Before beginning this unit on EM waves, students will have been exposed to the following vocabulary and concepts in a previous unit. What is a wave? A wave is a disturbance that transfers energy through matter or space. There are two types of waves: mechanical and electromagnetic (EM) waves. A mechanical wave is a type of wave that requires a medium for propagation while electromagnetic waves are a type of wave made of changing electric and magnetic fields and does not require a medium for propagation.1 Examples of mechanical waves include earthquakes, water waves, and sound waves because each of these waves requires a medium or matter to travel through. There are two types of mechanical waves longitudinal and transverse waves. Transverse waves are waves in which the wave motion is perpendicular to particle motion while longitudinal waves are waves in which wave motion is parallel to the particle motion.2 The difference between these two waves is nicely demonstrated with a slinky. Sound waves are examples of longitudinal while ripples on the surface of water are an example of transverse waves. All EM waves are transverse waves.
A transverse wave has the following properties and often looks like the following figure. The high point of a transverse wave known as the crest while the low point of a transverse wave is known as the trough. The amplitude of a wave is the greatest distance particles are displaced from their normal resting position. A wavelength is the distance between two identical parts of a wave. Frequency of a wave is the number of waves passing a given point in a given amount of time measured in the unit of hertz (Hz). A period is different from frequency because it is the time required for one full wavelength of a wave to pass a certain point.3
Figure 1: This diagram shows the characteristics of a transverse wave.
Another concept important to this unit about EM waves is wave interference. Interference occurs when multiple waves in the same location combine to create a single new wave that is different from the original waves.4 Two types of interference include constructive interference and destructive interference. When the crest of one wave overlaps the crest of another wave they reinforce each other and the amplitude increases. This type of interference is known as constructive interference. Destructive interference decreases the amplitude when the crest of one wave meets the trough of another. The resulting wave has an amplitude smaller than the amplitude of the larger of the two original waves. A third type of wave interference is the creation of a standing wave. A standing wave is the result of interference between a wave and its reflected wave. The result of a standing wave is that the medium vibrates in a stationary pattern that looks like a loop or a set of loops.5 These are all the vocabulary and concepts that students will need to learn before the unit on EM waves presented here.