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During my visit to the doctors', I recollect having seen the sign which says

Please remove all metallic objects from the body before the commencement of the X-Ray

How do metallic objects interfere with X-Rays? Don't they have metallic implants for certain kind of surgeries?

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It is well-known that x-rays are blocked by metal. [Ref: Kid's Science]

Obviously the doctor wants to look at your internal organs, unobscured by a fuzzy outline of your house keys and pendant.

So, the sign is requesting that you removing metal items from the external parts of your body, to allow visibility to the internal parts.

(MRI is a totally different story.)

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To add a slightly more detailed answer than yesterdays kids's answer:

X-rays are electromagnetic waves, like light. They're different from visible light because they have a higher frequency, and therefore a higher energy per photon. That also means they have a lower wavelength (since f*λ=c)

Metals are solid materials in which some electrons are not strongly bound to a single nucleus, but instead form a electron cloud dispersed throughout the whole object. These electrons can therefore move easily, which is why metals conduct electricity.

An x-ray photon that hits a metal object may have enough energy to shoot straight through. If not, it will hit an electron and kick it away at high speed. The energy and momentum is taken from the x-ray photon, which means it's scattered in another direction at lower energy (and could then be absorbed outright).

So, it's the metals electron (band) structure that causes the interference with x-rays.

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  • $\begingroup$ What in your answer is specific to metals? The same scattering off electrons works in any material. The issue with metals is that you can have attenuation due to gross electron motion, but this effect must go away as the X-ray wavelength becomes shorter than the inter-atomic distance. I don't know which limit medical X-rays are in, the ultraviolet full absorption limit, or the gamma-ray who-cares-that-its-a-metal limit. $\endgroup$ – Ron Maimon Jan 5 '12 at 16:34
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    $\begingroup$ @Ron: Not that much, actually, and that's why non-metallic objects will also scatter X-rays. It's a qualitative difference after all. That's why you see bones on X-rays: the atoms in there (e.g. calcium) have a higher electron density than hydrogen and oxygen. Wikipedia suggests medical X-rays are 20-150 kEV, which means you're dealing with both the photoelectric effect and Compton scattering. $\endgroup$ – MSalters Jan 6 '12 at 8:10
  • $\begingroup$ I see, it's not that its a metal per se, its that its a high Z material. You could use Xe atoms to get the same scattering then. Thanks for the clarification. $\endgroup$ – Ron Maimon Jan 6 '12 at 19:15
  • $\begingroup$ +1: Just to briefly defend my "kid's science" answer, that was written when the question was on Skeptics.SE - my goal was to drill straight to clarifying the claim. Now that it is on Physics.SE, it makes sense to elaborate on the physical aspects of X-rays and metal at a much deeper level. $\endgroup$ – Oddthinking Jan 9 '12 at 1:57
  • $\begingroup$ @RohanArnold your question was deleted because it was in the wrong place, as an answer. You could ask it as a question by choosing "ask question" on the top bar far right $\endgroup$ – anna v May 4 '14 at 4:30

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