𝗪𝗶𝗹𝗵𝗲𝗹𝗺 𝗖𝗼𝗻𝗿𝗮𝗱 𝗥𝗼̈𝗻𝘁𝗴𝗲𝗻 was born on March 27th, 1845. If you're German you will immediately recognize the name "𝗥𝗼̈𝗻𝘁𝗴𝗲𝗻" and think of "𝗥𝗼̈𝗻𝘁𝗴𝗲𝗻𝘀𝘁𝗿𝗮𝗵𝗹𝗲𝗻", the rays he discovered and that are named after him. We even use "𝗿𝗼̈𝗻𝘁𝗴𝗲𝗻" as a verb when x-rays are taken.
Röntgen was a German physicist who discovered mysterious rays that were invisible and could go through certain materials like soft tissue but not through others like bone. Since he didn't know what those rays were he called them 𝗫-𝗦𝘁𝗿𝗮𝗵𝗹𝗲𝗻 (x-rays) where 'x' stands for the unknown.
In November of 1895, he took the first photo with the rays, of his wife's hand (see pic). He didn't want to patent his discovery so that everybody could use the new rays. In 1901, Röntgen was awarded the first ever Nobel Prize. He died in 1923, either having burned all his lab notes by himself or ordered them to be burned after his death.
Interestingly, there are quite a few languages which also use "𝗥𝗼̈𝗻𝘁𝗴𝗲𝗻" to describe x-rays, especially the Germanic languages but also in Bosnian and Turkish.
What do you call x-ray in your language?