Download State University of New York at New Paltz

State University of New York at New Paltz
Department of Secondary Education
Fall 2009 Syllabus
Course: PHY206: Exploring the Universe
Time: TF 10:50 AM-12:05 PM
Room: WSB 115
Faculty Contact Information and Office Hours
Dr. Rosemary Millham
Phone: 845-257-3118
SFB Room 109
E-mail: [email protected]
Office Hours: Tuesday 2-3:30 PM, Wednesday 9-10:30 AM, and Thursday 8-9:00 and by
appointment.
Handout I:
Ways of Thinking About Light
You have probably heard two different ways of thinking about light:
* There is the "particle" theory, expressed in part by the word ‘photon’
* There is the "wave" theory, expressed by the term ‘light wave’
From the time of the ancient Greeks, people have thought of light as a stream of tiny
particles. After all, light travels in straight lines and bounces off a mirror much like a ball
bouncing off a wall. No one had actually seen particles of light, but even now it's easy to
explain why that might be. The particles could be too small, or moving too fast, to be
seen, or perhaps our eyes see right through them.
The idea of the ‘light wave’ came from Christian Huygens, who proposed in the late
1600s that light acted like a wave instead of a stream of particles. In 1807, Thomas
Young backed up Huygens' theory by showing that when light passes through a very
narrow opening, it can spread out, and interfere with light passing through another
opening. Young shined a light through a very narrow slit. What he saw was a bright bar
of light that corresponded to the slit. But that was not all he saw. Young also perceived
additional light, not as bright, in the areas around the bar. If light were a stream of
particles, this additional light would not have been there. This experiment suggested that
light spread out like a wave. In fact, a beam of light radiates outward at all times.
Albert Einstein advanced the theory of light further in 1905. Einstein considered the
photoelectric effect, in which ultraviolet light hits a surface and causes electrons to be
emitted from the surface. Einstein's explanation for this was that light was made up of a
stream of energy packets called photons.
Modern physicists believe that light can behave as both a particle and a wave, but they
also recognize that either view is a simple explanation for something more complex. In
this article, we will talk about light as waves, because this provides the best explanation
for most of the phenomena our eyes can see.
What is Light?
Why is it that a beam of light radiates outward, as Young proved? What is really going
on? To understand light waves, it helps to start by discussing a more familiar kind of
wave -- the one we see in the water. One key point to keep in mind about the water wave
is that it is not made up of water: The wave is made up of energy traveling through the
water. If a wave moves across a pool from left to right, this does not mean that the water
on the left side of the pool is moving to the right side of the pool. The water has actually
stayed about where it was. It is the wave that has moved. When you move your hand
through a filled bathtub, you make a wave, because you are putting your energy into the
water. The energy travels through the water in the form of the wave.
All waves are traveling energy, and they are usually moving through some medium, such
as water. You can see a diagram of a water wave in Figure 1. A water wave consists of
water molecules that vibrate up and down at right angles to the direction of motion of the
wave. This type of wave is called a transverse wave.
Light waves are a little more complicated, and they do not need a medium to travel
through. They can travel through a vacuum. A light wave consists of energy in the form
of electric and magnetic fields. The fields vibrate at right angles to the direction of
movement of the wave, and at right angles to each other. Because light has both electric
and magnetic fields, it is also referred to as electromagnetic radiation.
Light waves come in many sizes. The size of a wave is measured as its wavelength,
which is the distance between any two corresponding points on successive waves, usually
peak-to-peak or trough-to-trough. The wavelengths of the light we can see range from
400 to 700 billionths of a meter. But the full range of wavelengths included in the
definition of electromagnetic radiation extends from one billionth of a meter, as in
gamma rays, to centimeters and meters, as in radio waves. Visible light is one small part
of the spectrum.
Frequencies
Light waves also come in many frequencies. The frequency is the number of waves that
pass a point in space during any time interval, usually one second. It is measured in units
of cycles (waves) per second, or Hertz (Hz). The frequency of visible light is referred to
as color, and ranges from 430 trillion Hz, seen as red, to 750 trillion Hz, seen as violet.
Again, the full range of frequencies extends beyond the visible spectrum, from less than
one billion Hz, as in radio waves, to greater than 3 billion Hz, as in gamma rays.
As noted above, light waves are waves of energy. The amount of energy in a light wave
is proportionally related to its frequency: High frequency light has high energy; low
frequency light has low energy. Thus gamma rays have the most energy, and radio waves
have the least. Of visible light, violet has the most energy and red the least.
Light not only vibrates at different frequencies, it also travels at different speeds
depending on what it travels through. Light waves move through a vacuum at their
maximum speed, 300,000 kilometers per second or 186,000 miles per second, which
makes light the fastest phenomenon in the universe. Light waves slow down when they
travel inside substances, such as air, water, glass or a diamond. The way different
substances affect the speed at which light travels is key to understanding the bending of
light, or refraction, which we will discuss later.
So light waves come in a continuous variety of sizes, frequencies and energies. We refer
to this continuum as the electromagnetic spectrum, below, which is not drawn to scale,
the visible portion of light occupies only one-thousandth of a percent of the spectrum!
The Electromagnetic Spectrum
Electromagnetic Radiation
Radiation describes any process whereby energy emitted by one body, travels through a
medium or through space to be absorbed, reflected, or refracted by another body. What
makes it ‘radiation’ is that the energy radiates (it travels outward in straight lines in every
direction) from the source and can be described geometrically as waves. This wave
geometry naturally leads to a system of measurements and physical units that apply to all
types of radiation, as we will see later when we discuss electromagnetic radiation.
Electromagnetic is defined as magnetism produced by electrical charges in motion or,
electromagnetic radiation (also called electromagnetic energy).
Electromagnetic radiation is energy radiated in the form of a wave as a result of the
motion of the electrical charges. A moving charge creates a magnetic field. If the motion
of the electrical charges is changing (accelerated), then the magnetic field will change
and produce an electric field. These interacting electric and magnetic fields are at right
angles to one another (perpendicular) and to the direction the energy is moving as seen in
the illustration below. So, one type is moving up and down, and the other is moving from
side to side. They travel through empty space as well as through air and other substances
at about the same speed, 300,000 km per second (186,000 miles per second).
This figure illustrates the motion of the electrical field (in blue) relative to the magnetic
field (pink), and the direction that the energy is moving. The movement of the electrical
charges (energy) produces the magnetic field creating the electromagnetic radiation
spectrum.
Scientists have observed that electromagnetic radiation has a dual "personality." Besides
acting like waves, it acts like a stream of particles (called "photons") that have no mass.
The photons with the highest energy correspond to the shortest wavelengths, and the
photons with the least energy correspond to the longest wavelengths.
A wavelength of energy is commonly measured from crest to crest or trough to trough as
seen in the diagram below. These wavelengths are the identifying signature for different
types of electromagnetic energy much the same as your fingerprint identifies who you
are.
This figure illustrates a wavelength. All wavelengths of light look the same, EXCEPT that
the distance from crest to crest is different for every type of energy.
Radio waves have the longest wavelengths and gamma rays have the shortest. The waves
move in an up and down and side to side kind of motion, but the energy is traveling in a
straight line from a point ‘a’ to a point ‘b’ under normal circumstances. So, two kinds of
motion are occurring at the same time. The wave motion (up and down and side to side)
and the forward motion in the direction the energy is traveling.
The number of ups and downs over a given distance is called frequency (see the
frequency diagram below). Frequency is the number of waves per unit time. So, if light
travels at 300,00 km per second, only a few hundred radio waves would move in that
second compared to a trillion gamma ray waves. But, in a straight line, they are both
traveling at the speed of light and will both arrive at the same time.
In this figure we can see that wavelengths can vary greatly in size. In reality, the longest
radio waves could be the size of the diameter of the Earth, and the smallest gamma rays
could be smaller than the atomic nuclei, hence, this is not to scale.
So, if all of the wavelengths of energy are traveling at the same speed, the speed of light,
how do the gamma rays get to the same place at the same time as the radio waves when
they have so many ups and downs to cover at the same time they are moving forward!
How do they keep up? They have more energy! Let’s see how it works.
Let’s just say we have light energy coming in from 300,000 km (186,000 miles) away
from our home. Longer wavelengths of energy will have fewer numbers of waves over
that given distance than the shorter wavelengths (see the wavelength diagram above). The
number of waves over a given distance increases as the energy level increases. Radio
waves have the longest wavelengths so they have the least amount of energy and the least
number of waves over a given distance. Gamma Rays have the shortest wavelengths so
they have the greatest amount of energy and the greatest number of waves over a given
distance. This means that gamma ray wavelengths are so close together that they have
many more ups and downs to travel (frequencies) going from point ‘a’ to point ‘b’ than
any other type of wavelengths of energy. In order to get to the same place at the same
time as all of the other kinds of energy, they have to be even more energetic!
Another way to look at this would be to think about two people traveling the same
distance in a straight line, but one person has to climb up and down millions of mountains
500 meters high while the other person only has to climb and descend hundreds of
mountains of the same size. If both people start the trip at the same time and have to get
to the same place at the same time, the one going over millions of mountains has to move
with much more energy up and down the mountains than the one going over only
hundreds of mountains. They have both gone the same distance in a straight line, but one
has covered more frequencies (mountains)!
This concept can be visualized if you think about the speed of light. If all of the different
types of energy have different wavelengths and, therefore frequencies, and travel at about
300,000 km per second to reach the same spot at the same time, then the energy in
different types of light have to be different.
The only way that gamma rays can reach the same spot as the radio waves at the same
time is if they move very fast up and down the waves because they have so many more
waves to travel.
Here is another example: in the figure below the red visible light wavelengths have
longer wavelengths than the green or the blue or the violet. That means that the red
wavelengths have fewer frequencies per second to complete than the green and the blue
wavelengths. So, if all of those waves follow the law for the speed of light and reach the
same spot at the same time, which one of the visible wavelengths has the most energy?
In this illustration the wavelengths of visible light are represented by their color. Since
the wavelengths of visible light vary from the distance across a bacteria or virus to the
distance across a protozoan these wavelengths are obviously not to scale. However, it
does show that the frequency for each color is different. Which of the visible wavelengths
is the longest with the least amount of energy? The diagram is not to scale.
The electromagnetic spectrum (EMS) is the entire range of wavelengths and frequencies
of electromagnetic radiation (energy) from the shortest wavelengths (gamma rays) to the
longest wavelengths (radio waves). The wavelengths of energy that humans can see,
visible light, are a tiny fraction of the entire range of energy in the EMS. Look at the
diagram below. We have already discussed the wavelengths and frequencies, so lets look
at the numbers. What do they mean?
Under the names radio, microwave, infrared, visible, ultraviolet, X-ray and gamma ray
for the types of electromagnetic energy (specific ranges of wavelengths) are numbers
with exponents. These numbers tell us the size of the wavelength from crest to crest in
meters. 103 is 10 X 10 X 10= 1000 meters, so radio waves are pretty big. On the other
hand, gamma ray wavelengths are 10-12. That is one-trillionth of a meter! So our diagram
is a good visual, but we do not have enough room on it to show the real difference
between each of the kinds of energy, so we use exponents.
The numbers under the frequencies for each type of energy tells us the number of waves
that energy will cycle through for every distance traveled by all of the other wavelengths.
For example, if it takes 300 seconds for all of the wavelengths to get from point ‘a’ to
point ‘b’ traveling at 300,00 km per second, the radio wave frequency at 104 (10,000
waves) is a lot fewer waves than the waves for the same distance in gamma rays at 1020
(100,000,000,000,000,000,000)! Do you know the name of that huge number? It is 100
quintillions!
Now lets look at the real numbers. Each type of electromagnetic radiation on the EMS
has a range of wavelengths. We cannot show this on the diagram because there is not
enough room. We can, however, look at the wavelength ranges in numbers to get an idea
about the range of each energy wavelength on the EMS.
First, a micron, or micrometer, is 1,000,000th (one millionth) of a meter and has a symbol
m. A nanometer is 1,000,000,000th (one billionth) of a meter! Those are very small
measurements, so we use the decimal system or exponents to shorten them. So, 1 m is
equal to a micron (micrometer) or 10-6. The nanometer symbol is nm, and is represented
as 10-9. As you can see from the illustration above, every type of energy to the left of the
visible wavelengths on this EMS has very small wavelengths.
Now look at this EMS. It is the same thing, except that read from left to right the radio
waves come first. This is not important. What is important is that the wavelengths are
correct. So, the EMS can be illustrated, reading from left to right, from the radio waves to
the gamma rays, or the gamma rays to the radio waves. On another note, scientists are not
always in agreement as to where one type of energy wavelength ends, and another begins.
So, sometimes you will see different ranges for a particular type of energy. For example,
the gamma rays in the previous illustration ranges between 10-12 and 10-14 and the
illustration below has a different range. Not to worry. Scientists do not have all of the
answers yet!
We can look at the range of wavelengths for each type of energy individually as well.
Here are examples:
Gamma rays have the shortest wavelengths, < 0.01 nanometers (about the size of an
atomic nucleus). This is the highest frequency and most energetic region of the
electromagnetic spectrum. The range for gamma rays is 0.03nm to 0.003nm.
X-rays range in wavelength from 0.01 – 10 nm (about the size of an atom).
Ultraviolet radiation has wavelengths of 10 – 310 nm (about the size of a virus).
Visible light covers the range of wavelengths from 400 – 700 nm (from the size of a
single molecule to a protozoan). Our sun emits most of its radiation in the visible range,
which our eyes perceive as the colors of the rainbow. Humans can ‘see’ only these light
wavelengths. Human eyes cannot detect all of the smaller or larger wavelengths.
Infrared wavelengths span from 710 nm – 1 millimeter (from the width of a pinpoint to
the size of small plant seeds). At a temperature of 37 degrees C, our bodies give off
infrared wavelengths near 900 nm.
Microwaves range from 1 m down to 1 mm. We associate these waves with heating food
in our microwave ovens. Contrary to some beliefs, we cannot see microwaves. We can
only observe the results of microwaves as they heat food.
Radio waves are longer than 1 mm. They can actually be much larger than depicted here
on the diagram. Since these are the longest waves, they have the lowest energy and are
associated with the lowest temperatures. Radio wavelengths are found everywhere.
We can also isolate portions of wavelengths for each type of energy. Look again at the
infrared diagram above. The wavelengths range from 710 nm to 1 mm. That range of
wavelengths is divided again into near, mid, and far infrared radiation as shown on the
diagram. The division of specific types of energy into smaller portions is possible
because even the wavelengths in a given type of energy have different properties and
characteristics than neighboring wavelengths when interacting with objects on the surface
of Earth, and in our atmosphere. Green plants for example radiate infrared energy (IR) in
thermal IR (Far infrared), and help us to identify plant types.
Bands are a continuous group, or range, or wavelengths with an upper limit and a lower
limit. We can program instruments to detect a specific range of wavelengths depending
on what information we are trying to find out. This is a specific range of wavelengths is
called a band.
Special Note: Far-infrared waves are the longest of the infrared wavelengths and are
often referred to as Thermal Infrared because we perceive these waves as heat. We sense
heat from a fire or a light bulb because these objects are radiating far-infrared waves. We
cannot “see” these waves, but instruments that can sense heat, like thermometers, night
vision goggles or infrared cameras, allow us to “see” the existence of infrared waves
emitted from warm objects like animals and blacktop.
Sensors and Other Tools
A spectrometer (spectrograph or spectroscope) is an optical instrument used to measure
properties of light over a specific portion of the electromagnetic spectrum, typically used
in spectroscopic analysis to identify materials.
The variable measured is most often the light's intensity but could also, for instance, be
the polarization state. The independent variable is usually the wavelength of the light,
normally expressed in nanometers, but sometimes expressed as a unit directly
proportional to the photon energy, such as wave-number or electron volts, which has a
reciprocal relationship to wavelength.
A spectrometer is used in spectroscopy for producing spectral lines and measuring their
wavelengths and intensities. Spectrometer is a term that is applied to instruments that
operate over a very wide range of wavelengths, from gamma rays and X-rays into the far
infrared. If the region of interest is restricted to near the visible spectrum, the study is
called spectrophotometry.
A spectrograph is an instrument that separates an incoming wave into a frequency
spectrum. There are several kinds of machines referred to as spectrographs, depending on
the precise nature of the waves. The first spectrographs used photographic paper as the
detector. The star spectral classification and discovery of the main sequence, Hubble's
law and the Hubble sequence were all made with spectrographs that used photographic
paper. The plant pigment phytochrome was discovered using a spectrograph that used
living plants as the detector. More recent spectrographs use electronic detectors, such as
CCDs that can be used for both visible and UV light. The exact choice of detector
depends on the wavelengths of light to be recorded.
The forthcoming James Webb Space Telescope will contain both a near-infrared
spectrograph (NIRSpec) and a mid-infrared spectrometer (MIRI).
An echelle spectrograph uses two diffraction gratings, rotated 90 degrees with respect to
each other and placed close to one another. Therefore an entrance point and not a slit is
used and a 2d CCD-chip records the spectrum. Usually one would guess to retrieve a
spectrum on the diagonal, but when both gratings have a wide spacing and one is blazed
so that only the first order is visible and the other is blazed that a lot of higher orders are
visible, one gets a very fine spectrum nicely folded onto a small common CCD-chip. The
small chip also means that the collimating optics need not to be optimized for coma or
astigmatism, but the spherical aberration can be set to zero.
A spectrograph is sometimes called polychromator, as an analogy to monochromator.
Light Sources
Visit this website to find a fairly comprehensive list of natural light sources in our
universe: http://en.wikipedia.org/wiki/Astronomical_object
Please become familiar with these objects as we study the universe.