Jennifer B. Esty
Astronomical objects emit all kinds of light. Not all of the light reaches the surface of the Earth. Not all of the light can be detected by the human body. Of the light that can be detected by the human body, most of the information it contains can not be analyzed without tools. As a result, astronomy is full of gadgets and tools that make the information we receive from electromagnetic emission in the universe more accessible and more comprehensible. This section is largely about the tools astronomers use and the information astronomers glean from the electromagnetic energy that reaches Earth. This section also addresses the Earth science standards mentioned in the appendix.
Electromagnetic spectrum
The electromagnetic spectrum is the continuum of the different frequencies of electromagnetic waves that we find in the universe. Most basic science textbooks that cover electricity and magnetism will have a diagram similar to figure 2 showing the different types of electromagnetic energy.
When light interacts with an object several things can happen: it can bounce off, reflection; it can go straight through, transmission, it can go through and change direction, refraction; or it can get stuck, absorption. In astronomy, objects will also emit light. Emission, transmission and refraction, absorption, and reflection are discussed below.
Figure 2: Electromagnetic Spectrum with order of magnitude of wavelengths
(image available in print form)
Emission
As described above, when an electron moves from a high state of energy to a lower state of energy, a photon, with energy equal to that which the electron lost, is released. Neon signs are an example of this type of emission. Astronomically, emission by this mechanism is seen from tenuous, hot, gas clouds that surround massive stars.
Blackbody radiation
While some of the light in the universe comes from emissions as described above, most of the light we see in the universe comes from heated matter in the form of blackbody radiation. When matter is heated to a specific temperature, it glows. The red of iron in a blacksmith's forge is an example of this phenomenon. Different types of matter emit different wavelengths of light at different temperatures. This characteristic glow is used by astronomers to gain information about distance objects.
In a star, when an atomic nucleus is shattered, sometimes a photon with gamma ray intensity is released. Sometimes the photon finds its way out of the star intact. However, most of the time, the photon is absorbed by matter in the star and another photon of lower intensity is released. Eventually, the energy is released from the outer layers of the star. The journey of the energy from the interior of the star to the surface is described in detail in chapter 6 of Death by Black Hole and other cosmic quandaries. As a result, the star emits light at various characteristic wavelengths depending on the type of matter that the energy moved through. The set of wavelengths at which a star emits light is called its emission spectrum.
Transmission and Refraction
The human eye can detect electromagnetic energy only in the visible light range of the electromagnetic spectrum. Even in that range, the human eye is not equipped to make precise judgments about the exact frequency of a particular light wave. Furthermore, the human brain tends to reinterpret what the eye reports, so what we recognize as seeing is not always what is actually there. All of this is to say that humans need to use tools to interpret the electromagnetic energy that comes from astronomical observations.
When light encounters matter at an angle perpendicular to the matter's surface, the light will bounce off, be absorbed or go through. When the light goes through the matter without bending or distorting, the object is said to transmit light. Transmission is somewhat rare; more often light is refracted when it passes through matter.
One of the simplest tools used in the science classroom is the prism. When electromagnetic energy is transmitted at an angle through matter, it slows down. When the energy enters the matter at an angle it bends. In my classroom I use a toy car to demonstrate this phenomenon. The car is one of the ones that you roll a few times to prime the spring inside so that when you let it go, the car rolls at a constant speed under the influence of the spring. I point the car in the direction of a pile of sand so that the car hits the sand at a fairly acute angle. The car turns in toward the pile of sand when the tires start to get bogged down in the sand. In the case of the car, the car turns because the sand side wheels are moving more slowly than the table side wheels. In the same way that a car turns a corner more easily when it is traveling slowly than it does when it is traveling faster, the lower energy waves hitting a prism are slowed more and bend more than the higher energy waves. The prism causes the light to separate into its component parts according to the amount of energy each part contains.
A very detail description of refraction and Snell's law may be found on the physic's classroom website.
Laboratory Activity: Snell's Law
When light bends as it passes through matter, the phenomenon is called refraction. The amount that the light bends depends on the type of matter through which it is passing. The amount the light bends is called the index of refraction and is a fundamental property of the type of matter. Calculating the index of refraction of various substances to determine their composition is an easy lab for students to do.
For this experiment students will need paper, several different translucent substances of similar shape and thickness, a light source which emits a beam of light, a ruler and a protractor. I use a flashlight with two pieces of masking tape forming a slight through which light can pass. Something like electrical tape might work better because it is more opaque. When I have done this experiment, I used rectangles of different types of glass and plastic. In my case, the experiment and the materials came with the classroom, so I don't have any good advice as to where to find them. I suspect that the experiment and the materials are available online someplace, though.
In the experiment, the student traces the block of glass on a piece of paper. This line becomes the boundary line between the air and the glass. The student then shines the light towards the glass at a fairly acute angle. The student traces the path of the light from the flashlight to the edge of the glass, the incident ray. The ruler may be useful for this part. When the light hits the glass, it should change directions. The student should lay the ruler on the block of glass so that the edge of the ruler follows the edge of the light. The student should continue this line on to the paper beyond the glass. The glass is then removed. The line that followed the light's path through the glass should be connected to the inside of the boundary line. This line tracing the light's path inside the glass is the refraction line.
This is where the calculation part of the lab begins. A normal line is drawn through the point where the incident ray and the refracted ray meet at the boundary line. The angle of incidence is measured from the normal line to the incident line. The angle of refraction is measured from the normal line to the refraction line.
Snell's law relates the amount of refraction to the type of substance and is written as follows. n
1
sin(O
i n c i d e n t
) = n
2
sin(O
r e f r a c t i o n
) Where n
1
is the index or refraction for the substance where the light originates; in our case this substance is air. In the equation, n
2
is the index of refraction for the substance into which the light passes, in our case this substance is the glass. The thetas in the equation are the angles of incidence, measured as described above.
Using Snell's law, students will be able to come up with an index of refraction for their substance and determine the type of substance used. Because this lab is performed in air, whose index of refraction is 1, a variation on this experiment may be used as follows. The student should measure the angle of refraction for multiple angles of incidence. The sine of the angle of incidence may be plotted against the sine of the angle of refraction, forming a straight line. The slope of this line will yield the index of refraction for the substance.
Spectrophotometry
Spectrophotometry is the study of the electromagnetic spectrum. If you are very fortunate and have an electronic spectrophotometer at your disposal, this section of the unit will be fairly straight forward. If, like me, you do not have an electronic spectrophotometer, you will have to improvise a bit. A spectrophotometer uses refraction to separate light into its component parts.
Once light has been separated into its parts, it can be studied to see which pieces are present and which are missing. This task is made easier by the use of a spectrophotometer. A spectrophotometer, at least at the high school level, is used to study electromagnetic waves in the visible light range. Light from a particular source is passed through or reflected off a substance which refracts the light. The light is split into its component parts. The parts of the visible light spectrum that are visible from a particular light source create a sort of fingerprint which can be used to identify the substance that is emitting the light. So, for example, a neon light will not emit the same spectrum of light that a sodium vapor lamp or a tungsten filament lamp will emit. This is obvious from a simple observation of the color of the light emitted by each of the sources, even without a spectrophotometer; however, a spectrophotometer allows a scientist to observe the precise differences in the wavelengths of light that are emitted by each of the sources.
Building a Spectrophotometer Activity
When this unit is taught, the students will make their own spectrophotometers. There are very good directions for making a spectrophotometer on the sci-toys website. When I tried this demonstration with my fellow seminar participants, I made a few modifications to the directions on the sci-toys website. We used shoe boxes and cut a slit to allow the excess CD to protrude. We used electrical tape rather than foil tape. Finally, we used heavy card stock to make the light slit rather than razor blades. These modifications make the spectrophotometer less expensive and somewhat safer to manufacture. In fact the tape is the only thing that has to be purchased. In my classroom, I intend to have each of my students each make her own spectrophotometer. It is an inexpensive project and the spectrophotometers will be used in an experiment described later in this unit.
The spectroscope as I made it requires a shoebox with a lid, the inner tube from a toilet paper or paper towel roll, a dead CD, electrical tape, heavy card stock (a manila folder would work), and a box cutter (for construction). Before you build your spectrophotometer, I recommend that you check the sci-toys website, because they have good explanatory pictures on their site.
Start with the lid on the box. Hold the box so that the lid is on the top and you are looking at one of the long sides of the box. Slide the CD up between the lid and the side of the box so that one edge of the CD lines up with the top of the box and another edge lines up with the left edge of the side you are looking at. Make sure that the top of the box isn't lifted off as you do this. Trace the small hole in the center of the CD on to the side of the box. Remove the CD. Place the hole of the tube over the circle you have just drawn. Trace the outside of the tube. Move the tube a few centimeters to the right and trace the outside of the tube again. You should have something that looks like a Venn diagram drawn on the side of the box. Put the tube down and turn the box so that the short end of the box closest to the Venn diagram is facing you. Use the CD again and line it up so that one edge is lined up with the top of the box and one edge is lined up with the left edge of the short side. Trace the circle in the center of the CD onto the box. Remove the CD and draw a square about one centimeter on a side so that the right side of the square lines up with the left edge of the circle. Turn the box so that you are looking at the short end opposite the end with the square drawn on it. Line up the CD so that one edge lines up with the top of the box and one edge lines up with the
right
side of the short end. On the bottom of the box, where the CD sticks out beyond the short end of the box, draw a line tracing the width of the CD that extends beyond the box. Remove the CD. Using the box cutter, or an exacto knife, cut an oval that encompasses the two circles of the "Venn diagram". Cut out the square on the short side. Cut a slit along the line that was traced on the bottom of the box. Take the top off the box. Insert the CD into the slit and make sure that the box lid still fits. If it doesn't, make the slit longer until the CD fits. However, be sure that the CD still lines up with the side of the box. Once the CD fits, put a piece of tape across the top of the CD to attach the CD to the box. Make sure that the reflective side of the CD is pointing to the inside of the box. Tape the lid down to the box. Cut a piece of card stock that is large enough to cover the square you cut into the short end of the box opposite the CD. Cut your piece of card stock in half and tape the two halves to the box so that you have a vertical slit up the center of the square. Be sure to tape the card stock to the box above and below the square but not covering the square. Finally, position the tube in the oval so that the CD is visible. Tape the tube into place and cover up any places that light leaks in. When you look through the tube you should be able to see the spectrum of light that is emitted by the light source behind you.
If you have questions about these directions, I recommend that you visit the sci-toys website, http://scitoys.com/, because they have pictures to go along with their directions. Their directions are very similar to the ones I have discussed above.
Absorption
Sometimes electromagnetic energy does not make it through a particular substance. Two phenomena can cause this, absorption and reflection. When the wavelength of electromagnetic energy is just right, an atom or a molecule will catch and retain the photon that hit it. The energy contained in the photon inevitably causes some change in the molecule. Sometimes the molecule breaks apart. This frequently happens to gas molecules like ozone in the upper atmosphere when they are hit with ultraviolet light from the sun. In a plant cell, electromagnetic energy from blue and red light is absorbed by the chloroplast and is used to create sugars and starches. In the case of food in a solar cooker or a microwave oven, the electromagnetic energy causes the molecules, particularly the water molecules, in the food to move more rapidly and cook.
In astronomy absorption is often like the microwave oven example above. Electromagnetic energy is absorbed in one wavelength and is emitted by the same substance at a different wavelength. In the example of the solar cooker, visible light is absorbed by the food and infrared light is emitted; we know this because the food becomes hot after sitting in the sunlight. In the universe objects like dust clouds absorb electromagnetic energy at high frequencies and emit electromagnetic energy at different frequencies.
Reflection
Sometime light is neither absorbed nor transmitted by a substance. In this case it "bounces" off the substance. This is called reflection. My students are familiar with the concept of reflection from looking at things in mirrors and typically do an experiment with mirrors when we cover optics. Reflection is used in astronomy primarily in the construction of telescopes. However, reflection is also important in the study of reasonably close objects as it allows us to see objects like planets and moons. As telescopes, planets and moons are not the major focus of this curriculum unit, we will not spend much time on this topic.
