Watching Cherenkov Light Inside Eyes

For decades, radiotherapy patients receiving treatment for brain cancer reported seeing flashes of bright or blue light during their sessions. Researchers later connected those reports to Cherenkov radiation, the same phenomenon that produces the blue glow around nuclear reactors. In 2019, a team confirmed that this light appears when radiation passes through the fluids of the eye and moves faster than light can travel in that medium. Those brief, strange flashes turned out to be patients literally watching high-energy particles generate Cherenkov light inside their own eyes.

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