Working through this chapter of the study guide will enable you to:

1. Understand the general properties of both transverse and longitudinal waves.
2. Describe electromagnetic waves and list the various types of transverse waves that fit into this category.
3. Discuss sound waves in detail and understand their general properties and the way they propagate through air.
4. Explore the Doppler effect in both sound and light waves and explain how this effect helps us understand such diverse processes as the change in pitch of the siren of a moving ambulance and the overall expansion of the universe.
5. Show how standing waves are formed and how their presence can be related to resonance.

Discussion

Wave processes are a common occurrence in our everyday lives and are critical to the description and understanding of both sound and light. Although the form and method of propagation are quite different, both sound and light follow very similar rules when it comes to their wave characteristics, and that is what we shall study in this chapter.
Light and other types of electromagnetic radiation do not need any physical medium to support their propagation. Sunlight comes to us on Earth through essentially empty space. Light is a transverse wave, although no actual matter is displaced by its movement. When a light wave passes a given point, there are changes in the electric and magnetic properties of space perpendicular to the direction in which the light wave is traveling. This means that electromagnetic waves are considered to be transverse waves.

Sound waves, on the other hand, must have a medium such as air or water through which to propagate. Most of the sound waves that we deal with in our everyday lives travel through the air. Sound waves are longitudinal waves. This means the air particles move back and forth on paths that are parallel to the direction of wave travel and thus take the form of compressions and rarefactions of the molecules in the air itself.

After studying this chapter, you will have a much better understanding of the interaction of waves in our daily lives and of how the application of such topics can explain many seemingly diverse natural occurrences. Without wave propagation, we would not receive information about the world around us by either the processes of sight or hearing. As you can imagine, our perception of the world would be quite different without the sensory input provided to us by waves.


Section  6.1Waves and Energy Propagation

The transfer and propagation of energy in matter are not limited to temperature differences (Chapter 5). In many common cases, the energy is transferred by a disturbance and propagated as waves. We call this transfer of energy wave motion. Energy is transmitted from one particle to another in the medium (water, air, etc.) as the wave propagates outward from the disturbance. However, electromagnetic waves, such as visible light and radio waves, can propagate through a vacuum. They need no medium and thus can travel freely through space.


Section  6.2Wave Properties

Wave energy can be transferred as either a single pulse or as a series of periodic vibrations known as a wave train. In either case, two basic waveforms are involved: longitudinal and transverse. Longitudinal waves exhibit parallel motion of the particles in the medium relative to the direction in which the wave is propagating; transverse waves are characterized by perpendicular motion relative to the direction of wave travel. The direction of wave propagation is important here and is the direction in which the wave energy is traveling. Each type of wave travels at its own rate, known as the wave speed (v), which is expressed in units of length divided by time.

The periodic motion of a wave is described by using certain specific terms. Amplitude (A) is the maximum displacement of the wave from its normal equilibrium position. Wavelength (l ) is the distance between one point of maximum oscillation along a wave train and the next point of maximum oscillation. (Actually, the wavelength can be measured between any two adjacent similar points in a wave's oscillation pattern.) The period (T) is the time interval required for one complete oscillation of the wave to be completed, and the frequency (f) is the number of complete oscillations that occur in a given time interval, usually 1 second. The frequency and period are reciprocal to one another.

T	=	1 / f
f	=	1 / T

The wavelength multiplied by the frequency of the wave is equal to the wave speed for any wave.
l f = v
Using these basic concepts, it is usually possible to understand and describe even the most complicated wave motions found in nature.


Section  6.3Electromagnetic Waves

Electromagnetic radiation is an important wave type that includes gamma rays, X-rays, ultraviolet, visible light, infrared, microwaves, and radio waves. All electromagnetic radiation travels at the same wave speed, which for historical reasons is referred to as the speed of light. Visible light and the other types of electromagnetic radiation do not need a physical medium for their propagation. They can travel through a vacuum, although they can also travel through transparent media such as glass and water. Sunlight comes to us on Earth through essentially empty space, but it also penetrates through the gases of our atmosphere and can be detected at great depths in our lakes and oceans.

Light is a transverse wave. When a light wave passes a given point, changes occur in the electric and magnetic properties of space perpendicular to the direction in which the wave is traveling. That is why light is classified as a transverse wave even though no physical material is involved in the oscillations that occur. It is the variations in the electric and magnetic character of space that are responsible for the transfer of energy in all types of electromagnetic waves.

The frequency and wavelength for electromagnetic waves vary widely from one type to another, and every type has its own range of values for each of these properties. That is how we distinguish one type of electromagnetic radiation from another. Remember that frequency and wavelength are related to each another by the wave speed equation discussed in Section 6.2. All the types of electromagnetic radiation make up what is known as the electromagnetic spectrum. This complete spectrum is shown in Fig. 6.8 in the textbook, and this figure should be studied carefully so that you will be able to identify the various types of electromagnetic waves and their relationships to each other.


Section  6.4Sound Waves

In general, we will deal only with sound waves traveling through the air. Sound waves are best described as compressions and rarefactions of the medium through which they are propagating. In this process, the molecules themselves move back and forth, parallel to the wave direction, when a sound transfers energy from one place to another. This means that sound waves are classified as longitudinal waves.

Sound waves also make up a spectrum based on the frequency of their wave oscillation. This spectrum is somewhat simpler than that for electromagnetic waves in that it has only three sections: the infrasonic region below 20 hertz (Hz), the audible region between 20 and 20,000 Hz, and the ultrasonic region above 20,000 Hz. The frequency of a sound wave is generally perceived as the pitch of the sound, which is most relevant when the musical aspects of sound are considered.

The loudness of a sound is related to the amplitude of oscillation of the particles in the medium through which the sound is traveling. These variations in displacement take the form of compressions and rarefactions that can be quantitatively measured in terms of the intensity of the sound produced in units of either W/m² or decibels. Figure 6.13 in the textbook summarizes the sound intensity levels that we commonly encounter and gives us an indication of the precise loudness associated with a specific intensity level.


Section  6.5The Doppler Effect

The Doppler effect can be observed both in electromagnetic waves and in sound waves. The change in pitch of a car or truck horn when the vehicle is passing you is a common example of this effect. Whenever a wave source is approaching an observer, the frequency of the vibrations perceived by the observer will be higher. In sound waves, this means the listener will hear a higher pitch. If the source and observer are moving apart, the perceived frequency will decrease and, in the case of sound, a lower pitch will be heard. This is why the pitch of a passing car horn will suddenly drop to a lower frequency when the car passes you.

Electromagnetic radiation traveling to Earth after being emitted by distant galaxies also exhibits a Doppler shift toward the red end of the visible spectrum. This indicates that these galaxies are moving away from us, often at high rates of speed. The Doppler effect provides strong evidence to support our current belief that the entire universe is continuously expanding after its original formation in a gigantic Big Bang explosion (Chapter 18).

Doppler shifts are currently being used in radar units to measure the speed at which objects such as cars are moving. Such devices enable the police to check the speed of oncoming motorists. The speed of a pitched baseball or a tennis serve can also be determined by the use of Doppler shift radar guns like the ones you may have seen while viewing sporting events on television or while visiting a local carnival or fair. Even the wind speeds inside storms can be measured using "Doppler radar," which enables us to detect violent storms, such as tornadoes, and in general to predict the movement of complex weather patterns more accurately.


Section  6.6Standing Waves and Resonance

Often, when sound waves propagate through the air, they reach some surface from which they are reflected. An echo is a common example. If the incoming energy is in the form of a continuous wave train and the reflection is in exactly the opposite direction to that of the incoming sound wave, the reflected wave train may interact with the incident wave train to form fixed patterns of maximum displacement, called antinodes, and zero displacement, called nodes. The distance between adjacent nodes or antinodes in such a pattern is one-half of the wavelength of the wave.

Such fixed waveforms are called standing waves. Standing waves result from resonances that intensify the wave energy for those frequencies whose wavelengths happen to fit exactly into the standing wave pattern. Resonances are very important in the production of musical notes and in the formation of the sounds associated with human speech. Similar resonances occur in electrical circuits such as those used to tune radio or television receivers to a particular transmitting station. Large amounts of energy can be transferred by the resonance process, occasionally with quite disastrous effects. See Fig. 6.19b in the textbook for a dramatic example of destructive resonance.

Music is a natural extension of basic sound principles, although the terms used to describe it differ somewhat. The frequency of sound, and to some extent the loudness, are related to the concept of pitch, wave amplitude is related to the loudness of a sound, and the number and amplitude of the harmonics and overtones of a sound determine the musical quality, or timbre. Musical instruments create specific pitches because of standing waves that are produced on strings or in resonant air columns.

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