Roentgenor

Roentgenor stands in a dark lab and holds up a glowing screen. Behind him, a tube wrapped in black card. Between them, something nobody could see. X marks the unknown.

The Physics

On 8 November 1895, in Würzburg, Wilhelm Conrad Röntgen was working with a cathode-ray tube wrapped in thin black card. In the darkened room, a screen painted with barium platinocyanide began to glow. No visible light could get through the card, yet something did, crossed the room and lit the screen. He called the new rays "X-Strahlen", X-rays, and their nature was unknown. This November 8 marks the anniversary of that discovery.

His paper, "Ueber eine neue Art von Strahlen", was submitted on 28 December 1895 to the Physical-Medical Society of Würzburg (Sitzungsberichte, pp. 132–141); an English translation appeared in Nature on 23 January 1896. In 1901 he received the first Nobel Prize in Physics.

E = hc / λ λ_min = hc / (eV)

• E: photon energy; h: Planck's constant; c: speed of light; λ: wavelength
• λ_min: shortest wavelength from the tube; e: elementary charge; V: tube voltage

Today we know X-rays are electromagnetic waves, with wavelengths of about 0.01 to 10 nm (the borders with ultraviolet and gamma rays are a convention). Electrons braking in the tube's target give a continuous spectrum, and no photon can carry more energy than one electron gained from the voltage, so the tube voltage sets the shortest wavelength: the Duane–Hunt law of 1915.

Honest caveat: Röntgen's lab notes were not preserved, so the date 8 November 1895 comes from his later accounts. It is the standard date, but no notebook records it. In the paper, he says he chose the name "X-rays" to tell them apart from other rays.

On the shirt

Front: Roentgenor, the pixel-art character, with "X – 8 NOV 1895 / WUERZBURG / THE SCREEN GLOWED IN THE DARK" and the slogan "X MARKS THE UNKNOWN". Back: the black-card experiment, the name X-Strahlen, E = hc / λ, the Duane–Hunt limit, the 1895 paper and the 1901 Nobel Prize. Printed on a unisex tee.