Nature Jan 1 1941

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Stationary Electric and Magnetic Fields in Beams of Light

According to the electromagnetic theory of light (Maxwell, Hertz) the electric light vector and the magnetic light vector oscillated perpendicularly to the direction of propagation. The energy of the wave is given by Poynting's vector.

It is shown below, on the basis of experimental findings, that every wave of light possesses likewise a stationary field intensity E in its direction of its direction of propagation and also the stationary magnetic field of intensity H. That means there is a potential difference between two points on the ray of light. Accordingly, it should be possible to collect electricity from the ray under suitable conditions. A beam of light therefore constitutes a source of electricity; furthermore, light has magnetizing effects.

Experimental proof of this generalization was obtained from my investigations of the interaction between light and small particles of matter¹. This permits of the measurement of forces of the order of 10-9 to 10 -10 dynes. The sensitivity of measurements of forces is thus increased b my methods by a factor of 1,000-10,000.

When partials of matter are irradiated by sufficiently intense light of sufficiently small wave-length, regardless of the direction of the wave front normal, then positive or negative electric charges, or north or south magnetic poles, are inducted on these particles. Particles of otherwise identical properties move in a homogeneous electric or in a homogeneous magnetic field in or against the direction of the electric field (electro-photophoresis), or in or against the direction of the magnetic field (magneto-photophoresis). These induced temporary electric of magnetic ions exist as long as the particles are irradiated by sufficiently intense light. Furthermore, it can be observed that some particles stay at rest and that their motion commences suddenly, or that moving particles appear to change their velocity and even reverse it. These are due to changes of charge. The movement of magnetic ions in a homogeneous magnetic field is a magnetic current.

These phenomena are best observed when two fully symmetrical beams of light are directed against each and when the fields act perpendicularly, are reversible and free from residual magnetism and electricity, and are also homogeneous. The intensity of this motion depends upon the frequency of the light wave. It increases with the increasing frequency and is also dependent on the material. I have also found that, when using just one concentrated beam of light, without any field , small particles of matter of magnitude 10-4 to 10-5 cm. of the same kind as before moved in clean gases either away from the source of light (light positive, longitudinal photophoresis) or towards the source of light (light-negative, longitudinal photophoresis). This force increases with the intensity of the light and likewise depends upon frequency and material.

There are particles which do not show longitudinal photophoresis at first, but only after a certain time, and some which gradually lose it. I have shown in another paper also that radiometer forces cannot account for the effects2. Longitudinal photophoresis has also been found in liquids with particles of the same material. These particles moved in opposite directions (Satiendranat Ray, Lucknow; w. Barkas, London).

Since light makes particles of matter unipolar with respect to homogeneous electric fields, and since, when no such fields act, it makes them move in or against the direction of it wave front normal, there must be an electric field E coincident with the direction of the wave front normal. This means that electromagnetic waves possess longitudinal stationary components of E, and therefore potential differences between different points along the beam. The magnitude of those fields can be calculated from actual measurements.

These facts have been confirmed by further experiments by myself and some of my pupils. An electric field suitably arranged parallel to the wave front normal permits the acceleration or retardation or even reversal of positive or negative photophoresis. The superposed field alters the component of the electromotive force in the direction of the beam.

From similar experiments it can be concluded that stationary magnetic fields exist in the beam of light, since superposed magnetic fields accelerate or retard the magneto-photophoresis. Those stationary magnetic fields in the beam of light have a magnetizing effect on the material as above mentioned.

In conclusion, I find that the light beams have electric stationary components in the direction of the wave front normal, and that consequently there must be stationary electric potential differences between different points along the beam. There must also be a stationary magnetic field in the beam of light with potential differences.

Felix Ehrenhaft
New York City
Nov. 28.
1Ann. Phys., 13, 151 (1940).
2J. Franklin Inst. , 230, 381 (1940)

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