Physics
X-rays
Quick fact
X-rays were discovered in 1895 by Wilhelm Röntgen, who called them 'X' to indicate an unknown type of radiation. He earned the first Nobel Prize in Physics for this discovery.
Why this is interesting
You've seen them at the dentist—those ghostly images of your teeth. But how can invisible light take a picture of the inside of your body without cutting you open?
Read the full explanation
Understanding X-rays
Imagine throwing a handful of sand at a wall. Most sand bounces back, but if the wall is made of thin tissue paper, some might pass through. X-rays are like a super-energetic version of light—so energetic that they can pass straight through many materials. When you get an X-ray image, an X-ray beam is directed at your body. Some X-rays pass through, and some are blocked and absorbed by different tissues. Dense things like bones absorb more X-rays, casting a shadow on a detector behind you. That shadow—the areas where X-rays didn't get through—is what appears white on the image. That's how an invisible energy beam becomes a visible picture of your insides.
A deeper explanation
At the heart of X-rays is their nature as electromagnetic waves with very short wavelengths (roughly 0.01 to 10 nanometers) and therefore very high photon energy. This high energy allows them to knock electrons out of atoms (ionization), which is why they can penetrate matter. When a beam of X-rays hits a material, some photons are absorbed due to the photoelectric effect—their energy is transferred to electrons, causing ejection. The likelihood of absorption depends on the material's atomic number and density: denser and higher-atomic-number elements (like calcium in bone) absorb more X-rays, while softer tissues (like skin and muscle) are more transparent. This differential absorption is what creates contrast in an X-ray image. Moreover, X-rays are not just used for imaging; their ability to damage cellular DNA is harnessed in radiation therapy to kill cancer cells, while the same ionizing effect requires careful safety measures. Because X-rays also exhibit wave-like behavior, they can be diffracted by crystal lattices, enabling scientists to determine the atomic structure of materials—a technique known as X-ray crystallography. Thus, X-rays are a quintessential example of the wave-particle duality of electromagnetic radiation, and their interactions with matter underpin both their diagnostic and therapeutic applications.