An X-ray photograph of the leg you broke skiing down the expert slope with intermediate skill shows your bones most brightly because bone is dense and stops more X-ray particles than the flesh around it.

An X-ray camera fires electrons at a plate covered with silver halide crystals, which are sensitive to light. Your hapless leg is put in the way of the penetrating stream of particles. When an electron reaches the plate unimpeded, it turns a halide crystal black. The crystals that receive no electrons fall away when the plate is developed and leave that area transparent, or white under the light.

X-ray particles are so highly energized that most of them pass right through flesh, which is made mostly of water, leaving only a vague image on the film where a few of them were stopped. Bones, on the other hand, are very densely packed and contain large amounts of calcium and other heavy elements. They stop the X rays by absorbing them- the crack in your tibia shows up black on the plate because that’s where bone isn’t.

From: How Do They Do That? By: Caroline Sutton