ELECTRON OPTICS

Waikato Times, Volume 119, Issue 19882, 11 May 1936, Page 12

ELECTRON OPTICS

New Branch of Study

USEFUL APPLICATIONS Cathode rays are streams of elsctronl shot out at a high velocity from flat cathode or heated filament of on evacuated tube. Their behaviour is govern‘ ed by the construction and arrangement of other electrodes to which p 0: tentiols of various magnitudes may be applied. Early in the Work that was carried out it was suspected thnt they, resembled in some respects light beams. g For example, they would throw a. : “shallow" when some solid object was placed in the stream. Further, it was found that these electron beams could be refracted or bent in much the some , way that the path of light rays could 1 be changed. The similarity in tho behaviour of ‘ electron beams and light rays led to the application of such terms as elec~ tron optics, electron lenses, and images and others, borrowed from light, which ;in themselves indicate that those »heams can be subjected to concentrations and other processes of the kind. to which light beams can be subJectcd. Electron optics has now become I, brnnr-h of science \\hichy With "'0 growing importance of cathode ray tubes. valves, photo-electric cells, and similar devices. approaches in importance that of ordinary optics. There is this important difl'erence. however, between the two. A light ray is relatively unimpeded in its path by glass lenses of suitable composition, but an electron beam, being composed of tangible, although very Imlll masses, must be influenced by somo means which will not block its path. The desired efi‘ect is obtained by the use of electric and magnetic fields. The arrangements for providing these fields may take many forms. For example, in a cathode ray tube where the electrons are produced from n heated filament in a. part of the tube which is frequently known as the gun, find are caused to rush forward at o high velocity towards the fluorescent screen at the other and of the tube under the influence of I positively 'Phflrged body somewhere between the two, on initial concentration of the been of electrodes is carried out by the cylinder surrounding the uthodl. If it were not for this cylinder the electrons would diver a like water 'frorn a sprinkler, but %y the application of suitable repelling voltsges to the cylinder the electrons nro forced into n comparatively narrow pencil. Further concentration of the beam, which in a tube of this sort is most essential if the fluorescent spot on the screen is to be small and bright, may be efl‘ectod by either magnetic or else. trnstatie means. In the first place I coil is suitably placed so that n current pnssing through it forms a field which concentrates the electrons in the beam so that they are focused at a point which, if everything is nrrsnged properly, lies upon the screen at the end of the tube. In electrostatic mothods of concentration cylindrical nietallie bodies are placed concentrically round the line of the beam, and to these cylinders are applied difierent potentials.

The equipotential surface set up in the space inside the cylinders not as reflecting surfaces for the electrons, and as n rEsult of succesalve refractions the whole my is brought to a focus. Such arrangement; In these, although quite unlikc lenses in the ordinary sense of the word, are known as electron lenses, and the art which has grown up of investigating the balmviuur of electron beams, and the influence upon them of various deflecting and concentrating structures in known as electric optics. _

This new development in rapidldy becoming of great importance, en elthongh still in comparative infancy some astonishing re-ultr have been obtained. There wee recently described an arrangement known as “electron telescope." This device has a [urge cathode, upon which the image to he observed is focused optically. Streams of electrons are emitted from this cathode in accordance with the illumination thereon, and these stream- are controlled by mean: of focusing rings and so ml to throw the electron image upon a fluorescent screen at the other end of the tube, whereupon it is made visiblo

Among the useful applications of this “telescope" in the conversion of images expressed in infra red or ultra

violet rays into visible fight, no that, for example, for infra red cameras there in a means ut‘ finding out what the picture will look like before it is t'aken, the rnys which influence the negative therein being normally innsibla to the eye. The arrangement alsu opens the possibility of “seeing" through smoke and haze, of “dark" signalling, and of detecting movements of objects in the dark for alarm purposes.