Pieter
Zeeman – Nobel Lecture
Nobel Lecture, May 2, 1903
Light
Radiation in a Magnetic Field
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".
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.
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 was 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
Pieter
Zeeman was born on May 25, 1865, at Zonnemaire, a small village in the isle
of Schouwen, Zeeland, The Netherlands, as the son of the local clergyman Catharinus
Forandinus Zeeman and his wife, née Wilhelmina Worst. After having
finished his secondary school education at Zierikzee, the main town of the island,
he went to Delft for two years to receive tuition in the classical languages,
an adequate knowledge of which was required at that time for entrance to the university.
Taking up his abode at the house of Dr. J.W. Lely, conrector of the Gymnasium
and brother of Dr. C. Lely (Minister of Public Works and known for initiating
and developing the work for reclamation of the Zuyderzee), Zeeman came into an
environment which was beneficial for the development of his scientific talents.
It was here also that he came into contact with Kamerlingh Onnes (Nobel Prize
in Physics for 1913), who was twelve years his senior. Zeeman's wide reading,
which included a proper mastery of works such as Maxwell's Heat, and his
passion for performing experiments amazed Kamerlingh Onnes in no small degree,
and formed the basis for a fruitful friendship between the two scientists.