(S-5) Waves and Photons

Astronomers studying the Sun enjoy a big advantage: the object which they observe is very, very bright. It is therefore possible to extract from its light just one narrowly defined color--a single spectral line--and still have enough brightness left to give a detailed picture.

Ever since George Ellery Hale in 1892 found a way to observe the Sun in this manner, astronomers have used it to look at the Sun in the light of hydrogen, calcium or helium. The detailed pictures of the Sun presented (for instance) on the world-wide web, the ones that show clouds, streaks, tufts and other structures, are all created in this manner. Other monochromatic images are obtained in extensions of the visible spectrum, e.g. UV, EUV (extreme UV) and X-rays.

One feature shown in such pictures are prominences, large clouds of denser and cooler gas, rising high above the photosphere. Some of them stand out against the dark background sky on the visible edge ("limb") of the Sun, and if one watches them for a while, one can see material falling back towards the Sun. Others are seen in the middle of the solar disk, where they appear as dark filaments, because being cool, they absorb the spectral line in which the observation is made. Prominences turned out to be important for understanding coronal mass ejections, discussed here in a later section.

Electromagnetic Waves

With that term added (the "displacement current"), the equations of electricity and magnetism allowed a wave to exist, propagating at the speed of light. The drawing below illustrates such a wave--green is the magnetic part, blue the electric part--the term Maxwell added. The wave is drawn propagating just along one line: actually it fills space, but it would be hard to draw that.

Electromagnetic Wave (see text above)

Maxwell proposed that it indeed was light. There had been earlier hints--the velocity of light had appeared unexpectedly in the equations of electricity and magnetism--and further studies confirmed it. For instance, if a beam of light hits the side of a glass prism, only part of it enters--another part gets reflected. Maxwell's theory correctly predicted properties of the reflected beam. The next obvious question was: if this was an electromagnetic wave with wavelength around 0.5 microns, what about other wavelengths?

Heinrich Hertz in Germany calculated that an electric current swinging very rapidly back and forth in a conducting wire would radiate electromagnetic waves into the surrounding space (today we would call such a wire an "antenna"). With such a wire he created (in 1886) and detected such oscillations in his lab, using an electric spark, in which the current oscillates rapidly (that is how lightning creates its characteristic crackling noise on the radio!). Today we call such waves "radio waves". At first however they were "Hertzian waves, " and even today we honor the memory of their discoverer by measuring frequencies in Hertz (Hz), oscillations per second--and at radio frequencies, in megahertz (MHz).

Light and radio waves belong to the electromagnetic spectrum, the range containing all different electromagnetic waves. Over the years scientists and engineers have created EM waves of other frequencies--microwaves and various IR bands whose waves are longer than those of visible light (between radio and the visible), and UV, EUV, X-rays and g-rays (gamma rays) with shorter wavelengths. The electromagnetic nature of x-rays became evident when it was found that crystals bent their path in the same way as gratings bent visible light: the orderly rows of atoms in the crystal acted like the grooves of a grating.

[It should be stressed that such vaves are very different from sound waves, which have no connection to electricity but are pressure waves, associated with the elasticity of solids, liquids and gases. The wavelength ranges of radio waves and sound overlap. However, radio is stopped by a metal screen because it conducts electricity, while penetrating non-conducting solid walls (you can listen to your radio indoors!). Sound on the other hand is stopped by walls but penetrates the mesh of the screen.]

Photons

To find how a light wave passes through a telescope, one calculates its motion as if it filled the entire focusing mirror. Yet when that same wave gives up its energy to one individual atom, it turns out that it acts like a particle. Regardless of whether a light beam is bright or dim, its energy is always transmitted in atom-sized amounts, "photons" whose energy depends only on wavelength.

Observations have shown that such duality also existed in the opposite direction. An electron should in principle have at any time a well-defined location and velocity, yet experiments that measure them give a blurred result. Quantum physics tells us that arbitrary precision in such observations cannot be attained, but that the motion may be described by a wave.

This may be a good place for introducing new quantities and notations. An electromagnetic wave of wavelength l (lambda, small Greek L) covers a distance of c meters each second, where c is the velocity of light in space, close to 300,000,000 meters/second. Its frequency n (nu, small Greek N)--the number of up-and-down oscillations per second--is also the number of wave crests in that distance, and is therefore obtained by dividing c with the wavelength:

A basic quantum law then states that the energy E in joules of a photon of light of frequency n is

Exploring further: A web page on electromagnetic waves, part of an extensive and detailed site on "The Amazing World of Electrons and Photons". Click here for a map of that site.

Wavelength and Energy

The process also works the other way around: when "excited" atoms give up their excess energy to an electromagnetic wave (energy they might have received, say, through a collision with some fast atom in a glowing gas) they can only do so in photon-sized amounts. The fact that atomic emissions appear in narrowly defined "spectral lines" suggests that "excited" atoms cannot contain extra energy in arbitrary amounts, but must be in one of their "energy levels" which resonate with their structure, each associated with a precisely defined amount of energy.

Each atom also has a "ground state, " its lowest energy level and the one in which it prefers to stay. When it descends from some excited state to the ground state, the starting and final energies of the atom are precisely specified energy levels. The energy emitted, equal to the difference between the two, is thus narrowly defined, producing a photon with a precise wavelength. The great success of quantum mechanics has been its ability to calculate and predict the energy levels of various atoms and combinations of atoms.

The formula E = hn = hc/l means that the shorter the wavelength l, the more energetic the photon. A photon of UV contains more energy than one of visible light, and photons of X-rays and g-rays (gamma rays) are more energetic still. One therefore expects that hotter regions of the Sun, where individual particles have more energy, will emit electromagnetic radiation of shorter wavelength, and that is indeed observed.

The temperature of a gas is proportional to the average energy of each of its particles (the formula, by the way, is E = 3/2 kT, where T is the absolute temperature in degrees Kelvin--like Celsius, just different zero point--and k is a fixed number, "Boltzmann's constant."). Thus while the photosphere emits mainly visible light, the hot corona is better observed in EUV (extreme UV) or in long-wavelength X-rays. Flares give even higher energies to ions and electrons, and to trace locations where those particles are produced and absorbed, shorter X-rays and g-rays are needed. All these ranges have been observed by instruments aboard spacecraft. They cannot be studied from the ground, because all short-wavelength photons are easily absorbed by the atmosphere and do not reach ground level.