What does not allow X-rays to pass through?

X-rays are a form of electromagnetic radiation, like visible light, but with much shorter wavelengths. They are produced when high-energy electrons are rapidly decelerated, such as when they strike a metal target. X-rays can penetrate most materials, but some materials, such as lead, are opaque to X-rays. This is because the atoms in these materials have a high atomic number, which means that they have a large number of electrons. The electrons in these atoms absorb X-rays, preventing them from passing through.