The Electromagnetic Spectrum by Year 12

X- Rays

by Sambathkumar Uruthirakumar 12G

Brief description

X-rays are electromagnetic waves which have a very high frequency but a short wave length. They have wavelengths ranging from 0.01 to 10 nanometres with corresponding frequencies in the range 30 petahertz to 30 exahertz (3 × E16 Hz to 3 × E19 Hz) and energies in the range 100 eV to 100 keV. They can be found between gamma radiation and ultra-violet light.


X-rays are part of the electromagnetic spectrum, with wavelengths shorter than visible light. Different applications use different parts of the X-ray spectrum.


X-rays were found emanating from Crookes tubes, experimental discharge tubes invented around 1875, by scientists investigating the cathode rays that are energetic electron beams that were first created in the tubes. Crookes tubes created free electrons by ionization of the residual air in the tube by a high DC voltage of anywhere between a few kilovolts and 100 kV. This voltage accelerated the electrons coming from the cathode to a high enough velocity that they created X-rays when they struck the anode or the glass wall of the tube. Many of the early Crookes tubes undoubtedly radiated X-rays, because early researchers noticed effects that were attributable to them, as detailed below. Wilhelm Röntgen was the first to systematically study them, in 1895

X-radiation is often referred to as Röntgen radiation, after Wilhelm Röntgen, who is usually credited as its discoverer, and who had named it X-radiation to signify an unknown type of radiation.


Wilhelm Conrad Röntgen (27 March 1845 – 10 February 1923) was a German physicist, who, on 8 November 1895, produced and detected electromagnetic radiation in a wavelength range known as X-rays or Röntgen rays, an achievement that earned him the first Nobel Prize in Physics in 1901

He was working with various vacuum tubes in his laboratory and passed an electrical discharge through them. One of them contained a thin aluminium window to permit cathode rays to exit but a cardboard covering was added to protect the aluminium from damage by the strong electrostatic field that is necessary to produce the cathode rays. He knew the cardboard covering prevented light from escaping, yet Röntgen observed that the invisible cathode rays caused a fluorescent effect on a small cardboard screen painted with barium platinocyanide when it was placed close to the aluminium window. It occurred to Röntgen that the Hittorf-Crookes tube, which had a much thicker glass wall than the Lenard tube, might also cause this fluorescent effect.

In the late afternoon of 8 November 1895, Röntgen was determined to test his idea. He carefully constructed a black cardboard covering similar to the one he had used on the Lenard tube. He covered the Hittorf-Crookes tube with the cardboard and attached electrodes to a Ruhmkorff coil to generate an electrostatic charge. Before setting up the barium platinocyanide screen to test his idea, Röntgen darkened the room to test the opacity of his cardboard cover. As he passed the Ruhmkorff coil charge through the tube, he determined that the cover was light-tight and turned to prepare the next step of the experiment. It was at this point that Röntgen noticed a faint shimmering from a bench a few feet away from the tube. To be sure, he tried several more discharges and saw the same shimmering each time. Striking a match, he discovered the shimmering had come from the location of the barium platinocyanide screen he had been intending to use next.

That’s when he concluded that this was a new type of ray, which was capable of passing through many substances and causing a shadow of solid objects.

Nearly two weeks after his discovery, he took the very first picture using X-rays of his wife Anna Bertha’s hand. When she saw her skeleton she exclaimed “I have seen my death!”

Hand mit Ringen (Hand with Rings): print of Wilhelm Röntgen’s first “medical” X-ray, of his wife’s hand, taken on 22 December 1895 and presented to Ludwig Zehnder of the Physik Institut, University of Freiburg, on 1 January 1896.


Production of X-rays

X-rays can be generated by using an X-ray vacuum tube that uses a high voltage to accelerate electrons released by a hot cathode to a high velocity. The high velocity electrons collide with a metal target, the anode, creating the X-rays.


The type of material in the target can dictate the type of X-rays produced.



The maximum energy of the produced X-ray photon is limited by the energy of the incident electron, which is equal to the voltage on the tube times the electron charge, so an 80 kV tube cannot create X-rays with energy greater than 80 keV. When the electrons hit the target, X-rays are created by two different atomic processes.

The image below shows the spectrum of the X-rays emitted by an X-ray tube with a rhodium target, operated at 60 kV. The smooth, continuous curve is due to bremsstrahlung, and the spikes are characteristic K lines for rhodium atoms.


Characteristic X-ray emission: If the electron has enough energy it can knock an orbital electron out of the inner electron shell of a metal atom, and as a result electrons from higher energy levels then fill up the vacancy and X-ray photons are emitted. This process produces an emission spectrum of X-rays at a few discrete frequencies, sometimes referred to as the spectral lines. The spectral lines generated depend on the target (anode) element used and thus are called characteristic lines. Usually these are transitions from upper shells into K shell (called K lines), into L shell (called L lines) and so on.

Bremsstrahlung: This is radiation given off by the electrons as they are scattered by the strong electric field near the high-Z (proton number) nuclei. These X-rays have a continuous spectrum. The intensity of the X-rays increases linearly with decreasing frequency, from zero at the energy of the incident electrons, the voltage on the X-ray tube.

So the resulting output of a tube consists of a continuous bremsstrahlung spectrum falling off to zero at the tube voltage, plus several spikes at the characteristic lines. The voltages used in diagnostic X-ray tubes range from roughly 20 to 150 kV and thus the highest energies of the X-ray photons range from roughly 20 to 150 keV.

Both of these X-ray production processes are inefficient, with a production efficiency of only about one percent, and hence, to produce a usable flux of X-rays, most of the electric power consumed by the tube is released as waste heat. The X-ray tube must be designed to dissipate this excess heat.

X-rays can also be produced by fast protons or other positive ions. The Proton-induced X-ray emission or Particle-induced X-ray emission is widely used as an analytical procedure.

Use of X-rays

X-ray crystallography, in which the pattern produced by the diffraction of X-rays through the closely spaced lattice of atoms in a crystal is recorded and then analysed, to reveal the nature of that lattice. A related technique, fibre diffraction, was used by Rosalind Franklin to discover the double helical structure of DNA.


Each dot, called a reflection, in this diffraction pattern forms from the constructive interference of scattered X-rays passing through a crystal. The data can be used to determine the crystalline structure.

X-ray astronomy which is an observational branch of astronomy and deals with the study of X-ray emission from celestial objects


The above image is an X-ray image of a mini supernova

X-ray microscopic analysis, which uses electromagnetic radiation in the soft X-ray band to produce images of very small objects.

X-ray fluorescence, a technique in which X-rays are generated within a specimen and detected. The outgoing energy of the X-ray can be used to identify the composition of the sample.

Industrial radiography uses X-rays for inspection of industrial parts, particularly welds.

Industrial CT (computed tomography) is a process which uses X-ray equipment to produce three-dimensional representations of components both externally and internally. This is accomplished through computer processing of projection images of the scanned object in many directions.

Paintings are often X-rayed to reveal the underdrawing and pentimenti or alterations in the course of painting, or by later restorers. Many pigments such as lead white show well in X-ray photographs.

X-ray spectromicroscopy has been used to analyse the reactions of pigments in paintings. For example, in analysing colour degradation in the paintings of van Gogh

Airport security luggage scanners use X-rays for inspecting the interior of luggage for security threats before loading on aircraft.

Border control truck scanners use X-rays for inspecting the interior of trucks.

X-ray fine art photography by Peter Dazeley can be seen here

X-ray art and fine art photography, artistic use of X-rays, for example the works by Stane Jagodič

X-ray hair removal, a method popular in the 1920s but now banned by the FDA.

Shoe-fitting fluoroscopes were popularized in the 1920s, banned in the US in the 1960s, banned in the UK in the 1970s, and even later in continental Europe.

Roentgen stereophotogrammetry is used to track movement of bones based on the implantation of markers

X-ray photoelectron spectroscopy is a chemical analysis technique relying on the photoelectric effect, usually employed in surface science.


The biggest uses of X-rays are used in medicine

Medical imaging is the technique and process of creating visual representations of the interior of a body for clinical analysis and medical intervention.

A radiograph is an X-ray image obtained by placing a part of the patient in front of an X-ray detector and then illuminating it with a short X-ray pulse.


Mrs Hare’s repaired broken wrist

Computed tomography (CT scanning) is a medical imaging modality where tomographic images or slices of specific areas of the body are obtained from a large series of two-dimensional X-ray images taken in different directions.


Head CT scan (transverse plane) slice -– a modern application of medical radiography

Fluoroscopy is an imaging technique commonly used by physicians or radiation therapists to obtain real-time moving images of the internal structures of a patient through the use of a fluoroscope.

The use of X-rays as a treatment is known as radiation therapy and is largely used for the management (including palliation) of cancer; it requires higher radiation doses than those received for imaging alone. X-rays beams are used for treating skin cancers using lower energy x-ray beams while higher energy beams are used for treating cancers within the body such as brain, lung, prostate and breast.


The main benefit of using X-rays is that it enables objects to be examined without having to go inside the object


X-rays are considered to be safe for adults however; it is not safe for small children or the developing foetus. Diagnostic X-rays (primarily from CT scans due to the large dose used) increase the risk of developmental problems and cancer in those exposed. X rays are classified as a carcinogen by both the World Health Organization’s International Agency for Research on Cancer and the U.S. government. It is estimated that 0.4% of current cancers in the United States are due to computed tomography (CT scans) performed in the past and that this may increase to as high as 1.5-2% with 2007 rates of CT usage

Some X-rays require the use of a dye to make the image clear and it usually contains iodine. This can cause hives, itching, light-headedness, nausea and a metallic taste in the mouth. There is a remote chance that the dye can cause a severe reaction like an anaphylactic shock, very low blood pressure or cardiac arrest.

Other sources used:







2 thoughts on “The Electromagnetic Spectrum by Year 12

  1. I have just discovered that you have used one of Peter Dazeley’s copyrighted x-ray images of an x-ray of a needle fish. We have no record of permission being sought for this. Please explain how this has happened and where we should send the invoice for its use.


    1. This is entirely my fault as I should have checked what my student had asked for permission. Could you let me know how much this would cost. If it is too much then I will remove the photograph.


Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s