Editing X-ray (section)

==Visibility==
While they fall outside of the wavelengths that compose the visible light spectrum, in special circumstances X-rays can be detected by eye. Brandes, in an experiment a short time after Röntgen's landmark 1895 paper, reported after dark adaptation and placing his eye close to an X-ray tube, seeing a faint "blue-gray" glow which seemed to originate within the eye itself.<ref>{{Cite web | vauthors = Frame P |title= Wilhelm Röntgen and the Invisible Light |website= Tales from the Atomic Age |publisher= Oak Ridge Associated Universities |url= https://www.orau.org/health-physics-museum/articles/wilhelm-rontgen-invisible-light.html |access-date= 11 October 2021}}</ref> Upon hearing this, Röntgen reviewed his record books and found he too had seen the effect. When placing an X-ray tube on the opposite side of a wooden door Röntgen had noted the same blue glow, seeming to emanate from the eye itself, but thought his observations to be spurious because he only saw the effect when he used one type of tube. Later he realized that the tube which had created the effect was the only one powerful enough to make the glow plainly visible and the experiment was thereafter readily repeatable.{{Citation needed|reason=Did Röntgen publish work on the replication of this effect?|date=April 2025}} The knowledge that X-rays are actually faintly detectable to the dark-adapted naked eye has largely been forgotten today; this is probably due to the desire not to repeat what would now be seen as a recklessly dangerous and potentially harmful experiment with [[ionizing radiation]]. It is not known what exact mechanism in the eye produced the visibility described by Röntgen and Brandes, though [[Cherenkov radiation]] caused by the X-rays traveling through the vitreous humor of the eye is a likely explanation.<ref>{{cite journal |last1=Tendler |first1=Irwin I. |last2=Hartford |first2=Alan |last3=Jermyn |first3=Michael |last4=LaRochelle |first4=Ethan |last5=Cao |first5=Xu |last6=Borza |first6=Victor |last7=Alexander |first7=Daniel |last8=Bruza |first8=Petr |last9=Hoopes |first9=Jack |last10=Moodie |first10=Karen |last11=Marr |first11=Brian P. |last12=Williams |first12=Benjamin B. |last13=Pogue |first13=Brian W. |last14=Gladstone |first14=David J. |last15=Jarvis |first15=Lesley A. |title=Experimentally Observed Cherenkov Light Generation in the Eye During Radiation Therapy |journal=International Journal of Radiation Oncology*Biology*Physics |date=February 2020 |volume=106 |issue=2 |pages=422–429 |doi=10.1016/j.ijrobp.2019.10.031 |pmid=31669563 |pmc=7161418 }}</ref> Other potential explanations for the glow include direct excitation of retinal cells by X-rays, similar to some instances of light flashes seen during experiments regarding [[cosmic ray visual phenomena]].<ref>{{cite journal |last1=Charman |first1=W. N. |last2=Dennis |first2=J. A. |last3=Fazio |first3=G. G. |last4=Jelley |first4=J. V. |title=Visual Sensations produced by Single Fast Particles |journal=Nature |date=April 1971 |volume=230 |issue=5295 |pages=522–524 |doi=10.1038/230522a0 |pmid=4927751 |bibcode=1971Natur.230..522C }}</ref>

Though X-rays are otherwise invisible, it is possible to see the [[ionization]] of the air molecules if the intensity of the X-ray beam is high enough. The beamline from the [[Wiggler (synchrotron)|wiggler]] at the  [[European Synchrotron Radiation Facility]]{{fact|date=April 2025}} is one example of such high intensity.<ref>{{cite book |doi=10.1002/9781119998365 |title=Elements of Modern X-ray Physics |date=2011 |last1=Als-Nielsen |first1=Jens |last2=McMorrow |first2=Des |isbn=978-0-470-97395-0 |pages=40–41 }}</ref>