How do you think you can see your bone in an
X-ray? Do you suppose it's sort of like holding a
flashlight against your hand and seeing a little
bit of light come through, with more light in some
places than others? Maybe it's like a window, too - lots of light comes in through the window, but if there's a shade or curtain over part of the
window, not so much light comes through the shade
or curtain as through the plain window. And it's
even darker where the window frame is.
Our body is sort of like that - we have thin areas of skin where some light comes through, such as the skin between your thumb and forefinger. We have areas with bone under the skin in our hands, and less light comes through the bones, so we can 'see' the bones a little bit as dark areas against the lighter parts of our hands where there is skin but not bones for the light to go through. X-rays are related to light, and they can go through skin better than light.
A lot of the x-rays [the radiation] can go through a thin part of bone, but only a bit of radiation can go through a thick part of bone. This makes an X-ray image where the thick parts of bone are the darkest. Check this out here:
Bone radiography. The pictures show light bones, though, and dark tissue around them. Why do you think they look that way? Maybe that's what happens when you take the X-ray and make a print of it, like taking a photo negative and making a print of it.
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