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Note: This section is conceptually more difficult and may be skipped or skimmed.

(S-5) Waves and Photons


The Sun

S-2.Solar Layers

S-3.The Magnetic Sun

S-3A. Interplanetary
        Magnetic Fields

S-4. Colors of Sunlight

S-4A.Color Expts.

  S-5.Waves & Photons

Optional: Quantum Physics

Q1.Quantum Physics

Q2. Atoms   (and 6 more) ------------------------

Or continue here:

S-6.The X-ray Sun

S-7.The Sun's Energy

S-7A. The Black Hole at
        our Galactic Center

LS-7A. Discovery
      of Atoms and Nuclei

S-8.Nuclear Power
    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

Back now to an old question: what sort of wave is light? Remember the idea of Faraday which evolved into the "magnetic field" concept--that space in which magnetic forces may be observed is somehow changed. Faraday also showed that a magnetic field which varied in time--like the one produced by an alternating current (AC)--could drive electric currents, if (say) copper wires were placed in it in the appropriate way. That was "magnetic induction," the phenomenon on which electric transformers are based.

    So, magnetic fields could produce electric currents, and we already know that electric currents produce magnetic fields. Would it perhaps be possible for space to support a wave motion alternating between the two? Sort of:

magnetic field ---> electric current ---> magnetic field ---> electric current ---> ...

    There was one stumbling block. Such a wave could not exist in empty space, because empty space contained no copper wires and could not carry the currents needed to complete the above cycle. James Clerk Maxwell--the bright Scotsman who also proposed the three-color theory of perceived light--solved the riddle by proposing that the equations of electricity needed one more term, representing an electric current which could travel through empty space, but only for very fast oscillations.

    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 γ-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.

Further reading:

    --A personal, readable biography of Maxwell: "The Man who Changed Everything: The Life of James Clerk Maxwell" by Basil Mahon, Wiley 2003, 254 pp., $27.95. Reviewed in Nature, 425, p. 765-6, 23 October 2003.


    Waves and particles seem to be diametrically opposed concepts: a wave fills a region in space, while an electron or ion has a well-defined location. That, at least, was the view before the discoveries of the first half of the 20th century. Those discoveries suggested that on the atomic scale, the distinction became blurred: waves had some properties of particles, and vice versa.

    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 λ (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 ν (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:

ν = c/ λ

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

E = hν

where h = 6.624 10-34joule-sec is "Planck's constant", a universal constant that is fundamental to all quantum theory. It was introduced in 1900 by Max Planck, when he tried to explain the "black body" distribution of wavelengths in the light emitted by a solid hot object. Incidentally, it was the above formula, published by Albert Einstein in 1905, that later earned him the Nobel prize, not (as many still believe) his theory of relativity.

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

    Quantum physics is a huge subject, too big and too mathematical to cover here. It is only brought up because of its claim that the amount of energy which an atom can receive from an electromagnetic wave--its photon--depends only on that wave's length.

    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 = h ν = hc/λ means that the shorter the wavelength λ, the more energetic the photon. A photon of UV contains more energy than one of visible light, and photons of X-rays and γ-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 γ-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.

Further Exploration:     An Experiment on the Photoelectric Effect

    Einstein's relation suggests red photons have less energy than green ones, which have less than blue and UV ones. Frequently a photon of higher energy can initiate a chemical process which one of lower energy fails to do. Orthochromatic film (for black-and-white, the original one used) is not sensitive to deep red, so darkrooms handling film or paper with orthochromatic emulsions may have a deep-red "safelight" which helps the photographers' work but does not register on the film.

    An experiment demonstrating this color sensitivity was described by Michael Horton on PHYSHARE, a web listserver for physics teacher. In his words:

    I saw an excellent activity to show ... We took the green phosphorescent paper that glows in the dark and turned off all of the lights. We used a little pen flashlight to draw shapes on it and it glowed. Then we used a blue light (they're pretty easy to find at auto parts stores for keychains) and it glowed. Then we took a red laser and tried it and . . . nothing. No matter how long you held it there or how bright the laser was, it wouldn't glow because the light wasn't energetic enough. It was a fun activity that taught a good lesson. I bought some of the glow in the dark paper for this year along with some white, blue, yellow, and red lights. We'll see how it works with the students.
M. Horton
Chem/Phys/Comp. Repair teacher/Dept. Chair
Perris High School; Perris, CA
Questions from Users:   The response of solar cells to different colors
                                      (An unexpected result of a science project.)
                         Also:  What exactly is "radiation"?.
                      ***     Superposition of waves
                ***     How can radio waves carry sound?
             ***     Shielding out radio waves

Next Stop: (S-6) Seeing the Sun in a New Light

            Timeline                     Glossary                     Back to the Master List

Author and Curator:   Dr. David P. Stern
     Mail to Dr.Stern:   stargaze("at" symbol) .

Last updated: 12-12-2004
Re-formatted 26 March 2006