Margaret M. Loos
Our natural source of light is, of course, the sun. Sunlight may be examined from many approaches, but one of the most delightful experiences with light is with that of the rainbow created as the rays of white light pass through the droplets of a summer sun shower. Everyone can recall some of the colors, but perhaps not all of them, or in their correct order of appearance. To unify this experience prisms may be used to look into the light (never directly into the sun). The colors observed will finally be described as red, orange, yellow, green, blue, indigo, and violet, or ROY B. GIV. A scientist will refer to this rainbow as the spectrum of white light, so white light is actually the addition of many colors (those named). The triangular glass prism bends, or refracts the many colors, or their wave lengths, at different angles and spreads them out. Refraction is defined as the bending of light between two media, in this case, the air and the glass of the prism. Actually, the lightwaves are bent both when they enter the glass and when they exit it. Other media will also bend the wave lengths according to the media’s index of refraction. A common experience with this is the enlargement we see when an object is immersed in water in a clear glass container.
ACTIVITY: The use of prisms to determine what one is looking at as compared to what one sees. Students can estimate the angle between the two planes. A table of indices of refraction will be examined to try predicting the outcome if those media were used.
Prisms are employed in the study of light within our atmosphere, on the light from celestial bodies (our sun and other stars), and in light produced by heating elements to incandescence for analysis. One instrument used for these purposes is the spectroscope. It is comprised of a small telescope and a prism. Light is allowed to enter a tiny slit at one end, pass through a lens which arranges it into parallel rays which then pass through the prism and emerge as a band of color (spectrum). Sometimes the spectrum is projected onto a photographic plate making the spectroscope into a spectrograph.
(figure available in print form)
The longest waves (red) are about 1/30,000 of an inch long.
The shortest waves (violet) are about 1/70,000 of an inch long.
PRINCIPLE OF THE SPECTROSCOPE
Two prisms may be arranged to first break down white light into the various colors and then to unify the rays back into white light. Newton first did this in the early l8th century (1704). We will attempt it in a dark room with a different light source.
ACTIVITY
: Diagram in notebook.
Other forms of light may also be examined with our prisms. We may inspect the differences in the spectrum of fluorescent lighting, or in flames produced by the burning of copper sulphate, boric acid, sodium chloride, or magnesium, for instance. Certain elements give typical color flames when burned, for instance copper, sodium, lithium, potassium and strontium. By seeing this, students may be able to project why it is possible to recognize the spectra of elements that are heated to incandescence in far-off celestial bodies.
ACTIVITY
: Demonstration of flame tests and burning of some compounding solution by teacher. Chemicals should be available from chemistry teacher.
Diffraction is the bending of light around barriers placed in its path. Diffraction plates are used in newer spectroscopes. The most common experience we might have had is viewing the colored pattern when a bright light is seen through a fine screen. When we study wave action in water we will see a more concrete example of diffraction.
ACTIVITY
: Examination of light behavior on a diffraction sheet.
Polarization is another characteristic behavior of light (waves). Beams of light actually emerge in all directions. By using a material termed a polarizer on a reflected source, and looking through the polarizer at different angles we may find the light “lightening” and “dimming”. At one angle the light may actually be completely absorbed. This can also be accomplished with certain crystals. This behavior indicates that if light is indeed in waves, they must be transverse waves since longitudinal waves would not be deflected by the polaroid but pass through undiminished.
ACTIVITY
: Use of polaroids.
Interference is a phenomenon which is also exhibited by light. When we make soap bubbles we are creating a thin film of varying thickness and varying wave lengths will be reinforced or diminished and exhibit bands of the colors of white light. By reading through color filters students will get the idea of positive (reinforcing interference) and negative interference. Soap bubbles will be made with bubble pipes.
These characteristics and behavior patterns of light furnish us with some clues as to its nature. It would be nice in a science paper to give a definition of the subject. However, light is more primitive than any of the terms we might use to explain it. So we are describing its behavior and properties in order to compare them with better understood phenomena.
Early students of light have been quoted thusly: Plato said, “The law of proportion according to which the several colors are formed, even if a man knew, he would be foolish in telling, for he could not give any tolerable explanation of them.” c. 380 B.C.
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Aristotle is quoted: “Color sets in movement, not the sense, but what is transparent, the air, and that extending continuously from the object to the organ, sets the latter in movement.” Aristotle,
On
The
Soul
, 11,7.2
Two men, not specifically in search for the nature of light, were responsible for giving others, who followed, invaluable gifts. One was Copernicus who in 1543 proposed that the sun, and not the earth, was the center of the solar system, thus setting the proper “frame of reference.” This was the heliocentric theory. Galileo Galilei, the other, in 1609, constructed a light gathering instrument which he called the telescope. In the 17th century two theories of light were proposed. Sir Isaac Newton thought of light as a beam of small particles which were emitted by all light sources and which travel in straight lines. Huygens, on the other hand, thought of the movement of light as an impulse moving in all directions of space, and with each wave generating new waves (fronts).
(figure available in print form)
ACTIVITY
: Look up the definition of light in a two hundred year old encyclopedia.
This wave theory held sway for over one hundred and fifty years without modification. In the 19th century several new researchers in the field of light came forward. They included Young, George Green, and Fresnel. Fresnel, who like the others accepted the wave argument, established that the waves were transverse. He also deduced the laws of refraction and reflection in theory. Maxwell found that electrical oscillations would also have transverse wave form and would travel at the same rate as light. This established electromagnetivity as a theory and led to our present understanding of the electromagnetic spectrum.
Although Maxwell’s electromagnetic character of light gave more credence to the wave concept of light and explained many aspects of light’s behavior, we must still resort to the corpuscular, or particle, theory to understand other aspects. Those corpuscles, or particles, are now considered sophisticated discrete packets which have energy and momentum, and are termed photons. This dual nature of light exhibiting particle behavior and wave behavior is the basis of Quantum Mechanics. Many of the assumptions of Quantum Mechanics are based on findings of Einstein dating as far back as 1905 which indicated that light is concentrated in photons when it interacts with electrons. The energy is proportional to the frequency (number of waves) of the light. All of the energy of one photon is absorbed by the electron and the photon is annihilated. Planck calculated constant of proportionality between the frequency and the energy. This is too complex for this study but students can see that the dual nature in the explanation of light’s behavior has not yet been satisfactorily finalized.
ACTIVITY
: Look at some pictures of the Balmer Series in Chemistry textbooks and discuss it.
Many of these characteristics of light may be explored by examining the actions of waves in water. We will use a shallow, clear glass platter evenly curved upward on the outer rim. (Clear plastic would do as well.) We will fill this with a thin layer of water and place it on an overhead projector so the waves generated will be seen as light. Wave behaviors, such as wave generation, interference, refraction, and diffraction may be observed. Students should be reminded that light waves extend in all directions from the source (three dimensional).
ACTIVITY
: Lab writeup and drawings of the waves.
(figure available in print form)
1. Drop pebble
2. Drop 2 pebbles
3. Place cardboard with at same time 1/4” slit. Drop pebble in this location.
Wave length, frequency, and the speed of light can be related by the mathematical sentence C (Speed of light) = (Wave length* f (Frequency). The distance from the crest of one wave to the crest of the next (or one trough to the next) is one wavelength. It is signified by the Greek letter Iambda ( ). The number of wave lengths per period of time is the frequency. As Planck proposed energy is a function of frequency. The greater the frequency, the greater the energy. The speed of light is a constant, and is the product of the wave length times the frequency. The speed of light is approximately 186,000 miles per second or 300,000 kilometers per second. This is a good time to learn how to convert miles to kilometers by multiplying by 1.6.
MATH
ACTIVITY
: Find the frequency by knowing the wave length of the colors of the spectrum and the speed of light. Discuss the energy of the various colors. Are they in the order the students expected? Is red really a “hot” color? How does the energy or frequency of the wave length affect the refraction of that wave length?
Convert miles to kilometers.
Find the amount of time necessary for sunlight to reach the earth if the sun is at a distance of 93,000,000 miles sway.
Figure the distance in a light year. (The distance travelled by light in a year.)
Throughout these and other calculations students should be allowed to use a calculator and the computer to back up their work. The exponential function in the computer should be explained as well as the order of the functions of the computer. It should be impressed on the students that we use approximate values of the speed of light for convenience and that at this time the exact value of the speed of light is accepted as 2.997925+ or -0,000003 times 10
10
cm/second. Interpret this number for them thus making a first encounter with scientific notation.
Visible light makes up a very small portion of the electromagnetic spectrum (one octave out of one hundred octaves). By examining the chart of the entire spectrum, students will discover that wave lengths much longer than those our eyes can perceive exist. They may recognize the terms infrared, radar waves, microwaves, and radio waves. On the other end past the visible range they will see there are ultraviolet, x-rays, gamma waves and cosmic waves, some familiar terms and some, more obscure. It should be pointed out that no one unit of measurement will suffice for the tremendous range of the wave lengths, so we use scientific notation. This is based on the powers of ten and the magnitude or power is more important for accuracy than the actual numerical value.,
ACTIVITY
: Use of the book
Powers
of
Ten
to illustrate the dimensions of powers.
Consulting the electromagnetic chart, exercises in using scientific notation.
Light can be changed into other forms of energy. We have all experienced the warmth one feels in a black shirt on a hot sunny day, or sitting on the sunny side of a car with the solar insulation making us uncomfortable. The driver on the other side may not be uncomfortable at all and whether to use the air conditioner or open the windows becomes a matter for debate. A radiometer will clearly show that light energy may be changed to kinetic energy (energy of motion). Our calculators that derive their energy from light are other examples. We can also construct several devices using photovoltaic cells, such as music boxes.
How is energy lost when we produce light? If we turn on an electric light, the form of energy we wish is light. The electricity (a form of energy) is changed into light. But what else? Heat! All energy will continue to exist in one form or another. None of it can be created or destroyed. This is essentially the Law of Conservation of Energy. Students may be able to suggest other changes in forms of energy by following the path of electricity in their kitchen, for instance. They may also recognize wasted energy in inappropriate forms.
These conjectures about light are difficult to “see.” One of the limiting factors is, ironically, our connection with light and color, our eyes. How does the human eye “see?” A physiology teacher will say that the light impulses are changed to nerve impulses by receptors in the eye and are carried by the nerve fibers to the sight center in the occipital lobe of the brain where they are interpreted. This demands certain conditions. First: they eye must be so constructed that the light may reach the special cells that are the receptors and are located in the back of the eye. The light path must be through media that are transparent and translucent. By examining sheeps’ eyeballs, we may trace this path through the cornea (a type of lens), then through the anterior chamber, which is filled with a watery fluid called the aqueous humor (also a magnifier), the crystalline lens, the vitreous humor and finally reach the back of the sphere where the choroid layer acts as a shield that keeps the light from scattering and actually reflects it back to the receptors on the retina. Second: The light must be under control of some monitor so that one receives the proper amount of light. This is accomplished by a beautifully balanced set of muscles which comprise the iris, or colored part of the eye. It is innervated to open the pupil when the eye requires more light, and to constrict the pupil when the eye is receiving too much light. Third: There must be receptors to react to the light with “color” and light that can be described as “black and white” which really means shapes. The mapping of the receptors may be approximated by a simple diagram. Many excellent pictures of the rods (black and white receptors) and cones (receptors of color) are now available, thanks to the electron microscope. These visual aids and the tactile experiences of a lab with sheeps’ eyes should impress on students the suitability of the structure to the function. They can learn about accommodation and eye reflexes as well. A sheep’s brain would be helpful to show the crossover of the optic nerves and the area of sight interpretation in the brain.
ACTIVITY
: Sheep’s Eyes Lab.
DIAGRAM OF THE EYE: Students should be able to match the names to the structure. The light path should be indicated by arrows.
(figure available in print form)
COLOR
: Why do we see natural objects with one color or another? Why is the grass green? Newton said, “These colors arise because some natural objects reflect some rays, others other sorts, more copiously than the rest.” In 1777 George Palmer said, “ . . . each ray of light is compounded of three rays only one analogous to the yellow, one to the red and the other, blue.”
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In 1851 Maxwell constructed the color triangle, which is still used in art. However, only when light is composed of pure rays of light and free of our eyes’ limitations can we discover that certain colors are truly complimentary, that is, when they combine they reconstruct white light.
White can be produce by combining:
Red and greenish blue
Orange and cyan blue
Yellow and indigo blue
and other combinations.
Although color can not exist without an observer, observers do not perceive identical color. Individual differences exist in color perception. Some things are the same in most peoples visual makeup. however. One area of the retina is especially sensitive to color. It is called the fovea. It actually contains no rods in some parts. The complete retina may have as many as 130,000,000 rods over the rest of its surface. Cones, the color receptors, are confined in the central part of the retina and the fovea. While the cones react to brightness and motion as well as to colors, rods react mostly to brightness and to motion in subdued light. The cones number about 7,000,000, so we find an area, small, but crowded with color receptors, each with a nerve fiber of its own. These nerve fibers are part of the optic nerve which goes directly to the brain, therefore, it is remarkably sensitive to fine detail. In the brain only a small area is devoted to peripheral vision (rods) and a large area to foveae or color vision.
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Although eyes are marvelous, some of them may cause the image to be misfocused, so that it does not focus on the retina for the best reception. The most common problems are:.
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1. myopia—the image is projected by the lenses on a
-
point in front of the retina
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2. hyperopia—the image is projected behind the retina
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3. astigmatism—improper fusion of the observed stimuli
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4. presbyopia—”old-eye”—loss of the ability of the lens to adapt.
(figure available in print form)
Since we wish to have sight that is most useful, correction devices have become common. Optical devices are based on the use of lenses that refract light and therefore focus the image on the proper place on the retina, and otherwise adapt it to give the best vision possible.
ACTIVITY
: Drawing of anomalies, discussion of vision and using glasses with an optician (interview on cassette), and playing with optical illusions. Charts for identification of color blindness or those in an encyclopedia should also be used.