X-rays are photons, quanta of radiation, appearing on the electromagnetic spectrum between ultraviolet and gamma radiation. Nowadays we accept their existence as a matter of course. However, at the time of their discovery x-rays must have seemed miraculous since, at a stroke, in many cases, the requirement to cut open the body of a patient simply to diagnose an injury ceased.

The discovery of X-rays was one of those serendipitous moments in science. None of us can tell how much longer it would have taken had the world needed to wait for someone to predict and subsequently experimentally validate their existence. In this case, the fortunate scientist was physicist Wilhelm Röntgen, who was working at the University of Würtzburg in Germany. 

The discovery came on the 8th of November 1895 while he was experimenting with an improved design (by Philipp Von Lenard) of cathode ray tube - the earliest examples having been developed in the 1850s by Heinrich Geissler. It had previously been noticed that the then mysterious cathode rays (what we now know as electron beams), could partially pass through a thin aluminiumaluminium-covered covered aperture, and it was the possibility that they might also be able to permeate the glass wall of the tube itself, that Röntgen was investigating at the time. Despite having blacked out the tube in order to negate the fluorescent glow from the ionised gas within, Röntgen noticed a nearby barium platinocyanide-coated screen glowing with fluorescence. Such screens were routinely used for detecting cathode rays and this one was sitting prior to being positioned as part of Röntgen’s apparatus. Further investigation led Röntgen to realise he was dealing with a 'new' type of radiation, which, as it was unknown he designated the moniker X! 

Now, it should be pointed out that whilst Röntgen at the satisfyingly ripe age of 50 deservedly takes credit for this discovery having followed through on his unexpected observation, others before him (notably William Morgan, Humphrey Davy, Hermann Von Helmholtz, and even Röntgen’s own assistant, Ludwig Zehnder) had almost certainly previously observed the effects of X-rays without going on to conclusively investigate the phenomenon. Amusingly, X-rays were briefly known as chi-rays due to a misinterpretation of the letter X by some as the Greek letter χ!

The publication of Röntgen's discovery paper resulted in a veritable explosion of research into X-rays and their application to medicine in particular. Whilst under initial investigation, it was noted that they travelled in straight lines, unaffected by magnetic or electric fields, and they appeared, at first, not able to be reflected or refracted, something we now know to be incorrect.

Of course, as was mentioned at the top of this discussion, X-radiation quickly became a vital part of medical diagnosis; with the examination of everything from bullet wounds to broken bones suddenly becoming considerably less traumatic. Medical X-rays are produced using a high voltage to accelerate electrons that have been emitted by a heated cathode, towards a (usually) tungsten anode. A continuous spectrum of X-radiation occurs both as a combination of Bremsstrahlung radiation, due to the sudden “braking” of the negatively charged particles as they interact with the anode, and also characteristic radiation, due to ionisation of the tungsten atoms. As only a small fraction of the electrons’ kinetic energy is converted to X-radiation, the anode needs to be rotated to prevent overheating.

Medical X-ray images work by photons being absorbed en route to the detection plate, according to the attenuation coefficient of the intervening material; bone in particular being a good X-ray 'stopper' as it contains elements such as phosphorus and calcium whose atoms have large numbers of electrons which can absorb the high energy radiation. Of course, despite all the benefits that the likes of modern CT scanners have to offer, it still needs to be borne in mind that overexposure to X-rays may cause cellular damage leading to the development of tumours.

As hinted at above, X-rays can be refracted, although since their refractive index for most materials is very close to 1 focusing X-rays presents particular problems. They can however be readily diffracted, as in the field of X-ray crystallography, used to determine the atomic or molecular structure of a material. Famously, it was this method that ultimately led to the understanding of the structure of DNA. And, so-called grazing incidence reflection of X-rays in astronomical telescopes whereby the incoming X-rays 'glance' at extremely shallow angles off the inside of closely nested arrays of alternating parabolic and hyperbolic mirror 'cones' allows us to observe some of the highest energy processes across our galaxy and beyond - to galaxy clusters whose presence in x-ray images is betrayed by intracluster pools of hot gas.

On a final note, besides his indisputable legacy, Röntgen's contribution to science was at least for a time preserved in the name of the unit of X-ray exposure. This measurement (as it only applied to air) has since fallen into obsolescence.

Images

• X-ray wrist bones photo by Cara Shelton on Unsplash
• Barium Platinum Cyanide Screens and Fluoroscopes by Arallyn! on Flickr

• The X-ray diffraction photo of DNA that came from Rosalind Franklin by A. Barrington Brown © Gonville & Caius College/Science Photo Library / Anderson, Stephen P.; Fast, Karl, 2020