Class Notes #3 ALL NOTES ARE SUBJECT TO CHANGE
Chapter 5
Properties of Light
Students often start out with an idea that astronomy is all about lounging around a campfire, looking and stars and constellations, and telling stories about them. In fact, that may be how it all began thousands of years ago. Usually in the beginning, the study of the stars was not entirely an effort to understand what stars are, but rather what they mean to us on Earth -- how they affect our everyday lives and even human personality. In other words, in the early times, astronomy was intricately bound up with astrology. Today we know that astrology is basically nothing more than fortune-telling. There is no evidence that stars have any direct effect on our everyday lives. Despite all the claims, all the hype and the emotional stories some people tell, there is not one shred of evidence that the positions of the stars and planets have anything to do with life on Earth. Gradually, some began to separate the study of stars for forecasting ability from the study of stars in their own right. Unfortunately, the connection lasted a long time, though, all the way up to and including Kepler.
In order for stars to have any influence on Earth, there would have to be some significant force that could carry that influence from the heavens to Earth. And indeed there are two forms of energy that connect the stars with Earth -- light and gravity. Unfortunately for the astrologers, neither has enough influence to make any imapct at all on human life. (Some may see a seeming contradiction in the fact that light and especially gravity are too week to influence human life, especially when gravity holds the Universe together -- but in fact gravity has a significant influence only on very large and massive bodies -- not human bodies).
But light carries a tremendous amount of information, and with it, astronomers have opened up our knowledge of the Universe. Until relatively recently, light was all astronomers had to study. Since light is so essential to the study of astronomy, there are a few things that you need to know about it.
- Light is a form of Energy (radiant energy). A beam of light carries energy from one place to another. Go out and stand in the sunlight. Doubtless you will notice that the sunlight warms you up. In fact it does so with energy that left the surface of the Sun about 8 minutes earlier. Actually various wavelengths of light carry various amounts of energy. In terms of visible light, a photon or partical of red light, carries less energy than a photon of blue light. But of course it also depends on how many photons we are talking about .
- Light
comes in a range of Wavelengths Wavelength can be though
of as another way of saying "color," as shorter wavelengths are blue
and longer wavelengths are red. Light can be thought of as a
wave of energy with "highs" and "lows" which we call peaks and troughs.
One wavelength is the distance between two
successive peaks. Frequency is how many peaks pass a given point in one
second. You can think of frequency as the inverse of wavelength, in
that short wavelength light has high frequencies, and long
wavelength light has low frequencies.
In terms of scale, the wavelengths of visible light are very small, typically measured in millionths of a meter. However, there are much longer forms, such as radio waves, which can have wavelengths hundreds of meters long. And there are shorter wavelengths, such as ultraviolet, X-rays and gamma rays, the latter of which has wavelengths measured in trillionths of a meter. Taken together , these various forms of radiant energy (longest to shortest: radio, microwaves, infrard, visible, ultraviolet, X-ray and gamma rays) are called the Electromagnetic Spectrum. Here is a chart from Wikipedia:
- The duality of light Strangely, light can act as a wave, or it can act much like a particle. We call these particles of light photons. Do not confuse this word with proton, which is part of an atom. So you can think of the Sun (or anything that radiates light) as pumping out waves or streams of small particles. We don't need to worry about this at this point, but sometimes it is easier to view it as one or the other. The really weird thing is that light appears to be both a wave and a particle, and it doesn't become one or the other until we decide which way we want to view it. Of course this does not come up in real life, but it is something physicists consider, and it is just part of "quantum weirdness."
- Color
and Temperature Electromagnetic energy is formed when
atoms vibrate or change in certain ways. First off, atoms are small! -- An atom
is about 10-11 meters across, whereas just the
nucleus is roughly 10-15 meter across. For
instance, a hydrogen atom is about as big compared to a racquetball, as
the racquetball is to the Earth. The human body has about 3.4 times 1027
atoms (that's 3.4 billion billion billion atoms!). That's a
lot of atoms!
The way atoms vibrate determines what color of light ( wavelngth) they produce. Of course this may be invisible fornms of light such as X-rays or radio waves. The color of light or wavelngth they produce is dependent on temperature. Thus color is associated with temperature. [e.g., red stars are less hot, yellow stars are moderately hot, and blue stars are the hottest.] This is a basica paraphrase of Wien's Law. (pronounced, "veen's")
.
The Formation of Spectra
- The
Spectrum Is
aac Newton discovered that if you
passed white light from the Sun through a prism, it would spread out
into numerous colors (in particular: red, orange, yellow,
green, blue and violet). When the light smoothly grades from one color
to the next as in a rainbow (a natural spectrum of sunlight), it is
called a continuous
spectrum. Based on this priciple, scientists developed a
device called a spectroscope, which is a combination of a prism with a
telescope, to study light. Today this is done mostly with computers,
but the concepts are the same. White light from the Sun
contains many different colors because the wavelengths the Sun emits
are various lengths.
Some time later, scientists looked at the Sun's light with a sensitive spectroscope and noticed that there were hundreds of dark lines that cross it. These are called Fraunhofer lines after the German astronomer who first studied them. For a long time they were a mystery, but today we know that the pattern of lines is a bit like a fingerprint or a DNA sequence, in that it tells us just what elements are in the Sun or star. Here is a photo of the spectrum of the Sun:
- Kirchhoff's Laws A
German physicist determined that there are three types of
spectra formed under different conditions. These became known as
Kirchhoff's Laws.
1) A heated solid, liquid or compressed gas gives off a continuous spectrum when heated or energized. The Sun is a highly compressed gas, so it gives off a continuous spectrum (more on why there are dark lines below). A red-hot fireplace poker is a solid, so it gives off a continuous spectrum. Glowing lava also gives off a continuous spectrum.
2)
A non-compressed gas (sometimes said to be "transparent"), when heated
sufficiently or energized in another manner, such as passing an
electric current through it, emits only certain wavelengths of light,
and the pattern of emitted light is indicative of the gas producing it.
In a
spectroscope, it looks like a series of colored lines with black
between them. Each
non-compressed gaseous element, say hydrogen, when heated or otherwise
energized, gives off a different distinctive series of colors that is a
unique fingerprint of the element. (This is called an emission or bright line
spectrum. We
indicated non-compressed because if it is compressed or at high
pressure, it will give off a continuous spectrum.) If we observe light
with an emission pattern, we know that it was a gas that produced it.
3) If light from a continuous source passes through a cooler (not energized), uncompressed gas, the resulting spectrum will be continuous, expcet marked by dark lines at specific wavelengths that correspond to specific elements. For a given gas, the wavelengths of the dark lines are exactly the same that would produce bright lines if the gas had been heated. Thus a beam of light that is missing a particular pattern of wavelengths gives evidence that the light was passed through a particular gas. (This produces an absorption spectrum.) We observe particular chemical elements in stars through these patterns in their light . [It is exactly this process whereby we know the chemical compositions of the Sun and stars. Light from a hot, highly condensed gas (the surface of the Sun or a star) passes through a cooler,less compressed gas (the atmosphere of the Sun or a star) and specific wavelengths of light corresponding to specific elements are filtered out. We see these as the dark lines in an absorption spectrum. The exact positioning of the lines gives their wavelengths, which reveals their nature.] - Atomic formation of spectra
We know that atoms consist of a central, positively charged
nucleus and a number of negatively charged electrons that
occur at certain distances around it. Each element has a
particular number of protons, which is what gives it its chemical
properties. In an
electrically
neutral atom, there are as many electrons as protons. The electrons are
arrayed around the atom in a particular pattern that is indicative of
that particular element. For example, the arrangement of electrons
around an nitrogen atom is different from the arrangement of electrons
around a carbon atom. About 100 years ago, physicists discovered that
for a given element, only certain arrangements of electrons
were allowed. They discovered that the distances that the electrons
were from the nucleus corresponded to energy levels, and only certain
energies were allowed. Think of it like a set of stairs, with each step
representing a particular energy. For each step you climb on
the stairs, you get higher from the floor, and hence your gravitational
potential energy relative to the floor increases. But it can only
increase by the amounts represented by each step. In other
words, you can't take a half step because at the half step level there
would be nothing for you to step on. That is, you can't climb 4 and
half steps up the stairs.
In an uncompressed gas, electrons can move or "jump" only between the allowed levels. Thus, for a given element such as hydrogen electrons can exist with only a certain pattern of energy levels. In other words, the cannot have just any energy, only specific amounts. Each element and chemical compound has its own unique arrangement of allowed energy levels. (By the way, when an electron changes between energy levels, it is called a "quantum leap." The common usage of this term is misleading and wrong, and it has nothing to do with time travel.)
To change energy levels, one way an electron can "jump," is by absorbing or emitting a photon. The interesting thing is that it must absorb or emit only photons whose energy exactly corresponds to the energy difference between the two levels involved. Thus only photons with certain wavelengths (or color in the visible spectrum) can absorbed or emitted. This gives rise to the dark lines in an absorption spectrum, and the bright lines in an emission spectrum.
A continuous spectrum is produced in solids, liquids and compressed gases because the atoms are pressed so tightly together that in essence, the energy levels between neighboring atoms overlap. In that case, electrons can make fractional energy jumps between atoms. Since there is an effectively infinite number of energy level transitions here, photons of all wavelengths are emitted and there is a continuous spectrum.
- The Doppler Effect
Another useful aspect of light is the fact that it exhibits
the Doppler Effect. By Einstein's Special Theory of Relativity, in a
vacuum, light speed is invariant. In other words, it is always the same
-- it does not slow down and since it already is the fastest possible
speed, it cannot speed up. But if the source of light is moving toward
or away from the observer, the wavelengths can change.
(In the graphic, the point is
moving to the left, such at waves in front of it are compressed to
shorter or bluer wavelngths, and the waves behind it are stretched out
to redder wavelengths.)
For light that is moving toward the observer, the wavelengths are compressed, just as if they were being squeezed together in the direction of motion. For visible light, this causes the light to be shorter in wavelength than it otherwise would be, and therefore to be pushed more to the blue end of the spectrum. It is blue-shifted. By determining the amount of blue shift, astronomers can determine how fast the source of light, such as a star, is moving toward us. Interestingly, the lines in the spectrum of that light also are shifted by the same amount.
If the source of light is moving away from us, the wavelengths will be stretched out and everything will be shifted in the direction of red. It is red-shifted, and the amount of shift tells us the speed of the source away from us.
The red and blue shifts, which are analogous to the shift in pitch in the sound of a train or plane passing us, is a manifestation of the Doppler Effect. It is incredibly important in astronomy and physics. Typically the amount of shift is far too small to detect without sensitive instruments.
Telescopes
- Refractors
The
primary instrument of astronomy is, of course, the telescope. Credit
for invention goes to Dutch spectacle maker Hans Lippershey in 1608,
although Galileo is often mistakenly credited. But although Galileo did
not invent the telescope, he is the first to use it for systematic
study of the heavens, and in fact the standard type of telescope used
in those early years is often called a "Galilean" telescope.
Scientifically, it is a refractor telescope, because it uses a larger
lens to collect and "refract" light. To refract means to bend or cause
the change direction, which is necessary for the light to be magnified.
The simplest refractor has a large lens called an objective, which
collects light from some distant object and refract it into a conical
beam, this concentrates the light. At the other end is a small lens
that takes the concentrated light and turns it back into parallel rays
that form an image we see. The distant object appears larger
and brighter.
Refractors are a bit limited in usefulness because large glass lenses are expensive to make and can distort from their own weight. The telescope at Chamberlin Observatory in Denver is a refractor with a lens 20 inches in diameter.
- Reflectors
This
type of telescope was first designed by Isaac Newton. It, in numerous
variations, is the most common scientific telescope in use today
because it can be made much less expensively than a refractor of the
same size, and it is not subject to the distortions that plague large,
heavy refractors
The basic reflector uses a large mirror as its objective rather than a large lens. The mirror collects and bends the light by reflecting it rather than letting it pass through the glass. This does essentially the same thing as in a refractor, but just in a different way. One advantage of this is that there is less absorption of the light, making the image brighter. Also, in today's reflectors, the mirrors can be made in several pieces. One large telescope on a mountaintop in Hawaii has 36 mirrors all acting as if they were one larger mirror.
The largest optical telescopes in the world today are of this type, although there are many variations in design.
- Radio
A
radio telescope is similar to a reflector telescope except
that is collects long wavelength radio waves with a large metal mirror
or "dish." Typically it needs a much larger collector than optical
telescopes. Most appear much like a satellite dish, and are essentially
the same. The dish is the antenna for the radio telescope.
The most common type is a large metallic mesh dish, although some use
completely different types of antennas.
- Other
Types
There are other types of instruments designed to detect various types
of particles and radiation from space, which sometimes are loosely
referred to as "telescopes." These include instruments to detect cosmic
rays, neutrinos (more on these later) and gravitational waves. These do
not look at all like traditional telescopes. For example, one neutrino
"telescope" is a large vat of cleaning fluid a mile underground in
South Dakota! It works by trapping neutrinos (particles from the Sun
and stars) int he chemicals of the cleaning fluid. It is a mile
undergruond to shield it from more powerful and penetrating cosmic rays.
END
(Some graphics copyright by Richard W. Pogge and used with permission. Other graphics are copyright by Larry C. Sessions or are believed to be in the public domain. Contact me at the email address below with questions or if you find a copyrighted image uncredited or inappropriately used.)