at it with red lensed glasses. The whole counter will appear red. Have you ever seen the rainbow that forms when light hits a CD just right? Imagine that CD is in the middle of our red-appearing counter. The red portions of the rainbow the CD makes will be bright and the rest of the CD will be very dark, even more than the counter. The difference is scale: Radio waves are much bigger. $\endgroup$ – user28387 May 14 '18 at 17:50

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