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This Little Light of Mine - How Can I Make it Shine?

© Wesley Colley

Light, much like gravity, is something that most of humans take for granted as so intrinsically necessary for and integrated into our lives, that we may rarely stop to ponder what exactly light is. And, much as it took until at least the time of Galileo to say anything scientific and correct about gravity, it took until nearly this century to do the same for light.

Light, at its simplest, is a travelling vibration of the local electric and magnetic fields, as described mathematically by J.C. Maxell in the late 1800s. Light is always generated by an accelerating charge (positive or negative). For instance, radio towers move charge up and down the tower very rapidly to emit light in the radio part of the spectrum. In nature, light is generated when an electron interacts with the local electromagnetic field and becomes accelerated. Such acceleration causes emission of light with power

Power ~ accleration²

This is the Larmor formula. It applies in the limit that quantum effects are unimportant, and the radiation is continuous - during synchrotron radiation, for example, where the electron is interacting with a very strong magnetic field.

Very often, quantum effects are important, however, and in that case the electron emits the radiation due to acceleration in one big burp, called a quantum, or a photon. A typical case of quantum radiation occurs when an electron interacts with an atom, often a positively charged ion (an atom lacking one or more electrons). As an electron passes an ion, it feels an electrostatic force or Coulomb force, due to the opposite charges of the electron and the ion. As such, the heavy ion accelerates the fleet electron, and the electron burps out some of its orbital energy as a photon. The energy of the burp is determined by how successfully the atom captures the electron. If the atom very tightly binds the electron, the photon is very energetic (ultraviolet or even X-ray); if it barely snares it, the photon is low in energy (infrared to optical).

Most of the light we see is due to quantum changes in orbital energies of electrons due to bumps from other particles excited by heat or electricity. It was this realization, largely due to Planck, that first solved the pesky question of why a hot stove looks red?

Astronomers require a detailed knowledge of how different elements and molecules emit and absorb light under different conditions, because light is the only information we receive from the celestial objects beyond the solar system.

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