X-rays — Röntgen's discovery

What just happened?

You raised the density of the imaged material to a value typical of bone. More X-ray photons were absorbed in the sample, so fewer reached the detector — the spot on the “film” clearly brightened.

What are X-rays?

They are electromagnetic waves with a very short wavelength (and therefore — following $E = hf$ — a high photon energy), much shorter than visible light. Wilhelm Conrad Röntgen discovered them by accident in 1895 while studying cathode rays in a vacuum tube: he noticed that a nearby screen was glowing even though the tube was tightly shielded with black cardboard. Not knowing the nature of this radiation, he named it “X-rays”. For this discovery he received the first ever Nobel Prize in Physics, in 1901.

Why are bones bright in an X-ray image?

X-ray absorption grows with the density of the material (the more atoms per unit volume, the greater the chance of absorbing a photon):

where ρ — material density, μ — attenuation coefficient (depends on the material and the sample thickness), T — transmission (the fraction of radiation that passes through), A — absorption. Bones (rich in calcium) are much denser than soft tissue (mostly water) — they absorb more radiation, so less reaches the detector, and that spot comes out bright. Soft tissue lets the rays through almost unimpeded, so it looks darker in the image.

Why this matters for your exam:

X-rays are one example of electromagnetic waves in problems about the EM spectrum and photon energy — and Röntgen’s discovery is a classic example of how great discoveries can be accidental, yet understanding and describing them requires a rigorous scientific method.

Remember: X-rays = electromagnetic waves with a short wavelength and high photon energy; the greater the material density, the greater the absorption — bones are bright, soft tissue darker in an X-ray image. Röntgen’s discovery (1895) — the first Nobel Prize in Physics (1901).