Goals: The student will learn
- How a glass prism resolves light into its rainbow components, which can be recombined to create white light.
- The difference between spectral colors and the colors perceived by the eye.
- That hot solids (or dense gases) radiate a continuous spectrum, related to their temperature.
- That the colors of a glowing rarefied gas are characteristic of the atoms emitting them, with examples.
- That cold atoms in a gas absorb the same colors as the ones they emit when hot, and that such absorbtion causes dark lines in the Sun's spectrum.
- That light is a wave spreading through space, with a very short wavelength, or the order of a few microns (0.001 millimeter).
Note: This lesson can also be embedded in a course on optics. At this level, the student is told that the wave nature and wavelength are deduced from the way parts of a beam of light resonate with each other. The question of why these waves are called "electromagnetic" is left for the next lesson.)
Terms: [Refraction of light, dispersion of light], spectrum, spectral color, perceived color, 3-color theory, black body spectrum, spectral lines, diffraction grating, wave, wavelength, absorption spectrum.
Stories: Discovery of helium.
Starting the lesson:
(One way of introducing the subject of color is through the rainbow. Accordingly, unless time is short, the teacher may devote the beginning of the class to the phenomenon of the rainbow. The discussion here also extends into optics--the study of light.)
Today we will discuss the colors of sunlight, but before we can talk about the Sun, we need to get an idea of what color is. In earlier times, this question confused many people--not just scientists, but also artists, who studied color for their own reasons.
One striking phenomenon involving color is the rainbow. People generally divide the rainbow into 7 colors. Anyone knows what they are, in the order in which they appear?
(Students may answer--put on board, make sure class copies).
Red, orange, yellow, green, blue, indigo, violet
(All of you know "indigo"? It is the name of a dye. What color are blue jeans?
- dark ones are indigo, light ones are blue)
We obtain the same sequence when light passes a prism--red is bent least, violet the most (draw on board). Anyone knows why?
(students may answer)
Because (draw on board, students copy) light hitting a flat surface of glass gets bent by an angle. (That is known as refraction--"The beam of light is refracted.")
However, different colors are bent by different amounts. (That feature is known as the dispersion of light).
When a beam of light hits a glass windowpane (draw on the board) with two parallel sides, the beam is refracted one way as it enters the glass, then the opposite way as it leaves. The net result is that it emerges with the same direction as before. So, inside the glass diffect colors may be separated, but when they emerge again they all move again in the same direction, and no separation remains.
In a prism (refer to drawing already on the board), the entry side and the exit side are inclined, not parallel, and therefore the direction is changed--by a different amount for each color.
How about rainbows? When do you see rainbows?
- We see rainbows when the Sun is shining on falling rain. Typically, a rain cloud has passed over you and you are now in the sunlight again.
(Students may also mention rainbows in sprays from large waterfalls or from garden sprinklers.)
Has anyone noticed where in the sky the rainbow appears?
- Always on the side opposite the Sun. In fact, it is part of a circle whose center is exactly in the opposite direction from the Sun. And the outermost color is always red.
Now, the big question: why the colors?
Raindrops are tiny spheres (draw on board) and act very much like prisms. The light that is reflected inside is separated into colors, and when it comes out--at a point where the angle of the surface is different, just as it is in a prism--the different colors are separated.
Sometimes you see a second, outer rainbow--fainter, larger, with red now the innermost color. Any guess how it gets produced?
Let me give you a hint. When a beam of light goes through a windowpane (refer the drawing on the board), not all the light goes through. Some of it, instead of being refracted the second time, is reflected back, as if the back of the glass was a mirror.
(add the reflected beam to the drawing on the board, a broken line.)
Given this hint--does anyone know how the colors arise, or can you
(end of optional section)
- By light reflected once inside the raindrop, as in the drawing.
How did Newton show that white light was a mixture of all colors?
He used a second prism (upside-down to the first) to bring all the colors together again, and the result was white light, like the one which entered the first prism.
In 1800 William Herschel, the astronomer who discovered the planet Uranus, made the following experiment. He split a beam of sunlight using a prism, and let the spectrum coming out of it--the spread of colors--fall on a table. (Draw on the board, adding details as described below.)
He then placed a thermometer on the place where the spectrum fell, and noted that the temperature rose.
He then moved the thermometer past the red edge of the spectrum, and noted that even there a higher temperature was recorded. What did this mean?
- Answer: The heating of the thermometer in the middle of the spectrum meant that light carried heat (or in our terms, carried energy).
- The heating of the thermometer past the edge of the spectrum suggested that light was shining there as well--a new invisible color which the eye could not see, but the prism was able to separate. Today we would call it infra-red ("infra" means "below"--related to "inferior")
So we have now 7 colors in the rainbow, plus perhaps others which we cannot see (but instruments can). Now let us look at a TV screen.
In a TV with a black-and-white picture tube, the screen contains many small dots which glow when a beam of electrons passes them. The beam sweeps over the picture many times each second, and by controlling its strength, each dot it passes over can be made to shine brightly, dimly or not at all. This way a picture is painted on the screen. The newer flat screens have no electron beams, but the glowing dots are directly connected electrically--otherwise it is very similar.
Anyone knows how a color TV gets its color?
- The screen of a color TV has 3 types of dots, each glowing in a different color, each connected separately to a source of electricity. At any time only one color is connected, and only it responds to the beam. Then the next color is switched on, then the third, and then the cycle repeats. The electrons therefore always paint, very quickly, a red picture, a yellow one and a blue one, and the eye sees the three combined.
But then we can only see three possible colors! How can we see all colors of the rainbow?
- Our eye's ability to tell colors apart is limited. It contains three types of cell, each sensitive to a different range of colors. Any color--of the rainbow or otherwise-- is therefore seen as equal to some combination of the 3 "primary colors. " Color printers, too, need only use inks in three colors.
Does combining yellow and red produce orange?
- To the eye, yes. In spectrograph, an instrument where colors are separated by a prism orange creates one response, red plus yellow, a completely different one. Orange is one color, red and orange, two.
Who discovered the "three color theory"--that the eye sees everything as the combination of 3 colors?
- Tell about James Clerk Maxwell. (see story from the web page)
What did he show happened when all 3 colors were put together (in proper proportion?)
- A combination of the 3 colors produced white. (and that is how a color TV or computer screen produces white, too.)
What is a spectrum?
- A distribution of colors, as obtained (for instance) by a prism.
From now on we only discuss "spectral color" as seen by a prism instrument ("spectrometer"), rather than color observed by the eye.
You are given a closed box with a hole, from which a beam of light emerges. You are given a spectrometer and asked to tell whether the light comes from a filament lightbulb or a fluorescent lamp. How can you tell?
- The light of a glowing filament has a smooth continuous spectrum ("black body spectrum"), most intense at a color which depends on temperature. (see here; much but not all of sunlight is of this type, too.)
- The light of glowing rarefied gas, like that in a fluorescent tube, has a "line spectrum" concentrated in "spectral lines" of well-defined colors, characteristic of the gas. If you see them in the spectrometer, the light source in the box is a fluorescent tube; if they are absent, the light comes from a hot filament.
(The fluorescent coating inside the tube absorbs light and re-emits it in a broad continuous spectrum, so in addition to spectral lines, you will also see a continuous spread.)
The star Antares (in the constellation of Scorpio) is reddish, Sirius is white and our Sun is yellow by comparison. How would the surface temperature of the Sun compare to the ones of the two stars?
- The Sun is hotter than Antares, not as hot as Sirius.
When the battery in your flashlight runs down, the light it produces looks
more orange than yellow. Why?
- Because the battery cannot supply as much electric current as before, the filament in the lightbulb isn't as hot as it should be.
Why are most flames yellow?
(Here may be the place to discuss neon lights, mercury and sodium streetlights, etc., all of which get their colors from spectral lines.)
- Heat easily makes sodium atoms emit a distinct yellow "spectral line" (actually two lines, very close in color). Firewood, paper, candle wax and most other combustible materials contain at least traces of table salt (sodium chloride), which colors the flame.
(optional, by the teacher)
The polar aurora or "northern lights" is emitted by the high atmosphere (around 100 km or 60 miles) when beams of fast electrons from space hit the edge of the atmosphere. These electrons are guided by the Earth's magnetic field lines and therefore are generally observed only in an "auroral zone" around the magnetic pole, typically 2500 km from the pole. At locations like Fairbanks, Alaska, aurora is not all that rare.
here for more about the aurora, and
here about the auroral zone.)
Light is emitted when those electrons collide with atoms, a bit like the way light is emitted when a beam of fast electrons inside a TV picture tube hits the screen. Different spectral lines are observed, but the brightest and most common one is a green spectral line at a wavelength of 5.577 micron. A red line at 6.300 micron is also sometimes emitted.
These emissions used to puzzle scientists, because they failed to match any spectra in the laboratory. Did the upper atmosphere contain any new unknown substance?
Around 1925, the answer was found--both colors came from oxygen. When an oxygen atom was activated ("excited") by a collision in a certain way, it took about half a second before it emitted light, an unusually long time. In laboratory experiments, the gas used was so dense that other atoms usually collided with the excited atom and carried away its extra energy. Only high in the atmosphere could the emission process proceed undisturbed.
(end of the optional section)
A spectrometer can use a prism to separate light into its other components. Do any other ways exist?