The Physics of X-Rays
X-ray imaging is shadow photography with high-energy photons. A tube accelerates electrons into a tungsten target, and the resulting X-rays pass through you — casting shadows where dense tissue blocks them.
Bremsstrahlung
Electrons decelerating violently in the target's electric field radiate a continuous spectrum of X-rays ('braking radiation'). Higher tube voltage means more energetic electrons and more penetrating X-rays.
Characteristic lines
Some electrons knock inner-shell electrons out of target atoms; when outer electrons drop in to fill the gap, they emit X-rays at exact fingerprint energies — tungsten's K-lines, for example.
Differential absorption
Bone (calcium, Z=20) absorbs far more X-rays than soft tissue (mostly low-Z elements) because photoelectric absorption scales roughly as Z³. The detector records the difference as light and dark — anatomy as an absorption map.
CT: many angles, one volume
A CT scanner rotates the tube around you, collecting hundreds of projections. Mathematics (the Radon transform) reconstructs a 3D volume from the shadows — same physics, computed tomography.
Try this yourself
- Compare a dental X-ray: fillings (dense metal) glow white, cavities (less dense) go dark.
- Note the lead apron at the dentist — lead's high Z makes it an excellent X-ray shield.