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(S-4) The Many Colors of Sunlight

    Index

The Sun

S-1. Sunlight & Earth

S-1A. Weather

S-1B. Global Climate

S-2.Solar Layers

S-3.The Magnetic Sun

S-3A. Interplanetary
        Magnetic Fields

S-4. Colors of Sunlight


Optional: Doppler Effect

S-4A-1 Speed of Light

S-4A-2. Frequency Shift

S-4A-3 Rotating Galaxies
            and Dark Matter
------------------------

  S-5.Waves & Photons

Optional: Quantum Physics

Q1.Quantum Physics

Q2. Atoms

Q3. Energy Levels

Q4. Radiation from
        Hot Objects

Color: What is it?

    The colors of the rainbow are the "basic spectrum" from which all the light we see is composed. Although these colors merge smoothly, they are sometimes divided into red, orange, yellow, green, blue, indigo and violet (and other names). Just as various musical sounds contain the tones of the basic scale (often combinations of tones, e.g. chords), so any colored light is made up of its "spectral components. "

    Isaac Newton showed that not only can a triangular prism separate a beam of sunlight into rainbow colors (that had already been known), but also that, when a second prism brings the different colors together again, white light is once more obtained. Therefore white light is a combination of all the rainbow colors, and the prism separates its colors because the angle by which a beam of light is bent, when it enters glass, differs from one color to the next.

        [For the same reason, a simple glass lens brings different colors to a focus at different distances. In Newton's time, if an astronomer focused a telescope to give (say) a sharp yellow image of a star, that image would be surrounded by unfocused patches of red and green. Newton thought the problem was insoluble, and proceeded to invent a new kind of telescope, based not on lenses but on concave mirrors, which reflect all colors equally. In later times optics were created which focused all colors together, using a combination of several lenses made of different kinds of glass, and these are nowadays found in cameras, projectors and small telescopes. However, all big modern telescopes follow Newton's idea and use mirrors.]
 
 

Perceived Color

    Even with the rainbow explained, the puzzle of color still baffled scientists. The riddle was solved around 1860 by James Clerk Maxwell (pictured on the left), the brilliant Scottish physicist who also gave us the basic equations of electricity, the ones that predicted electromagnetic waves (see Section S-5). Maxwell showed, while still a student, that two kinds of color existed, depending on whether it was perceived by an instrument or by the human eye:
  1. "Spectral color," i.e. the colors of the rainbow and their combinations. The amount which each part of the rainbow spectrum contributes to a beam of light can be determined by splitting the beam with a prism.

  2. "Perceived color" reported by the human eye to the brain.

    An instrument using prisms ("spectrograph") will reveal that the human eye can be fooled: different combinations of rainbow colors may look the same to the eye.

    Our eye contains three kinds of light-sensitive cells, each sensing a different band of colors--one band centered in the red, one in the green and one in the blue. Any color which we see--including brown, olive-green and others absent in the rainbow--is an impression our brain conveys as it combines signals from these 3 color bands. Color-blind persons lack some types of eye cells, and their world lacks certain colors, or even (for those having only one kind of cell) any color at all. Color blindness is much more prevalent in men; in women, on the other hand, a rare mutation exists whose eyes have four different kinds of receptor cells. The rest of us can only guess what colors those ladies must be able to see!

    [Birds, too, seem to have 4-color vision: see What Birds See by Timothy H. Goldsmith, Scientific American, July 2006, p. 69-75. Humans can see red but deer can't, so deer hunters wear red jackets for their own safety, without their prey noting.]
    That is why color TV and color printers can be based on the three "primary colors" red, green and blue, or 3 combinations of those colors. Such 3-color combination will not in any way reproduce the true spectral color of the objects they show, but for most colors, our eyes alone won't notice a difference. However, given any choice of 3 "basic" colors, we usually cannot mimic any color perceived by the eye. An e-mail discussion of this (involving physics teachers) is linked here.

    You can find on the web programs which let you experiment with 3-color combinations, using the color monitor of your computer. A somewhat simple program for doing so was provided with the original version of this web page, but be aware that it requires you to go into the "source code" of the HTML page, the text page which specifies the contents of the web page. (You will get instructions). It is linked at the end of this web page.

The Spectrum

    Any color discussed from now on will be a spectral color. Two kinds of color distributions are important in nature:

    (1) In light emitted from solids, liquids or extensive bodies of dense gas such as the Sun, the colors are distributed continuously. Their exact distribution ("black body spectrum") depends on the temperature at which it is produced--a warm hand radiates mostly in the infra-red, a glowing bar of iron is cherry-red, a lightbulb filament is bright yellow, and sunlight is white-hot.

        [Also of this type is the distribution of microwave radiation left over from the "big bang" when the universe apparently began, a radiation observed by NASA's COBE satellite, the Cosmic Background Explorer. When the observed COBE spectrum was first shown before a meeting of astronomers, it caused a great stir. Observed values generally show some experimental error, but here they were so close to the predicted theoretical curve that the first impression of the viewers was that the presenters had drawn the curve first and then placed their points on top of it.]

  Spectra of selected elements,
  © Donald E. Klipstein (see here for more)

    (2) The colors of light emitted by individual atoms or molecules in a rarefied gas are not distributed continuously, but are concentrated in narrow ranges of the spectrum. The colors are characteristic of the type of atom or molecule emitting them, just as the tone of any tuning fork is characteristic of its size, thickness and metal. These narrow ranges are known as spectral lines, because in most spectrographs light enters through a narrow slit, so that each emission appears as a line in the resulting image.

    Most light sources we use are of two kinds: The light is created either by heat--e.g. in the hot filament in a common flashlight--or by glowing gas, as in fluorescent lighting. Sources of the second kind give more light per unit energy, but that light is concentrated in a few preferred colors, so colored objects may appear somewhat un-natural.

    For street lighting true color is less important, so in general glowing vapors of soldium or mercury is used. Sodium emits orange-yellow light, and a spectrograph reveals the color comes from two closely spaced spectral lines (see middle part of 2nd strip in the image above). Mercury vapor emits a bluish light (see middle of bottom strip above), but no red.

    Because of the lack of red in such a light, pink skin seen by it seems unnaturally pale. Fluorescent lightbulbs also contain mercury (a spectroscope will show mercury "lines"), but to create softer and more pleasant light (and to put the UV light, usually wasted, to good use), they have a fluorescent coating ("phosphor") inside the glass, which absorbs the harsh mercury colors (including UV) and re-radiates them in a more even distribution of color(in above image see "Mercury W/phos"). Neon lights operate in a similar way, with small amounts of other gases producing appropriate colors.

The Wave Nature of Light

    Prisms and slits can be used to filter light, leaving only the "monochromatic" light of a single, well defined spectral color. Studies with such light have shown that light propagates like a wave, an oscillatory disturbance propagating in space Its wavelength, the distance from crest to crest, is rather tiny, typically 0.5 micrometers or microns (millionths of a meter).

        [We postpone addressing the question "crest of what? " Early physicists did not know the answer, either--they just knew that when two crests overlapped, the light was brighter, while when crest met "valley" (crest in the opposite direction), the waves cancelled each other, giving a darkening.

        We also skip the question "what exactly defines a wave?" The proper answer is, "the wave equation" expressed in terms of 3-dimensional differential calculus, a tool not available here. In qualitative terms, however, it is an oscillation which spreads without losing energy, keeping its profile-shape the same while it advances, although the height of that profile can decrease as the wave spreads over a growing volume in space.

       In these terms, a shock front is not a wave, since it heats the air and loses energy. Neither is a breaker on the seashore, which changes shape and finally collapses. "Real" waves however can be added, combining their wave shapes into a single one, shaped differently: musical chords are one example of this.]

    The wavelength determines the extent to which a wave can be confined to certain locations. Because light waves are so short, we can also visualize a light wave limited to a well-defined beam. However, outlines begin to blur when we look at small objects through a powerful microscope, magnifying several thousand times, because light waves cannot define details smaller than their wavelength. That is where electron microscopes become useful, using not light but beams of electrons.

    A variety of instruments allow physicists to actually measure the wavelength of light. The one most likely to be used by students is a diffraction grating, a plate ruled with fine parallel grooves, with a constant distance between each one and the next. Inexpensive plastic gratings are available, pressed from a metal grating and mounted on cardboard frames like photographic slides. The incoming wave resonates with the spacing between the grooves and some of it is deflected, by an angle which depends on the wavelength, and knowing the angle and the spacing allows the wavelength to be calculated. Thus gratings can separate a beam of light into its colors the way prisms do, and they are often used in spectroscopes.

        [Lit from the side with a reflecting surface behind them, gratings will shimmer in many colors, making them a popular item of costume jewelry. The same process is responsible for the shimmering of laser disks used in recording music and computer data, which also contain many narrow parallel grooves.]

Spectra

    19th century scientists, in particular Robert Bunsen (1811-99) and Gustav Kirchoff (1824-87), observed and catalogued the spectra of many substances. That provided a tool for analyzing the composition of metals and other substances, still widely used.

    The Sun, too, emits spectral lines. The ones noted first were dark lines (named Fraunhofer lines after their discoverer), suggesting increased absorption of light, not increased emission. Cool atoms absorb the same wavelengths as the ones they emit when hot--for instance, light from a filament bulb, shining through a tube with mercury vapor too cool to emit light, will develop dark lines at the same wavelengths as those emitted by hot mercury vapor. In the case of sunlight, it turned out that the absorbtion occured not in the Earth's atmosphere (as one might have guessed) but in the Sun's.

        In addition, however, sunlight also contains many bright emission lines, characteristic of hydrogen, calcium and other elements. One yellow line, discovered in 1868, was first identified as the yellow line of sodium, but it did not have the proper frequency and did not fit the spectrum of any other known substance. The British astronomer Norman Lockyer finally proposed that here was a new substance, unknown on Earth, and he was right: "helium" (from "helios", the Sun) was identified in terrestrial material by William Ramsay in 1895 and was later isolated by him.

Further Exploration

        http://umbra.nascom.nasa.gov/images/ is a useful compilation of solar information, including the most recent images of the Sun available from various observatories. The images are taken through filters which isolate narrow ranges of color emitted by selected substances (i.e. "spectral lines"), and show much detail about active solar regions.

Questions from Users:   How much do different elements contribute to sunlight?
                        Also:   What does the color of the Sun tell about its temperature?
                            ***   The Color Indigo
                                  ***       Why does sunlight have a continuous spectrum?