| To get an idea of how complex the answer to that question is, search the web for wave-particle duality. |
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We will
keep our definition of light as simple as possible and leave the
question of wave-particle duality to the quantum mechanicians. For
our purposes, light consists of
subatomic particles called photons.
Photons are emitted when there is a change in the energy level of an
electron, either a free electron or a member of the indeterminate cloud
of electrons that surrounds the nucleus of an atom. At the atomic level,
such a change in energy levels occurs in a piece of tungsten wire (the
filament) when you turn on an incandescent lamp; in the atoms of a
gas when you turn on a fluorescent lamp; when you put on a sweater
in dry weather; and even when you comb your hair.* The process also
occurs in stars, which are the ultimate source of almost all of
the visible
light in the Universe. In each instance cited above, electrons
are colliding with other electrons and imparting their energy to
the electrons that they strike. The electron that receives the additional
energy moves into an orbit farther from the nucleus of the atom.
That is not the electron’s preferred location, however, because the
atom needs to balance its energy and return to the neutral state
that it was in before the collision. That state of neutrality is
known as the ground state. In order for the atom to return to the
ground state the electron that received the additional energy must
discharge the additional energy in some manner. An electron cannot
manufacture another electron, but it can discharge excess energy
in the form of photons. If those photons fall within certain parameters,
we call them visible
light. Two characteristics of light that are very important to astronomers are that the path along which a photon is traveling can be altered by passing the photon through various media such as air, water, or the glass in a telescope; and that photons may also have their paths altered by reflecting them from a mirror (such as the mirror in a telescope). These properties of photons, that their paths can be bent and that they can be reflected, are what enable us to build optical devices of every kind, from bifocals to telescopes. In spite of what is going on at a deeper level (the quantum, or subatomic level) telescopes are relatively simple devices; one can build a telescope (grind a lens or a mirror) with the aid of simple instructions without ever having heard of quantum mechanics. |
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| I have been referring to visible light as if there were another kind—invisible light. In fact, most of the light in the Universe is invisible to our eyes. Radio waves in the AM and FM range, microwaves, cell-phone signals, x-rays, visible light, and gamma rays are all forms of light; they are all part of a continuous electromagnetic spectrum and the carrier of all of those forms of light is the photon. The only difference between visible light and other kinds of light is the amount of energy carried by the photons. Low-energy photons carry radio waves. Boost the energy a bit and you have visible light. The lowest-energy visible light photons are those that we see as red. Continue to increase the energy and you will move through all the colors of the spectrum and move back into the invisible region, starting with ultraviolet light. Keep increasing the energy and you have x-rays. Farther up the spectrum are the most energetic photons in the Universe, the gamma rays. Gamma rays are about a million times more energetic than visible light. |
