How do X-rays work?

An X-ray machine heats a filament to release electrons, which are then accelerated by tens of thousands of volts into a metal target, usually tungsten. As the electrons slam to a halt, they give off X-rays. Bones contain calcium, which absorbs X-rays strongly, so they appear white, while air-filled lungs look dark. Only about 1 percent of the electrons’ energy becomes X-rays; the rest turns into heat, so many tubes use a spinning target to avoid melting. A typical chest X-ray gives a radiation dose of about 0.1 millisieverts, similar to around ten days of natural background radiation.