OK , I already know:
Fast electrons collide with metal atoms ,creates x-rays .
But the questions is how those collisions create X rays. What I think (don't know if it right ! So I need help ) :
Electrons carry high energy ( due to high speed ) after collisions :Those energy transform into 2 part
- first main part is heat : most of energy from electron transports to metal atoms make those atoms move faster and produce heat.
Second part is photon ( x-rays ) : there some electron collide with metal atoms but some don't collide at all . It loses its speed due to electrostatic attraction between atoms and electron , and since they haven't collide those energy cant transports to metal atoms => According to conserve law those speed or energy lost in elec have to go somewhere and that is photon right ?

If what I think is correct then :
what if A ship (space ship ) in space with high speed , get pulled by gravity of a planet . It's does not collide , just lose of some it's speed then those energy lost will turn into to photons ray?

  • $\begingroup$ Accelerating charges produce radiation (check out synchrotrons). $\endgroup$
    – lemon
    Feb 21, 2016 at 16:20
  • 1
    $\begingroup$ Well described here. Bremsstrahlung (braking radiation) and characteristic X-rays hyperphysics.phy-astr.gsu.edu/hbase/quantum/xtube.html $\endgroup$
    – Farcher
    Feb 21, 2016 at 16:32
  • 1
    $\begingroup$ A tip on your English: The occasional grammatical mistake is not a big deal. But no matter what your native language is, I know it doesn't just use excessive capitalization, random line breaks, multiple question marks in a row, or inconsistent spacing around punctuation. These are traits of informality, not unfamiliarity, and they make your post difficult to read (in any language). $\endgroup$
    – user10851
    Feb 21, 2016 at 19:47
  • $\begingroup$ I am sorry .I thought if there are some line breaks it will be easier to read . ( I still dont think those line breaks are random though ) $\endgroup$
    – Jack Lee
    Feb 23, 2016 at 16:39

3 Answers 3


The highly energetic incident electrons can lose energy and produce x-rays in following two processes:

  1. Atomic Processes: The incident electrons cause excitation of inner shell electrons, leaving behind a vacancy. When this vacancy is filled by an electron of the atom from higher energy level, you get characteristic x-rays.
  2. Deceleration of electrons or bremsstrahlung: This is central to your problem I guess. In this process, the highly energetic electrons are decelerated by target nuclei. Not every decelerating/accelerating particles/bodies emit electromagnetic (EM) radiations, only charged particles do. Some details can be found here. This phenomenon gives the continuous spectra of x-rays produced by this method. The space ship will not be emitting the x-rays because it is overall neutral (almost all macroscopic bodies are by default neutral, equal amount of positive and negative charges).

The other answers cover the production of X-rays by the bremsstrahlung process just fine. Let me address this point about a decelerating spaceship not producing photons.

A classical approach uses Larmor's formula to calculate the power lost by electromagnetic radiation, and it is proportional to the second time derivative of the dipole moment squared, $\ddot{p}^2$. Only for isolated charged particles can we equate this to $q^2 a^2$, where $q$ is the charge and $a$ is the acceleration.

In bremsstrahlung, an electron is accelerated in the electric field of a nucleus and the dipole moment (charge times separation) of the pair is rapidly changing. Hence radiation is produced.

A decelerating spaceship and its constituent atoms (containing both positive and negative charges) do not experience a change in their dipole moments due to this acceleration and therefore there is no radiation emitted.


X-rays were first produced in some discharge tube cathode rays experiments performed by Roentzen in 1905. As the nature of radiation was not known and it was invisible to the eye He called it unknown (X) -rays.

Its presence was felt by the experimenter as his stacks of Photoplates kept in the drawers of his table got blackened as if exposed to some radiation- later he saw that these rays pass through the muscles of his hand and an image of bones get formed- that is historically discovery of X-rays.

Physically what was happening that electrons getting very high kinetic energy through a potential difference of say 20-30 kilovolts are accelerated and get close to the atom but penetrate the outer atomic shells and if they knock out an electron from the K shell which are the closest to the nucleus- can knock out the electron to create a vacancy in the shell.

this absence of say a K-shell electron may be filled up by electrons from higher shells naturally and this transfer (jump) of electron from outer to K-shell leads to emission of good amount of energy in the form of electromagnetic radiation of wavelengths say 1-2 angstrom (1 angstrom =10^-10 meter). These are called characteristic X-rays whose wavelength depend on the nature of the material on which the electrons are hitting.

However when energetic electrons who could not knock out inner electrons but were dissipating their energy in the repulsive field of atomic electron cloud- were decelerated and such charge particles emit radiations which were of higher wavelengths say 10-200 Angstrom. These were called soft x-rays which did not have much penetrating power as compared to characteristic X-rays.

So, the mechanism of production of X-rays have been given above -i think other questions come in secondary comments - these rays have been used extensively by scientific investigators as well as in Medical investigative technologies.

  • $\begingroup$ "electrons who could not knock out inner electrons but were dissipating their energy in the repulsive field of atomic electron cloud- were decelerated and such charge particles emit radiations which were of higher wavelengths say 10-200 Angstrom ". Is it also mean everthing else get decelerated without collide will shoot out photons ? I mean the speed lost has to go somewhere right ? $\endgroup$
    – Jack Lee
    Feb 23, 2016 at 16:33
  • $\begingroup$ No, it does not mean that" everything else on acceleration/deceleration emit radiations" . Only charged particles when they accelerate/decelerate emit electromagnetic radiations-usually when their energies are lower radio waves of large wavelengths are produced. As the cathode ray electrons have about 25 keV energies when they declerate produce soft X-rays of larger wavelengths than characterisic X-rays- these soft X-rays have less penetrating power and get absorbed by human tissues and more harmful to us. $\endgroup$
    – drvrm
    Feb 23, 2016 at 17:02
  • $\begingroup$ The X-ray tube where the electrons get accelerated and impinges on target has an arrangement to cool the target by flowing water as in this process of production of X-rays a good amount of heat radiations are also produced -that is part of the kinetic energy gets converted into heat energy. $\endgroup$
    – drvrm
    Feb 23, 2016 at 17:11
  • $\begingroup$ "The electrons do not get attracted by the atoms of the target material" as proposed in the question -as all the atoms have charge balance and are neutral in nature-the electrons get its energy to hit the atoms and penetrate to inside shells by outside arrangement of potential difference to the tune of 20-25 kV by step up transformers. $\endgroup$
    – drvrm
    Feb 23, 2016 at 17:17

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