X-rays are part of the electromagnetic spectrum, which includes light. X-rays are invisible, but behave like the light we’re familiar with. We use X-rays in medicine because X-rays can pass through soft tissues like skin and muscles, but are stopped by dense materials like bones.

We can use photosensitive films, therefore, to take pictures of bones in the body without needing to do surgery by shining X-rays through the body and seeing what part of the film gets exposed.

X-ray image of human lungs. Credit: Sudraben CC By-SA 4.0