Department of Energy Argonne National Laboratory Office of Science NEWTON's Homepage NEWTON's Homepage
NEWTON, Ask A Scientist!
NEWTON Home Page NEWTON Teachers Visit Our Archives Ask A Question How To Ask A Question Question of the Week Our Expert Scientists Volunteer at NEWTON! Frequently Asked Questions Referencing NEWTON About NEWTON About Ask A Scientist Education At Argonne Light and Fields
Name: Matthew W.
Status: student
Age: 15
Location: N/A
Country: N/A
Date: Thursday, June 06, 2002


Question:
If light does not have any electrons, than why does it have magnetic properties, when magnetism is cause by the motion of electrons in atoms?


Replies:
Matthew,

Actually, light has both electric and magnetic properties. There are two things that can make an electric field: electric charge and a changing magnetic field. There are two things that can make a magnetic field: a moving electric charge and a changing electric field. Light is first produced by a moving electric charge. The charge gives off some of its energy as changing electric and magnetic fields. These changing fields then transfer their energy to new changing electric and magnetic fields a little further from the original charge. This continues, fields progressing at the speed of light, until the fields crash into a charged particle. This is how light can carry energy from one location to another. An example is light from the Sun bringing energy to the Earth.

Dr. Ken Mellendorf
Physics Instructor
Illinois Central College


Having electrons is not a prerequisite for magnetic behavior. It is true that SOME motions of electrons in atoms produce a magnetic field. There are also some other motions of electrons in atoms that do not produce a magnetic field. Light (and all electromagnetic radiation from x-rays to microwaves) is composed of an alternating electric and magnetic field. James Maxwell described classical electromagnetic behavior in terms of oscillating electric and magnetic fields. Its predictions have been validated by millions of experimental observations, so there is very little doubt about its correctness on the macroscopic scale. The interaction of light and electrons on the atomic scale is described by a theory with the daunting name "quantum electrodynamics -- or QED for short, which takes into account the theory of relativity and the laws of quantum mechanics. The predictions of that theory have been shown to agree with experiment to within a few parts in 10^12. This is in about the same ratio as the width of a human hair to the distance between New York and Los Angeles, so that theory describing the interaction of electromagnetic fields with electrons is consistent with experimental measurements to an extraordinary accuracy. If the photon is zero rest mass, travels at a constant speed in a vacuum (about 3x10^8 meters/sec), why does it have spin = 1, which in a "classical" analogy would suggest that it is spinning?

No one knows, but that is the prediction of the theory and the result of experimental measurements. The only thing left out of all this theory is how does gravity behave on a quantum mechanical level. To my knowledge no one has incorporated that interaction into the picture satisfactorily. You may find a short book entitled: QED by Richard Feynman interesting reading. It is an excellent presentation of the state of knowledge about all this stuff without too much math, and even if you do not understand all of the math it is still fascinating reading because he was such a remarkable teacher.

Vince Calder



Click here to return to the Physics Archives

NEWTON is an electronic community for Science, Math, and Computer Science K-12 Educators, sponsored and operated by Argonne National Laboratory's Educational Programs, Andrew Skipor, Ph.D., Head of Educational Programs.

For assistance with NEWTON contact a System Operator (help@newton.dep.anl.gov), or at Argonne's Educational Programs

NEWTON AND ASK A SCIENTIST
Educational Programs
Building 360
9700 S. Cass Ave.
Argonne, Illinois
60439-4845, USA
Update: June 2012
Weclome To Newton

Argonne National Laboratory