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Light Radiation in a Magnetic Field

Light Radiation in a Magnetic Field

Nobel Lecture

Pieter Zeeman

Nobel Lecture, May 2, 1903


As Professor Lorentz told you last  December, immediately after hearing of the great and very  honourable distinction awarded to us, we set to work to see how  best to co-ordinate our two lectures. To my great regret I was  unable to be present here for Professor Lorentz's lecture, but he  was able to report to you on present electron theory from his  viewpoint, only briefly touching on the experimental  investigations which have occupied me in recent years. I hope  that you will allow me therefore to emphasize these experimental  investigations. Two fields of physics, light and magnetism, are  combined in the subject of today's lecture, whose history dates  only from the days of Michael Faraday. The wonderful discovery of  the connection between light and magnetism, which he made in  1845, was the reward for an investigation carried out with  indefatigable patience and tenacity. Today we call this  connection the magnetic rotation of the polarization plane.  Faraday succeeded in showing that the plane in which light  oscillations take place, is rotated as soon as light passes  through special magnetizable bodies along the lines of force.  Faraday himself called his discovery the magnetization of light  and the illumination of magnetic lines of force. His contempories  did not understand this name, which perhaps corresponded more to  what he was searching for than to what he found. Throughout his  life his hopes, desires and yearnings led him to make repeated  investigations into the connection between light, magnetism, and  electricity.

The last experiment recorded in Faraday's  laboratory notebook and ostensibly the last in his life, gives an  indication of the extent to which his spirit was still occupied  with the boundary region of possible phenomena.

It was on March 12, 1862, in the laboratory  of the Royal Institution that Faraday carried out this  experiment. The notes in his notebook, although not quite clear,  leave no doubt that he was attempting to demonstrate by means of  a spectroscope that magnetism has a direct effect on a light  source. The result was however absolutely negative, and Faraday  writes in his notebook "not the slightest effect demonstrable  either with polarized or unpolarized light".

ZeemanEffect Perhaps it was because of this observation  that Maxwell, at a meeting of the British Association in  Liverpool on September 15, 1870, said of the light-radiating  particles in a flame "that no force in nature can alter even very  slightly either their mass or their period of oscillation", a  statement which, coming from the mouth of the founder of the  electromagnetic light theory and spoken with such intensity, must  really surprise present-day physicists.

Image : A photo Zeeman took of the Zeeman effect.

It was not simply out of a spirit of  contradiction that I exposed a light source to magnetic forces.  The idea came to me during an investigation of the effect  discovered by Kerr on light reflected by magnetic mirrors.

When it is a question of splitting up the  light of a luminous gas into very fine detail, the simple glass  prism of Newton and Frauenhofer is of no use, and the physicist  has recourse to the excellent aid which we owe to Rowland: the  concave grating. Most physics institutes possess this polished  metal mirror with a very large number of grooves, say 50,000 over  a width of 10 cm scratched on by means of a diamond. A beam of  compound light is no longer reflected by the lined surface in the  ordinary way; instead each special kind of light follows its own  path.

Of course the light source must be very  restricted for the large number of beams corresponding to the  various kinds of light to appear separately. This is ensured by  placing the light source behind an opaque screen with a linear  slit. The spectral image produced can be observed, and from the  location and intensity of the linear-slit images we can determine  how the different kinds of light in the light being studied are  distributed on the basis of the period of oscillation and  intensity. A further main advantage of Rowland's grating is that  it is now no longer scratched on plane surfaces, but on spherical  concave surfaces with a radius of say 3 metres, so that real  images are produced of luminous lines without the need for the  insertion of lenses. Moreover, photography has made it possible  to fix these images and now provides us with a permanent record  of each observed spectrum, which can be measured out at any  time.

When we study the well-known Bunsen sodium  flame by means of Rowland's grating, we see a spectrum consisting  mainly of two separate sharp yellow lines, which in our grating  lie about I mm from each other. We see that sodium radiation  consists of two kinds of light, the periods of oscillation of  which differ only very slightly (1 in 1000) from each other. We  confined our attention exclusively to one of these two lines.

I must ask you now to go with me into the  Physics Institute of Leiden University. In August, 1896, I  exposed the sodium flame to large magnetic forces by placing it  between the poles of a strong electromagnet. Again I studied the  radiation of the flame by means of Rowland's mirror, the  observations being made in the direction perpendicular to the  lines of force. Each line, which in the absence of the effect of  the magnetic forces was very sharply defined, was now broadened.  This indicated that not only the original oscillations, but also  others with greater and again others with smaller periods of  oscillation were being radiated by the flame. The change was  however very small. In an easily produced magnetic field it  corresponded to a thirtieth of the distance between the two  sodium lines, say two tenths of an Angstrom, a unit of measure  whose name will always recall to physicists the meritorious work  done by the father of my esteemed colleague.

Had we really succeeded therefore in  altering the period of vibration, which Maxwell, as I have just  noted, held to be impossible? Or were there some disturbing  circumstances from one or more factors which distorted the  result? Several of such might be mentioned.

We doubted the result. We studied the light  source in the direction of the magnetic force, we perforated the  poles of the magnet; but even in the direction of the magnetic  lines of force we found that our result was confirmed. We also  studied the reverse phenomenon, the absorption of light in sodium  vapour, and this too satisfied our expectations. We then asked,  do different substances behave in different ways? What happens  when the magnetic force is raised to the maximum attainable  values? How do different lines of the same substance behave? But  before these questions could be answered, theory took over.

I was in fact able to verify experimentally  some conclusions which followed from the theory of optical and  electrical phenomena of my esteemed teacher and friend Professor  Lorentz. This theory assumes that all bodies contain small  electrically charged mass particles, "electrons", and that all  electrical and optical processes are based on the position and  motion of these "electrons". Light oscillations result from the  vibration of the "electrons". On the basis of Lorentz's theory, if  we limit ourselves to a single spectral line, it suffices to  assume that each atom (or molecule) contains a single moving  electron.

Now if this electron is displaced from its  equilibrium position, a force that is directly proportional to  the displacement restores it like a pendulum to its position of  rest. In this model the electrons are represented by the red  balls and the direction of the magnetic force by the arrows. Now  all oscillatory movements of such an electron can be conceived of  as being split up into force, and two circular oscillations  perpendicular to this direction rotating in opposite directions.  In the absence of a magnetic field the period of all these  oscillations is the same. But as soon as the electron is exposed  to the effect of a magnetic field, its motion changes. According  to well-known electrodynamic laws, an electron moving in a  magnetic field is acted upon by a force which runs perpendicular  to the direction of motion of the electron and to the direction  of the magnetic field, and whose magnitude is easily determined.  Here the rectilinear oscillation is not changed by the magnetic  field, the period remains the same; on the other hand the two  circular oscillations are subjected to new forces which, running  parallel with the radius, either increase or decrease the  original central force. In the first case the period of  oscillation is reduced, in the second it is increased.

Now it is easy to determine the light  motion to which this type of motion of the electrons will  lead.

Let us consider first what happens in a  direction running perpendicular to the lines of force. To  the three electron motions there correspond three electrical  oscillations, or in terms of the electromagnetic light theory  three light oscillations of different periods. Thus the light  source will emit three-colour light instead of the  original one-colour light. Therefore, instead of the  single non-polarized spectral line we shall see three separate  lines when we place the light source in a magnetic field. The  different directions of oscillation of the electrons affect the  polarization state of the emitted light. The light of the middle  component oscillates in parallel with, and that of the outer  components perpendicular to the lines of force.

I will presently show you as an  illustration a line which actually displays this behaviour  postulated by Prof. Lorentz's theory.

But let us first consider the rays which  run parallel with the lines of force. For this purpose I  will rotate the model so that the arrow points in your direction.  The opposite circular oscillations of the electrons excite two  circularly polarized rays rotating in opposite directions, one  having a longer and the other a shorter period of oscillation  than the original spectral line. The original spectral line  splits up under the action of the magnetic field into two  components which are circularly polarized in opposite directions.  The light source emits two-colour light.

I would now like to project for you two  enlargements of photographs taken with the aid of Rowland's  grating.* The lines are cadmium lines. In  the first half of the picture you can see the unchanged line, and  in the second rectilinear oscillations occurring in the direction  of the magnetic lines of half the line changed by magnetic  forces, the so-called triplet, which we see in the direction  perpendicular to the lines of force.

Secondly I will project for you a cadmium  line observed in the direction of the lines of force. The first  half of the picture shows the unchanged line, and the second half  the double line or doublet produced by the magnetic forces.

You see how beautifully the consequences  following from Prof. Lorentz's theory were confirmed by  observation in these cases. I should point out, however, that at  first some difficulty was experienced in observing the phenomena  predicted by the theory, owing to the extreme smallness of the  variations in the period of oscillation.

I have just said that the change was  extremely small; but it could be said that it was unexpectedly  large. The magnetic cleavage of the spectral lines is dependent  on the size of the charge of the electron, or, more accurately,  on the ratio between the mass and the charge of the electron. Let  us see what the observations teach us. When Prof. Lorentz  published his theory in 1895, no data were available from which  to estimate the ratio between the mass and the charge of the  light-exciting particles, and in this theory the ratio was left  undefined. We can now calculate this ratio from the magnitude of  the magnetic splitting of the spectral lines: it is  107 c.g.s. units per gram, a colossal number even for  the physicist, since it is 1,000 times as great as the similar  number which was known from electrolysis phenomena in the case of  hydrogen atoms. This makes it most probable for the physicist  that in the luminous particles only ca. 1/1000th of the atom  oscillates, and that the main mass of the atom remains virtually  stationary. The oscillating electrons and the electrolysis ions  were found to be not identical with each other; if they had been,  the splitting of the spectral lines would have been only one  thousandth of that observed, and then I should not have had the  honour of addressing you in Stockholm today.

A further question must also be answered  here and now, namely, are the oscillating particles positively or  negatively charged?

We observed the doublet in the direction of  the magnetic lines of force and studied the sign of the  polarization. Then I suddenly resolved the problem: the  oscillating electrons are negatively charged. We now know  that cathode rays, which can occur in tubes filled with highly  rarefied gases, are negative particles with the same high  charge/mass ratio. We can conclude that that which vibrates in  the light source is the same as that which travels in cathode  rays.

We can hardly avoid recalling the two  titles of Faraday's basic work: "Magnetization of light",  "Illumination of lines of force" They appear to us to be almost  prophesies, because we have now seen that light can in fact be  magnetized, and according to Prof. Arrhenius's theory we have in  nature itself, in the northern lights, an example of illumination  of the magnetic lines of force of the Earth by the electrons  escaping from the sun.

Nature gives us all, including Prof.  Lorentz, surprises. It was very quickly found that there are many  exceptions to the rule of splitting of the lines only into  triplets. The French physicist Cornu was perhaps the first to  observe that, contrary to the elementary theory, in some cases  splitting into four lines, a quadruplet, occurs. In other cases  splitting into five, six or even nine lines can be observed. In  the line-rich spectrum of iron we find a whole selection of  different forms. Very soon a number of physicists were working in  the extended field; I need only name Becquerel, Cotton,  Michelson, Preston, Righi, Runge, and Paschen. If I had more time  at my disposal, I would gladly deal in greater detail with the  work of the last-mentioned investigators. For the present,  however, I must confine myself to projecting a cadmium line for  you, which is split up into four lines, and negatives of a  mercury line which has split up into nine components, and for  which I am grateful to Prof. Runge. But despite this very  complicated splitting-up, even when larger aids are used, the  division into three groups of oscillations, two perpendicular to  and one parallel with the lines of force, assumed in Lorentz's  elementary theory, remains valid, as this photograph of the nonet  shows.

It was natural that, soon after I had  succeeded in splitting up lines, I should also study how the  different lines behave in this respect. I was soon able to show  by investigating the zinc lines that there are great differences  in the splitting-up of different lines of a substance.  Particularly great differences were found in lines belonging to  different series, the discovery of which we owe to the lucid  investigations of your countryman Prof. Rydberg, and in  particular Professors Kayser and Runge.

I found very great differences in the lines  of the different series, and it appeared that the splitting-up,  contrary to the postulations of the elementary theory, expressed  in the scale of oscillation frequencies, is not the same  for all lines in the same magnetic field. We can conclude from  this on the basis of Lorentz's theory that the charge/mass ratio  is not the same for all electrons.

I would now like to talk about three  separate phenomena, first a phenomenon which I have not been able  to observe, secondly phenomena which I have hardly been able to  verify, and thirdly a surprising phenomenon.

All the results which have been discussed  so far have related to line spectra; but in the case of many  bodies we also know of the existence of band spectra. Here a  difference is found : the band spectrum displayed by iodine  vapour or bromine vapour as an absorption medium at low  temperatures, remains unchanged in a magnetic field; I personally  have been unable to bring about any change in the extremely  accurate images which Prof. Hasselberg has given us of the  absorption spectra of bromine and iodine vapour, even with the  strongest magnetic fields.

We are indebted to Prof. Voigt in  Göttingen for a comprehensive theory of magneto-optical  phenomena. The triplet you have seen today was absolutely  symmetrical, as postulated in the elementary theory. Now on the  basis of his theory Prof. Voigt was able to predict that as a  result of the action of weak magnetic forces asymmetry  should occur. According to him, the two external components  should have different light intensities and be at different  distances from the centre line. In the case of iron, zinc, and  cadmium lines I was able to observe both asymmetries; because of  their extreme smallness, however, I cannot demonstrate the  phenomena in the projector.

However, another phenomenon, which will  give you some idea of the scope of Voigt's theory, is more  striking. In this phenomenon Faraday's magnetic rotation of the  polarization plane and the magnetic splitting of the spectral  lines, are intimately connected with each other.

The rotation of the polarization plane is  extraordinarily small in all gases, thus also in sodium vapour.  As Macaluso and Corbino found, it is only in the case of those  colours which lie close to an absorption band of the vapour that  the rotation is very great, of the order of 180°, and the  rotation takes place in the positive direction, the  direction of the current exciting the magnet.

What about the rotation inside the  absorption band?

Prof. Voigt was able to predict that in the  case of highly rarefied vapours the rotation must be negative in  the zone between the two components of the doublet, i.e. opposite  in direction to that outside the band, and also very great. I had  the pleasure of confirming this theoretical finding in  experiments on sodium vapour. Provided that the vapour is highly  rarefied, the rotation in very strong fields between the lines of  the doublet can rise to -400°.

To give you some idea of the distribution  of the rotations, I will show you two negatives connected with  this investigation.

The magnetic field is not set  up.

The two dark vertical lines are the  absorption lines of sodium vapour, the well-known D-lines. The  reason why they are so broad is that the vapour was very dense.  The horizontal bands are interference bands, which were produced  by means of a special device. They indicate the points where the  direction of oscillation is the same. The directions of  oscillation in each of the successive bands differ by  180°.

Now as soon as the magnetic field is set up  we get the image now being projected. On each side of each of the  D-lines the bands bend upwards, the more so the smaller  the distance, because the rotation in the vicinity of the bands  grows very rapidly and reaches almost 180° in the immediate  neighbourhood of the bands. Within the bands a blurred band only  is visible.

The phenomenon becomes far clearer once the  vapour is highly rarefied. The bands bounding the components   rise as before. At the same time, however, the inner band  becomes detached; it has fallen, the rotation is negative.  In our third image the rotation in the case of one of the D-lines  is about -90°, in the other everything is more blurred, the  rotation is about -180°.

Summarizing briefly the results of the  tests described in the light of Lorentz's theory, it can be  stated that firm support has been found for the assertion that  electricity occurs at thousands of points where we at most  conjectured that it was present. Innumerable electrical particles  oscillate in every flame and light source. We can in fact assume  that every heat source is filled with electrons which will  continue to oscillate ceaselessly and indefinitely.

All these electrons leave their impression  on the emitted rays. We can hope that experimental study of the  radiation phenomena, which are exposed to various influences, but  in particular to the effect of magnetism, will provide us with  useful data concerning a new field, that of atomistic astronomy,  as Lodge called it, populated with atoms and electrons instead of  planets and worlds.

I count myself fortunate to be able to  contribute to this work; and the great interest which the Royal  Swedish Academy of Sciences has shown in my work and the  recognition that it has paid to my past successes, convince me  that I am not on the wrong track.



* A  number of lantern slides were projected in the course of the  lecture.

From Nobel Lectures, Physics 1901-1921, Elsevier Publishing Company, Amsterdam, 1967

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