Many people would probably imagine that the history of the X-ray was purely about the medical applications that we know so well today, but as I will relate, the X-ray also played a significant part in helping our theoretical understanding of light itself.
X-rays are very short wavelength waves in the electromagnetic spectrum and were discovered by Wilhelm Konrad Roentgen on November 8th 1895, confirming the earlier predictions of James Clerk Maxwell. The new rays were highly penetrative, even passing through metal. Doctors began to use them for medical examination looking inside the body within days of them being announced.
Max von Laue discovered in 1912 that X-rays act as ordinary light using crystals to create diffraction patterns, proving that they were a part of the electromagnetic spectrum like any other.
Arthur Holly Compton in 1922 calculated that a discrete increase of wavelength occurs to an X-ray quantum when it collides with an electron. He subsequently established this experimentally. This X-ray scattering is now known as The Compton Effect.
This confirmed Albert Einstein's light quanta theory, that light travels in discrete packets known as quanta. Using X-rays was vital for this experiment because they were the only electromagnetic waves available with a short enough wavelength. The result allowed the calculation of the speed of light in a vacuum, known as c (for constant), one of the most fundamental and important numbers in the whole of science.
Despite the significant part played in the development of theoretical physics, it is for their medical applications that X-rays are far more well known today by the general public.
Within three months of their discovery by Roentgen an X-ray was used in the treatment of patient, Eddie McCarthy's, broken arm. You have to wonder just how many years the technology would have taken to get safety clearance had it just been discovered today.
In 1903 a further medical use of X-rays was suggested by George Perthes who found that they have an inhibitory effect on tumors and therefore proposed that they can be used for cancer treatment.
Another significant breakthrough occurred in 1949 when a synchrotron was used to generate X-rays for the first time for medical diagnosis and treatment.
X-ray technology has continued to be used and improved upon right up to the present day. But despite these impressive and common uses in medical applications, the importance of X-rays in the development of theoretical physics should not be forgotten.