For decades, people undergoing radiotherapy, which is used to treat cancer, have reported a bizarre phenomenon: Seeing flashes of light in their eyes, even when their eyes are closed.

Patients documented in the medical literature have described a ‘‘ray of blue light” and ‘‘seeing a blue neon light”, sometimes accompanied by a “white smell” during the delivery of radiation, lasting for a fraction of a second. There have been several theories for why this could be happening, including retinal pigments inside patients’ eyes being stimulated during the therapy, or that Cherenkov light or Cherenkov radiation – the same effect that makes nuclear reactors glow blue when they’re underwater – is produced inside the eyeball itself.

Now scientists have captured this strange light for the first time, producing the first photographic evidence that the phenomenon is in fact Cherenkov light.