Jennifer B. Esty
Electromagnetic waves are a strange phenomenon. As I stated earlier, electromagnetic energy has a dual nature; it is both finite like a particle, in the form of a photon, and infinite, in the form of a wave. It moves in a straight line, yet it also exists as a field. This duality is discussed in the sections that follow.
This part of the curriculum unit addresses the physics standards listed in the appendix. These are fundamental concepts, so you will probably find similar standards listed in other regions.
Waves
Waves come in a variety of different forms. As stated in the standard quoted in the appendix, all waves transport energy from one place to another. In fact, only traveling waves transport energy; standing waves do not. However, in this unit, we will not be discussing standing waves; we will only be discussing traveling waves. For example, a sound wave is generated when air is vibrated; the wave continues to vibrate other air molecules until the energy of the moving molecules is absorbed by some object like an ear drum or acoustic tile. While all waves share similar anatomical characteristics, which are discussed below, there are important distinctions which need to be considered.
Three waves which are most familiar to my students are sound waves, surface waves in water and visible light waves. In two of these cases, my students generally don't realize that the phenomena are actually waves, but they recognize their interactions with the waves. This prior experience makes these three waves a good starting point for discussions. In two of the three cases, the waves are mechanical, meaning the wave requires a medium through which it can travel. The third wave is a non-mechanical wave, meaning that it does not require a medium for existence. Both sound waves and water waves are mechanical; the visible light wave is non-mechanical. Because there is very little concentrated matter in space, the waves used to study astronomy are necessarily non-mechanical.
Anatomy
All waves share a similar anatomy. As illustrated in figure 1, all waves have crests and troughs. The length of a wave, one of its defining features is the distance between on crest and the next or between one trough and the next. In the figure below, the wavelength is measured from one crest to the next. The amplitude of the wave is half the distance from the height of the crest to the depth of the trough. The amplitude is another defining feature of a wave. The next defining feature of a wave, its rate, can be defined in two ways. It can be defined as a frequency or as a period. The frequency tells how many crests or troughs pass a certain point in a given amount of time; it is generally measured in Hertz (crests/second) abbreviated as Hz. The period tells how long it takes an entire wave (two crests or two troughs) to pass a given point; it is generally measured in some form of time (minutes, second, hours, etc.). The period and frequency are inversely related. The appendix contains a list of variables and formulas generally used to work with waves. It may be helpful to introduce the Greek alphabet before teaching this section.
Figure 1: Anatomy and Wave
(image available in print form)
Transverse, Longitudinal, Circular and Twisting Waves
There are several forms of waves. Electromagnetic waves are transverse waves, but the others are useful as comparisons. Transverse waves are waves that look like the pictures in the upper part of figure 1. In this type of wave, the amplitude is perpendicular to the direction of the motion of the wave. They are two dimensional. In the case of electromagnetic energy, electric and magnetic waves form fields that are perpendicular to each other around the central axis of both waves. Both fields travel in the same direction at the same time. The structure somewhat resembles a double helix. There is a picture of an electromagnetic wave in Universe in the chapter on light, optics and telescopes. There are also a number of pictures of the electromagnetic waves online.
Longitudinal waves, sometimes called compression waves, are a pulse of compressed material. In these waves, the crests and troughs of the wave are parallel to the direction of this motion of the wave. Sound waves are a good example. It is easy to demonstrate a longitudinal wave in a classroom. Have the students hold a hand up about an inch from their mouths and speak. The compressed air emanating when the students speak are an obvious examples of a longitudinal wave. Alternatively, a slinky or a fairly loose spring can be used to create a longitudinal wave. Either way, the wave form may be seen in figure 1. As you can see from figure 1, the crest of a longitudinal wave is a denser region and the trough is a less dense region. These concepts may be a bit more difficult for lower level students to understand, but a diagram like the one above combined with the aforementioned demonstration should help to get the ideas across.
Circular, sometimes called elliptical or surface, waves will be familiar to students from the beach. Students, however, probably will not recognize the waves they see at the beach as circular rather than transverse. In the example of a wave at the beach, any given molecule of water moves in a circle. The series of circles next to each other give the appearance of a longitudinal wave on the surface. However, if it were actually a longitudinal wave, there would be no undertow and the water would not recede back from the shore after each wave. Another way to picture this type of wave is to think of the air valve on a bicycle tire as the tire is rolling down the road. If a time-lapse photograph of the tire were taken, the valve would appear to move in a series of connected circles. A line drawn on the top of the circles would look like a transverse wave, but the valve itself is actually moving in a circular wave.
Twisting or torsion waves tend to be found in structures. If you wring out a wet cloth the twisting action is similar to a twisting wave. These waves are generally caused by natural forces acting on large structures. The example that comes to mind is the video of the Tacoma narrows bridge failing due to sympathetic oscillations. The twisting, rolling waves seen in the classic film footage is a good example of this type of wave.
Photons
Electromagnetic radiation is not simply a wave. Electromagnetic radiation always exists in the form of a wave and a photon. A photon is a very small, discrete packet of energy. To really understand photons and electromagnetic energy, though, you must understand how photons interact with atoms.
Photons and atoms interact in illuminating ways. An atom has a nucleus in the center with electron orbitals surrounding it. Think of the atom as a stadium: the nucleus is the stage, the orbitals are the rows of seats. Clearly, this is a very strange stadium as there are only two seats in each section, but never mind that. As in any concert experience, the seats right down in front, closest to the nucleus, are the most valuable. In a concert, they would be the most expensive. At this concert, though, the only currency is energy. So, the two electrons that are in the orbital closest to the nucleus, the ones with the best seats, have the least energy, because the best seats are the most expensive. If an electron "wants" more energy, it can accept a photon with a certain amount energy, but then it has to move to a specific higher orbital, further away from the nucleus. Similarly, if the electron "wants" to move in closer to the nucleus, it must give up a corresponding amount of its "currency", its energy, in the form of a photon. When an atom absorbs light, it is taking in photons which cause its electrons to move to a higher orbital. When an atom emits light, its electrons release energy in the form of photons, aka light, and the electrons move to a lower energy orbital. In the same way that tickets to different concerts cost different amounts, different atoms will react to different amounts of energy. The amount of energy in a given photon depends on the frequency of the electromagnetic wave. The different sources of astronomical electromagnetic energy are discussed below.
This topic has the potential to be a very interesting lesson. A staged "play" showing how electrons and photons interact might be effective in a normal sized class. I can see a situation where Nerf balls representing photons could be tossed into an "atom" made of students representing electrons in the atom. It has the potential to be a very memorable experience. My physics classes tend to be somewhat smaller with heavily pregnant students (being pregnant is a prerequisite for being a student in my school), so I might have the students do a somewhat less active activity like draw a cartoon comic book sequence instead.
Fields
An energy field is an area where a particular force has an influence on objects. In a way it is analogous to a cold-war sphere of influence. Warsaw Pact countries would be under the influence of the Soviet Union. NATO countries would be under the sphere of influence of the United States. A place like Berlin would be under both spheres of influence. In the same way, a paper clip dangling from a magnet would be in the magnetic field of the magnet, but it would also be in the Earth's gravitational field. Magnetic fields are easy to illustrate using iron filings. However, I will caution you that the iron fillings are very hard to get off the magnet if they come into direct contact with it. In my classroom, I generally use a child's toy for illustration. The toy is a piece of cardboard with a face printed on it. The face is covered by a piece of plastic. Between the plastic and the cardboard is a pile of iron filings. The toy comes with a magnet that is used to drag the iron filing around the face. The iron filings look like dark facial hair. In my classroom I evenly distribute the filing over the cardboard and then put a bar magnet underneath the cardboard. It works quite well. A detailed study of fields is possible, but the mathematics required for it is beyond the ability of most of my students.
