X-rays are similar to visible light but are more penetrating and cause ionisation.

For further information on electromagnetic spectrum, click

A discharge tube contains a gas at low pressure and two metal plates with a voltage across them..

Cathode rays are in fact particles called electrons

Stoney, an Irishman, came up with the name electron.

Photons release electrons from a metal in the photoelectric effect, whereas electrons release photons in x-ray production

Flesh is transparent to x-rays, but bones are not.

Radioactivity is a natural phenomenon whereby radiation (more penetrating than x-rays) is released spontaneously.

The term radioactivity was coined by Marie Curie.

Electromagnetic Spectrum

X-rays are electromagnetic radiation. Therefore they travel with the speed of light. They have much in common with visible light, radio waves and microwaves, but are more penetrating than these.

Induction Coil and Vacuum Pump

Callan’s invention of the induction coil in 1836 and Geissler’s invention of an efficient vacuum pump, laid the foundations for many of the scientific discoveries at the end of the nineteenth century. Callan’s coil provided the high voltages, and Geissler’s pump the low pressures, that led to the investigation of electric discharges in gases.

Discharge Tubes

When experimenting with discharge tubes,
Geissler discovered that rays were emitted from the cathode.
Goldstein named these rays “cathode rays”.

Cathode Rays

Cathode rays were found to be negatively charged and consequently were deflected by electric and magnetic field.
Cathode rays were also found to have momentum and energy.
In 1897 J.J. Thomson measured the specific charge (q /m) for cathode rays and concluded that the cathode rays were in fact particles which he called electrons (after an earlier suggestion that this term be used for the smallest unit of negative charge, by an Irishman, Stoney (1826-1911).

Photoelectric Effect

Late in the nineteenth century a series of experiments revealed that electrons are emitted from a metal surface when light of sufficiently high frequency falls upon it. This phenomenon is known as the photoelectric effect. (It seemed logical that light waves carry energy, which is absorbed by the metal and that electrons on the surface gather some of the energy and use it to escape. However the true explanation is far more complicated)


The discovery of x-rays was one of those lucky scientific ‘accidents’. On 8 November 1895 Roentgen was working with a cathode ray tube when he noticed an unexpected glow about two metres away from the tube, coming from some barium platinocyanide [BaPt(CN)4 ]. This was a fluorescent material that was often used to detect cathode rays. He discovered that human flesh is transparent to it, but that bones are not.
X-rays may be seen as the converse of the photoelectric effect in so far as electrons release waves in the former and waves release electrons in the latter.


In 1896 Bequerel discovered that some materials emitted an even more penetrating radiation than x-rays. These new waves were called gamma rays. They were found to be emitted as a consequence of the material being inherently unstabe. This new phenomenon was called radioactivity and was later shown to be a nuclear process unlike x-ray emission.

Nuclear Atom

In 1911 Rutherford proposed the nuclear model of the atom as a result of the alpha scattering experiment conducted by his students.