An X-ray is a high frequency, high energy wave form. It cannot be seen with the naked eye, but can be picked up on photographic film. Although you may think of an X-ray as a picture of bones, a trained observer can also see air spaces, like the lungs (which look black) and fluid (which looks white, but not as white as bones).
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This page was last updated at 3:14PM on October 7, 2014. This information is reviewed and edited by X-ray.