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A mirror is under normal circumstance used to reflect Electromagnetic radiation also known as photons (light) and in airport security or medical facilities, they use X-rays to detect anomalies inside objects or bodies to detect narcotics or injuries. However, I always wonder what if I add a mirror inside the luggage or put a mirror in front of me during scanning?

That in mind, how would an X-ray scanner see the mirror? Would it be invisible? I am sure I am not the first one to think of this, as a lot of security and criminals thought of this, however I never got an answer, so can someone tell me please?

If there are X-ray reflecting mirrors? Why don't Airport security ban these items and Mirrors all together? Would X-Ray mirror look like a normal mirror? Do they reflect visible light spectrum as well?

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up vote 30 down vote accepted

The thing that makes a mirror a mirror is a that it has a high reflectivity (and is very smooth of course, but that doesn't enter into this issue), but all optical properties including reflectivity are functions of wavelength.

The mirror is not reflective in the x-ray band, so it looks like a layer of glass (moderately dense) and a very thin layer of heavy metal (rather denser). It can be seen in the image but is in no way remarkable.

It will look just like any other thin layer of moderately dense material.

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dmckee points out that an ordinary mirror doesn't reflect X-rays, but if you could find an X-ray mirror and put it in your case it would just appear black.

When you look at yourself in the mirror you're seeing light from your skin/clothes that hits the mirror and is reflected back towards your eyes. But airport X-ray machines work by passing X-rays through the luggage. The X-rays are emitted on one side and detected on the other. An X-ray mirror would reflect the transmitted X-rays and stop them passing through your case, so the X-ray detector would just see it as a black sheet.

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Unless of course they are back-scatter X-Ray machines. – placeholder Jul 21 '14 at 3:38
@placeholder: in airport luggage scanners? – John Rennie Jul 21 '14 at 4:57
Hi JR - just to be clear for anyone reading, indeed the "TSA" in the usa went crazy for backscatter scanners, for a while there. After a "last straw" public outcry, they were recently removed. – Joe Blow Jul 22 '14 at 9:24
@JoeBlow: ah, yes. Weren't those the ones that could allegedly photograph you naked? With that sort of scanner the (hypothetical) X-ray mirror would appear to be glowing. – John Rennie Jul 22 '14 at 9:28
Right - (I guessed you were not in the US from the comment! Any yankee friend would have said something like "those #^%$@#£ things!") You know, I think they do pretty much, indeed did, create a kind of paradise for disturbed paedophiles, etc. Fascinating on the glow. I guess they still use very large ones for trucks/trains? etc. – Joe Blow Jul 22 '14 at 9:33

As others have noted, an ordinary mirror will not reflect x-rays. X-ray mirrors do exist, but will also probably not do what you want here. It's very difficult to manipulate the optical trajectory of an x-ray. The critical angle of typical metal foils at x-ray wavelengths is a few degrees at most -- that means that the x-ray is only reflected if it hits the mirror surface at a very, very shallow angle, and its direction only changes by a few degrees. If the x-ray hits the mirror head-on, as it would if you were holding the mirror up in front of yourself while going through the scanner, the x-ray would just pass through the mirror rather than being reflected.

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Can you (anyone reading this) direct me to resources to learn more about why x-rays can only be reflected at a shallow angle? – Tim S. Jul 21 '14 at 2:48
Basically, it boils down to the fact that the index of refraction for most materials at x-ray wavelengths is less than 1. If you want to get into why that is the case, I think Attwood's book on Soft X-rays and EUV radiation or Als-Nielsen's intro to modern x-ray physics have good explanations. (Unfortunately my copies are in storage right now and I can't check for sure.) – Adrian Jul 21 '14 at 3:14
An alternate way (that is a particle physicist's way) to look at it is in terms of the momentum transfer from elastically scattering an x-ray. Even at moderately shallow angle that momentum is more than enough to scatter the electron right off the atom, but that would make the event inelastic which means it is not "reflection" at all. To get reflective behavior you need quite shallow angles indeed. – dmckee Jul 21 '14 at 3:18
Wikipedia talks about mirrors for x-ray for low and very high energy. So what x-rays do the luggage x-ray machines employ? – Trilarion Jul 21 '14 at 11:47

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