View source for Cathode ray

{{Short description|Beam of electrons observed in vacuum tubes}}
{{More citations needed|date=February 2011}}
[[File:Cyclotron motion smaller view.jpg|thumb|upright=1.2|A beam of cathode rays in a vacuum tube bent into a circle by a [[magnetic field]] generated by a [[Helmholtz coil]]. Cathode rays are normally invisible; in this demonstration [[Teltron tube]], enough gas has been left in the tube for the gas atoms to [[luminescence|luminesce]] when struck by the fast-moving electrons.]]

'''Cathode rays''' are streams of [[electron]]s observed in [[vacuum tube|discharge tube]]s. If an evacuated glass tube is equipped with two [[electrode]]s and a [[voltage]] is applied, glass behind the positive electrode is observed to glow, due to electrons emitted from the [[cathode]] (the electrode connected to the negative terminal of the voltage supply). They were first observed in 1859 by German physicist [[Julius Plücker]] and [[Johann Wilhelm Hittorf]],<ref>{{Citation
 | first =Andre
 | last =Martin
 | contribution =Cathode Ray Tubes for Industrial and Military Applications
 | editor-last =Hawkes
 | editor-first =Peter
 | title =Advances in Electronics and Electron Physics, Volume 67
 | publisher =Academic Press
 | year =1986
 | isbn =9780080577333
 | page =183
 | quote ="Evidence for the existence of "cathode-rays" was first found by Plücker and Hittorf ..."
}}</ref> and were named in 1876 by [[Eugen Goldstein]] ''Kathodenstrahlen'', or cathode rays.<ref>E. Goldstein (May 4, 1876) [https://books.google.com/books?id=7-caAAAAYAAJ&pg=PA279 "Vorläufige Mittheilungen über elektrische Entladungen in verdünnten Gasen"] (Preliminary communications on electric discharges in rarefied gases), ''Monatsberichte der Königlich Preussischen Akademie der Wissenschaften zu Berlin'' (Monthly Reports of the Royal Prussian Academy of Science in Berlin), 279-295. From page 286:  "''13. Das durch die Kathodenstrahlen in der Wand hervorgerufene Phosphorescenzlicht ist höchst selten von gleichförmiger Intensität auf der von ihm bedeckten Fläche, und zeigt oft sehr barocke Muster.''"  (13. The phosphorescent light that's produced in the wall by the cathode rays is very rarely of uniform intensity on the surface that it covers, and [it] often shows very baroque patterns.)</ref><ref>Joseph F. Keithley ''The story of electrical and magnetic measurements: from 500 B.C. to the 1940s'' John Wiley and Sons, 1999 {{ISBN|0-7803-1193-0}}, page  205</ref> In 1897, British physicist [[J. J. Thomson]] showed that cathode rays were composed of a previously unknown negatively charged particle, which was later named the ''electron''. [[Cathode-ray tube]]s (CRTs) use a focused beam of electrons deflected by electric or magnetic fields to render an image on a screen.

==Description==
[[File:Crookes tube2 diagram.svg|thumb|A diagram showing a [[Crookes tube]] connected to a high voltage supply. The metal Maltese cross in the tube, with no external connection to the circuit, casts a shadow on the glowing wall.]]

Cathode rays are so named because they are emitted by the negative electrode, or [[cathode]], in a vacuum tube. To release electrons into the tube, they first must be detached from the [[atom]]s of the cathode. In the early experimental [[cold cathode]] vacuum tubes in which cathode rays were discovered, called [[Crookes tube]]s, this was done by using a high electrical potential of thousands of [[volt]]s between the anode and the cathode to [[ionization|ionize]] the residual gas atoms in the tube. The positive ions were accelerated by the [[electric field]] toward the cathode, and when they collided with it they knocked electrons out of its surface; these were the cathode rays. Modern vacuum tubes use [[thermionic emission]], in which the cathode is made of a thin wire [[Electrical filament|filament]] which is heated by a separate [[electric current]] passing through it. The heat provides enough energy for  electrons to escape from the surface of the filament, into the evacuated space of the tube.

Since the electrons have a negative charge, they are repelled by the negative cathode and attracted to the positive anode. They travel in parallel lines through the empty tube. The voltage applied between the electrodes accelerates these low mass particles to high velocities. Cathode rays are invisible, but their presence was first detected in these Crookes tubes when they struck the glass wall of the tube, exciting the atoms of the glass and causing them to emit light, a glow called [[fluorescence]]. Researchers noticed that objects placed in the tube in front of the cathode could cast a shadow on the glowing wall, and realized that something must be traveling in straight lines from the cathode. After the electrons strike the back of the tube they make their way to the anode, then travel through the anode wire through the power supply and back through the cathode wire to the cathode, so cathode rays carry electric current through the tube.

The current in a beam of cathode rays through a vacuum tube can be controlled by passing it through a metal screen of wires (a [[Control grid|grid]]) between cathode and anode, to which a small negative voltage is applied. The electric field of the wires deflects some of the electrons, preventing them from reaching the anode. The amount of current that gets through to the anode depends on the voltage on the grid. Thus, a small voltage on the grid can be made to control a much larger voltage on the anode. This is the principle used in [[vacuum tube]]s to [[Amplifier|amplify]] electrical signals. The [[triode]] vacuum tube developed between 1907 and 1914 was the first electronic device that could amplify, and is still used in some applications such as [[radio transmitter]]s. High speed beams of cathode rays can also be steered and manipulated by [[electric field]]s created by additional metal plates in the tube to which voltage is applied, or [[magnetic field]]s created by coils of wire ([[electromagnet]]s). These are used in [[cathode-ray tube]]s, found in televisions and computer monitors, and in [[electron microscope]]s.

<gallery widths="150">
  File:Katódsugarak mágneses mezőben(1).jpg|Crookes tube. The cathode (negative terminal) is on the right. The anode (positive terminal) is in the base of the tube at bottom. 
  File:Katódsugarak mágneses mezőben(2).jpg|Cathode rays travel from the cathode at the rear of the tube, striking the glass front, making it glow green by [[fluorescence]]. A metal cross in the tube casts a shadow, demonstrating that the rays travel in straight lines.
  File:Katódsugarak mágneses mezőben(3).jpg|A magnet creates a horizontal magnetic field through the neck of the tube, bending the rays up, so the green spot is higher.
  File:Katódsugarak mágneses mezőben(4).jpg|When the magnet is reversed, it bends the rays down, so the green spot is lower. The pink glow is caused by cathode rays striking residual gas atoms in the tube.
</gallery>

==History==
After the invention of the [[vacuum pump]] in 1654 by [[Otto von Guericke]], physicists began to experiment with passing high voltage electricity through [[rarefied air]]. In 1705, it was noted that [[electrostatic generator]] sparks travel a longer distance through low pressure air than through atmospheric pressure air.

===Gas discharge tubes===
[[File:Гейслерова трубка (2).jpg|thumb|upright=1.1|Geissler tube, in daylight and lit by its own light]]
[[File:Glow discharge regions.jpg|thumb|upright=1.4|[[Glow discharge]] in a low-pressure tube caused by electric current.]]

In 1838, [[Michael Faraday]] applied a high voltage between two metal [[electrode]]s at either end of a glass tube that had been partially evacuated of air, and noticed a strange light arc with its beginning at the cathode (negative electrode) and its end at the [[anode]] (positive electrode).<ref>Michael Faraday (1838) [https://books.google.com/books?id=ypNDAAAAcAAJ&pg=PA125 "VIII. Experimental researches in electricity. — Thirteenth series.,"] ''Philosophical Transactions of the Royal Society of London'', '''128''' :  125-168.</ref> In 1857, German physicist and glassblower [[Heinrich Geissler]] sucked even more air out with an improved pump, to a pressure of around 10<sup>−3</sup> [[atmosphere (unit)|atm]] and found that, instead of an arc, a glow filled the tube. The voltage applied between the two electrodes of the tubes, generated by an [[induction coil]], was anywhere between a few [[kilovolt]]s and 100 kV. These were called [[Geissler tube]]s, similar to today's [[neon sign]]s.

The explanation of these effects was that the high voltage accelerated free [[electron]]s and electrically charged [[atom]]s ([[ion]]s)  naturally present in the air of the tube.{{Citation needed|date=June 2012}} At low pressure, there was enough space between the gas atoms that the electrons could accelerate to high enough speeds that when they struck an atom they knocked electrons off of it, creating more positive ions and free electrons, which went on to create more ions and electrons in a chain reaction,{{Citation needed|date=June 2012}} known as a [[glow discharge]]. The positive ions were attracted to the cathode and when they struck it knocked more electrons out of it, which were attracted toward the anode. Thus the ionized air was electrically conductive and an electric current flowed through the tube.

Geissler tubes  had enough air in them that the electrons could only travel a tiny distance before colliding with an atom. The electrons in these tubes moved in a slow [[diffusion]] process, never gaining much speed, so these tubes didn't produce cathode rays. Instead, they produced a colorful glow discharge (as in a modern [[neon light]]), caused when the electrons struck gas atoms, exciting their orbital electrons to higher energy levels. The electrons released this energy as light. This process is called fluorescence.

===Cathode rays===
[[File:Ampola de Crookes.gif|thumb|A beam of cathode rays being bent by a magnetic field. Cathode rays are normally invisible; the path of this beam is revealed by having it strike a card with a fluorescent coating]] 
By the 1870s, British physicist [[William Crookes]] and others were able to evacuate tubes to a lower pressure, below 10<sup>−6</sup> atm. These were called [[Crookes tube]]s. Faraday had been the first to notice a dark space just in front of the cathode, where there was no luminescence. This came to be called the "cathode dark space", "Faraday dark space" or "Crookes dark space". Crookes found that as he pumped more air out of the tubes, the Faraday dark space spread down the tube from the cathode toward the anode, until the tube was totally dark. But at the anode (positive) end of the tube, the glass of the tube itself began to glow.

What was happening was that as more air was pumped from the tube, the electrons knocked out of the cathode when positive ions struck it could travel farther, on average, before they struck a gas atom. By the time the tube was dark, most of the electrons could travel in straight lines from the cathode to the anode end of the tube without a collision. With no obstructions, these low mass particles were accelerated to high velocities by the voltage between the electrodes. These were the cathode rays.

When they reached the anode end of the tube, they were traveling so fast that, although they were attracted to it, they often flew past the anode and struck the back wall of the tube. When they struck atoms in the glass wall, they excited their orbital electrons to higher [[energy level]]s. When the electrons returned to their original energy level, they released the energy as light, causing the glass to [[fluoresce]], usually a greenish or bluish color. Later researchers painted the inside back wall with fluorescent chemicals such as [[zinc sulfide]], to make the glow more visible.

Cathode rays themselves are invisible, but this accidental fluorescence allowed researchers to notice that objects in the tube in front of the cathode, such as the anode, cast sharp-edged shadows on the glowing back wall. In 1869, German physicist [[Johann Hittorf]] was first to realize that something must be traveling in straight lines from the cathode to cast the shadows. [[Eugen Goldstein]] named them ''cathode rays'' (German ''Kathodenstrahlen'').

===Discovery of the electron===
[[File:JJThomsonGasDischargeTubeElectronCavendishLab2013-08-29-17-11-41.jpg|thumb|upright=1.3|J. J, Thomson's electric deflection tube, in which he showed that a beam of cathode rays was bent by an electric field like matter particles. The cathode is on R. The electron beam is accelerated passing through the cylindrical high voltage anode (''center''), bent by a voltage on the deflection plates (''center L''), and strikes the back wall of the tube causing a luminous glow.]]   
At this time, atoms were the smallest particles known, and were believed to be indivisible. What carried electric currents was a mystery. During the last quarter of the 19th century, many historic experiments were done with Crookes tubes to determine what cathode rays were. There were two theories. Crookes and [[Arthur Schuster]] believed they were particles of "radiant matter," that is, electrically charged atoms. German scientists Eilhard Wiedemann, [[Heinrich Hertz]] and Goldstein believed they were "aether waves", some new form of [[electromagnetic radiation]], and were separate from what carried the electric current through the tube.

The debate was resolved in 1897 when [[J. J. Thomson]] measured the mass of cathode rays, showing they were made of particles, but were around 1800 times lighter than the lightest atom, [[hydrogen]]. Therefore, they were not atoms, but a new particle, the first ''[[subatomic particle|subatomic]]'' particle to be discovered, which he originally called "''corpuscle''" but was later named ''electron'', after particles postulated by [[George Johnstone Stoney]] in 1874. He also showed they were identical with particles given off by [[Photoelectric effect|photoelectric]] and radioactive materials.<ref name="Thomson1">{{cite journal
  | last = Thomson
  | first = J. J.
  | title = On bodies smaller than atoms
  | journal = The Popular Science Monthly
  | pages = 323–335
  | publisher = Bonnier Corp.
  |date=August 1901| url = https://books.google.com/books?id=3CMDAAAAMBAJ&pg=PA323
  | access-date = 2009-06-21}}</ref> It was quickly recognized that they are the particles that carry electric currents in metal wires, and carry the negative electric charge of the atom.

Thomson was given the 1906 [[Nobel Prize in Physics]] for this work. [[Philipp Lenard]] also contributed a great deal to cathode-ray theory, winning the Nobel Prize in 1905 for his research on cathode rays and their properties.

===Vacuum tubes===
The gas ionization (or [[cold cathode]]) method of producing cathode rays used in Crookes tubes was unreliable, because it depended on the pressure of the residual air in the tube. Over time, the air was absorbed by the walls of the tube, and it stopped working.

A more reliable and controllable method of producing cathode rays was investigated by Hittorf and Goldstein,<ref>{{Cite book |last=Eucken |first=Arnold |url=https://www.google.com/books/edition/Fundamentals_of_Physical_Chemistry_for_S/qLI5AAAAMAAJ?hl=en&gbpv=1&dq=cathode+rays+produce+hittorf+goldstein&pg=PA516&printsec=frontcover |title=Fundamentals of Physical Chemistry for Students of Chemistry and Related Sciences |date=1925 |publisher=McGraw-Hill |language=en}}</ref> and rediscovered by [[Thomas Edison]] in 1880. A cathode made of a wire filament heated red hot by a separate current passing through it would release electrons into the tube by a process called [[thermionic emission]]. The first true electronic vacuum tubes, invented in 1904 by [[John Ambrose Fleming]], used this [[hot cathode]] technique, and they superseded Crookes tubes. These tubes didn't need gas in them to work, so they were evacuated to a lower pressure, around 10<sup>−9</sup> atm (10<sup>−4</sup> Pa). The ionization method of creating cathode rays used in Crookes tubes is today only used in a few specialized [[gas discharge tube]]s such as [[krytron]]s.

In 1906, [[Lee De Forest]] found that a small voltage on a grid of metal wires between the cathode and anode could control a current in a beam of cathode rays passing through a vacuum tube. His invention, called the [[triode]], was the first device that could [[amplifier|amplify]] electric signals, and revolutionized electrical technology, creating the new field of ''[[electronics]]''. Vacuum tubes made [[radio broadcasting|radio]] and [[television broadcasting]] possible, as well as [[radar]], talking movies, audio recording, and long-distance telephone service, and were the foundation of consumer electronic devices until the 1960s, when the [[transistor]] brought the era of vacuum tubes to a close.

Cathode rays are now usually called electron beams. The technology of manipulating electron beams pioneered in these early tubes was applied practically in the design of vacuum tubes, particularly in the invention of the cathode-ray tube (CRT) by [[Ferdinand Braun]] in 1897, which was used in [[television set]]s and [[oscilloscope]]s. Today, electron beams are employed in sophisticated devices such as electron microscopes, [[electron beam lithography]] and [[particle accelerator]]s.

== Properties of cathode rays and the experiments that revealed them ==
During the last quarter of the 19th century dozens of historic experiments were conducted to try to find out what cathode rays were.<ref name="Brona">{{cite web | last=Brona | first=Grzegorz | title=The Cathode Rays | work=Atom - The Incredible World | url=http://library.thinkquest.org/19662/high/eng/cathoderays.html | access-date=2008-09-27 | display-authors=etal | url-status=dead | archive-url=https://web.archive.org/web/20090211185645/http://library.thinkquest.org/19662/high/eng/cathoderays.html | archive-date=2009-02-11 }}</ref> There were two theories: British scientists [[William Crookes|Crookes]] and [[C. F. Varley|Cromwell Varley]] believed they were particles of 'radiant matter', that is, electrically charged [[atoms]]. German researchers E. Wiedemann, [[Heinrich Hertz]], and [[Eugen Goldstein]] believed they were '[[Luminiferous aether|aether]] vibrations', some new form of [[electromagnetic wave]]s, and were separate from what carried the current through the tube.<ref name="Pais" />{{rp|79-81}}<ref name="Thomson" >{{cite book
  | last = Thomson
  | first = J. J.
  | title = The Discharge of Electricity through Gasses
  | publisher = Charles Scribner's Sons
  | date = 1903
  | location = New York
  | language = 
  | url = https://archive.org/details/bub_gb_Ryw4AAAAMAAJ/page/n5/mode/2up
  | archive-url= 
  | archive-date=
  | doi = 
  | id = 
  | isbn = 
  | mr = 
  | zbl = 
  | jfm =}}</ref>{{rp|189-190}}<ref name="Brona" /> The debate continued until [[J. J. Thomson]] measured cathode ray’s mass, proving they were a previously unknown negatively charged particle in an atom, the first [[subatomic particle]], which he called a 'corpuscle' but was later renamed the 'electron'.

===Straight line motion===
[[Julius Plücker]] in 1858/59 built a tube with an anode shaped like a [[Maltese Cross]] facing the cathode. It was hinged, so it could fold down against the floor of the tube. When the tube was turned on, the cathode rays cast a sharp cross-shaped shadow on the fluorescence on the back face of the tube, showing that the rays moved in straight lines.<ref name="Pais">{{cite book | last = Pais | first = Abraham | author-link = Abraham Pais | title = Inward Bound: Of Matter and Forces in the Physical World | publisher = Oxford Univ. Press | year = 1986 | location = UK | url = https://books.google.com/books?id=mREnwpAqz-YC&pg=PA81 | isbn = 978-0-19-851997-3 }}</ref>{{rp|79}} This fluorescence was used as an argument that cathode rays were electromagnetic waves, since the only thing known to cause fluorescence at the time was [[ultraviolet]] light. After a while the fluorescence would get 'tired' and the glow would decrease.<ref name="Thomson" />{{rp|143}} If the cross was folded down out of the path of the rays, it no longer cast a shadow, and the previously shadowed area would fluoresce more strongly than the area around it.

===Perpendicular emission===
[[file:Crookes tube for heating 1879.png|thumb|upright=0.4|Crookes tube with concave cathode]]
[[Eugen Goldstein]] in 1876 found that cathode rays were always emitted perpendicular to the cathode's surface.<ref name="Thomson" />{{rp|138}}<ref>Goldstein E. (1876). ''Monat der Berl. Akad''., p. 284.</ref> If the cathode was a flat plate, the rays were shot out in straight lines perpendicular to the plane of the plate. This was evidence that they were particles, because a luminous object, like a red hot metal plate, emits light in all directions, while a charged particle will be repelled by the cathode in a perpendicular direction.  Cathode rays heat matter which they strike.<ref name="Thomson" />{{rp|145}}  If the electrode was made in the form of a concave spherical dish, the cathode rays would be focused to a spot in front of the dish.<ref name="Brona" />  This could be used to heat samples to a high temperature.
{{breakafterimages}}

===Electrostatic deflection===
Cathode rays path can be deflected by an [[electric field]].  [[Heinrich Hertz]] built a tube with a second pair of metal plates to either side of the cathode ray beam, a crude [[Cathode-ray tube|CRT]]. If the cathode rays were [[charged particle]]s, their path should be bent by the [[electric field]] created when a [[voltage]] was applied to the plates, causing the spot of light where the rays hit to move sideways. He did not find any bending, but it was later determined that his tube was insufficiently evacuated, causing accumulations of [[surface charge]] which masked the electric field. Later Arthur Schuster repeated the experiment with a higher vacuum. He found that the rays were attracted toward a positively charged plate and repelled by a negative one, bending the beam. This was evidence they were negatively charged, and therefore not electromagnetic waves.
{{breakafterimages}}

===Magnetic deflection===
[[file:Crookes magnetic deflection tube.png|thumb|Crookes magnetic deflection tube]]
The rays path can be deflected by a [[magnetic field]].  [[William Crookes|Crookes]] put a [[magnet]] across the neck of the tube, so that the North pole was on one side of the beam and the South pole was on the other, and the beam travelled through the [[magnetic field]] between them. The beam was bent down, perpendicular to the magnetic field.<ref name="Thomson" />{{rp|150-151}} To reveal the path of the beam, [[William Crookes|Crookes]] invented a tube ''(see pictures)'' with a cardboard screen with a [[phosphor]] coating down the length of the tube, at a slight angle so the electrons would strike the phosphor along its length, making a glowing line on the screen. The line could be seen to bend up or down in a transverse magnetic field. This effect (now called the [[Lorentz force]]) was similar to the behavior of electric currents in an [[electric motor]] and showed that the cathode rays obeyed [[Faraday's law of induction]] like currents in wires. Both electric and magnetic deflection were evidence for the particle theory, because electric and magnetic fields have no effect on a beam of light waves in vacuum.

===Paddlewheel===
[[file:Crookes paddlewheel tube.png|thumb|Crookes's paddlewheel tube, from his 1879 paper ''On Radiant Matter'']]
[[William Crookes|Crookes]] put a tiny vaned [[turbine]] or [[paddlewheel]] in the path of the cathode rays, and found that it rotated when the rays hit it.<ref name="Thomson" />{{rp|146-149}} The paddlewheel turned in a direction away from the cathode side of the tube, suggesting that the force of the cathode rays striking the paddles was causing the rotation. [[William Crookes|Crookes]] concluded at the time that this showed that cathode rays had [[momentum]], so the rays were likely [[matter]] particles. However, later it was concluded that the paddle wheel turned not due to the momentum of the particles (or electrons) hitting the paddle wheel but due to the [[Crookes radiometer|radiometric effect]].<ref name="Brona" />  When the rays hit the paddle surface they heated it, and the heat caused the gas next to it to expand, pushing the paddle. This was proven in 1903 by [[J. J. Thomson]] who calculated that the momentum of the electrons hitting the paddle wheel would only be sufficient to turn the wheel one revolution per minute.<ref name="Brona" /> All this experiment really showed was that cathode rays were able to heat surfaces.

===Negative electric charge===
[[Jean-Baptiste Perrin]] wanted to determine whether the cathode rays actually carried negative [[Electric charge|charge]], or whether they just accompanied the charge carriers, as the Germans thought. In 1895 he constructed a tube with a 'catcher', a closed aluminum cylinder with a small hole in the end facing the cathode, to collect the cathode rays.<ref name="Thomson" />{{rp|161-165}} The catcher was attached to an [[electroscope]] to measure its charge. The electroscope showed a negative charge, proving that cathode rays really carry negative electricity.

===Anode rays===
[[file:Anode Ray Tube.jpg|thumb|upright=0.75|Special tube with perforated cathode, producing anode rays ''(top, pink)'']]
Goldstein found in 1886 that if the cathode is made with small holes in it, streams of a faint luminous glow will be seen issuing from the holes on the back side of the cathode, facing away from the anode.<ref>Goldstein E. (1886) Berliner Sitzungsberichte, 39, p.391</ref><ref name="Thomson" />{{rp|158-159}} It was found that in an electric field these [[anode ray]]s bend in the opposite direction from cathode rays, toward a negatively charged plate, indicating that they carry a positive charge. These were the positive [[ion]]s which were attracted to the cathode, and created the cathode rays. They were named ''canal rays'' (''Kanalstrahlen'') by Goldstein.<ref>{{cite web | title = Concept review Ch.41 Electric Current through Gasses | work = Learning Physics for IIT JEE | year = 2008 | url = http://iit-jee-physics.blogspot.com/2008/03/concept-review-ch41-electric-current.html | access-date = 2008-11-11}}</ref>

===Spectral shift===
[[Eugen Goldstein]] thought he had figured out a method of measuring the speed of cathode rays. If the [[glow discharge]] seen in the gas of Crookes tubes was produced by the moving cathode rays, the light radiated from them in the direction they were moving, down the tube, would be shifted in [[frequency]] due to the [[Doppler effect]].<ref name="Brona" /> This could be detected with a [[spectroscope]] because the [[emission line]] [[spectrum]] would be shifted. He built a tube shaped like an "L", with a spectroscope pointed through the glass of the elbow down one of the arms. He measured the spectrum of the glow when the spectroscope was pointed toward the cathode end, then switched the power supply connections so the cathode became the anode and the electrons were moving in the other direction, and again observed the spectrum looking for a shift. He did not find one, which he calculated meant that the rays were traveling very slowly. It was later recognized that the glow in Crookes tubes is emitted from gas atoms hit by the electrons, not the electrons themselves. Since the atoms are thousands of times more massive than the electrons, they move much slower, accounting for the lack of Doppler shift.

=== Lenard window ===
[[file:Lenard window tube labeled.svg|thumb|Lenard window tube ]]
[[Philipp Lenard]] wanted to see if cathode rays could pass out of the Crookes tube into the air. See diagram. He built a tube with a "window" ''(W)'' in the glass envelope made of [[aluminum foil]] just thick enough to hold the atmospheric pressure out (later called a "Lenard window") facing the cathode ''(C)'' so the cathode rays would hit it.<ref name="Thomson" />{{rp|182-188}}  He found that something did come through. Holding a fluorescent screen up to the window caused it to fluoresce, even though no light reached it. A [[photographic plate]] held up to it would be darkened, even though it was not exposed to light. The effect had a very short range of about {{convert|2.5|cm|in|2}}. He measured the ability of cathode rays to penetrate sheets of material, and found they could penetrate much farther than moving atoms could. Since atoms were the smallest particles known at the time, this was first taken as evidence that cathode rays were waves. Later it was realized that electrons were much smaller than atoms, accounting for their greater penetration ability. Lenard was awarded the [[Nobel Prize in Physics]] in 1905 for his work.

Wilhelm Conrad Röntgen repeated Lenard's experiments and, on the evening of November 8, 1895, as darkness fell, discovered the emission of X-rays from the glass wall of his tube as he further reduced the residual gas pressure and increased the high voltage. He found that, under optimal conditions, the emission was accompanied by a yellowish glow from the wall struck by the cathode rays. Geissler had already observed a similar yellowish glow 35 years earlier without noticing X-rays. Röntgen was awarded the first [[Nobel Prize in Physics]] in 1901.

===Wave-particle duality===
[[Louis de Broglie]] later (1924) suggested in his doctoral dissertation that electrons are like photons and can act as [[Matter wave|waves]]. The wave-like behaviour of cathode rays was later directly demonstrated using reflection from a nickel surface by [[Davisson–Germer experiment|Davisson and Germer]],<ref>{{Cite journal |last1=Davisson |first1=C. |last2=Germer |first2=L. H. |date=1927 |title=Diffraction of Electrons by a Crystal of Nickel |journal=Physical Review |volume=30 |issue=6 |pages=705–740 |doi=10.1103/PhysRev.30.705|doi-access=free |bibcode=1927PhRv...30..705D }}</ref> and transmission through celluloid thin films and later metal films by [[George Paget Thomson]] and Alexander Reid<ref>{{Cite journal |last1=Thomson |first1=G. P. |last2=Reid |first2=A. |date=1927 |title=Diffraction of Cathode Rays by a Thin Film |journal=Nature |language=en |volume=119 |issue=3007 |pages=890 |doi=10.1038/119890a0 |issn=1476-4687|doi-access=free |bibcode=1927Natur.119Q.890T }}</ref> in 1927. (Alexander Reid, who was Thomson's graduate student, performed the first experiments but he died soon after in a motorcycle accident<ref>{{Cite journal |last=Navarro |first=Jaume |date=2010 |title=Electron diffraction chez Thomson: early responses to quantum physics in Britain |url=https://www.cambridge.org/core/product/identifier/S0007087410000026/type/journal_article |journal=The British Journal for the History of Science |language=en |volume=43 |issue=2 |pages=245–275 |doi=10.1017/S0007087410000026 |issn=0007-0874|url-access=subscription }}</ref> and is rarely mentioned.)

==See also==
{{Portal|Physics}}
{{col div|colwidth=30em}}
*[[Beta particle|β (beta) particles]]
*[[Cathode-ray tube]]
*[[Electron beam processing]]
*[[Electron diffraction]]
*[[Electron microscope]]
*[[Electron beam melting]]
*[[Electron beam welding]]
*[[Electron beam technology]]
*[[Electron gun]]
*[[Electron irradiation]]
*[[Ionizing radiation]]
*[[Particle accelerator]]
*[[Sterilisation (microbiology)]]

{{colend}}

==References==
{{Reflist}}
* General Chemistry (structure and properties of matter) by Aruna Bandara (2010)

==External links==
*[https://web.archive.org/web/20140701141404/http://www.crtsite.com/page3.html The Cathode Ray Tube site]
*[https://www.youtube.com/watch?v=Xt7ZWEDZ_GI Crookes tube with maltese cross operating]

{{Authority control}}

{{DEFAULTSORT:Cathode Ray}}
[[Category:Electromagnetism]]
[[Category:Electron beam]]
[[Category:Articles containing video clips]]