By the end of this section, you will be able to do the following:. The learning objectives in this section will help your students master the following standards:. In addition, the High School Physics Laboratory Manual addresses content in this section in the lab titled: Waves, as well as the following standards:.
Review properties of waves—amplitude, period, frequency, velocity and their inter-relations. Sound is a wave. More specifically, sound is defined to be a disturbance of matter that is transmitted from its source outward. A disturbance is anything that is moved from its state of equilibrium. Some sound waves can be characterized as periodic waves, which means that the atoms that make up the matter experience simple harmonic motion. A vibrating string produces a sound wave as illustrated in Figure This creates slightly higher and lower pressures.
The higher pressure The pressure disturbance moves through the air as longitudinal waves with the same frequency as the string. Some of the energy is lost in the form of thermal energy transferred to the air. You may recall from the chapter on waves that areas of compression and rarefaction in longitudinal waves such as sound are analogous to crests and troughs in transverse waves.
The amplitude of a sound wave decreases with distance from its source, because the energy of the wave is spread over a larger and larger area. But some of the energy is also absorbed by objects, such as the eardrum in Figure Figure From this figure, you can see that the compression of a longitudinal wave is analogous to the peak of a transverse wave, and the rarefaction of a longitudinal wave is analogous to the trough of a transverse wave. Just as a transverse wave alternates between peaks and troughs, a longitudinal wave alternates between compression and rarefaction.
Ask them why the sound of thunder is heard much after the lightning is seen during storms. This phenomenon is also observed during a display of fireworks. Through this discussion, develop the concept that the speed of sound is finite and measurable and is much slower than that of light.
The speed of sound varies greatly depending upon the medium it is traveling through. The more rigid or less compressible the medium, the faster the speed of sound. The greater the density of a medium, the slower the speed of sound. The speed of sound in air is low, because air is compressible. Because liquids and solids are relatively rigid and very difficult to compress, the speed of sound in such media is generally greater than in gases.
Table Students might be confused between rigidity and density and how they affect the speed of sound. The speed of sound is slower in denser media. Solids are denser than gases. However, they are also very rigid, and hence sound travels faster in solids. Stress on the fact that the speed of sound always depends on a combination of these two properties of any medium. Sound, like all waves, travels at certain speeds through different media and has the properties of frequency and wavelength.
Sound travels much slower than light—you can observe this while watching a fireworks display see Figure The relationship between the speed of sound, its frequency, and wavelength is the same as for all waves:.
Recall that wavelength is defined as the distance between adjacent identical parts of a wave. The wavelength of a sound, therefore, is the distance between adjacent identical parts of a sound wave. Just as the distance between adjacent crests in a transverse wave is one wavelength, the distance between adjacent compressions in a sound wave is also one wavelength, as shown in Figure The frequency of a sound wave is the same as that of the source.
For example, a tuning fork vibrating at a given frequency would produce sound waves that oscillate at that same frequency. The frequency of a sound is the number of waves that pass a point per unit time. Fret placements on instruments such as guitars, banjos, and mandolins, are mathematically determined to give the correct interval or change in pitch. When the string is pushed against the fret wire, the string is effectively shortened, changing its pitch. Ask students to experiment with strings of different lengths and observe how the pitch changes in each case.
How does gravity affect the universe? What does red shift mean? When looking at a spectrum of light from a star, how can we tell that the light has undergone What is redshift and blueshift? What is the redshift of the CMB surface? What is the redshift of the Andromeda galaxy? How do scientists know that there is redshift from a star going away and EM waves have changed lengths? How does redshift differ from blueshift? As is true for all waves, light travels in straight lines and acts like a ray when it interacts with objects several times as large as its wavelength.
However, when it interacts with smaller objects, it displays its wave characteristics prominently. Interference is the hallmark of a wave, and in Figure 1 both the ray and wave characteristics of light can be seen.
The laser beam emitted by the observatory epitomizes a ray, traveling in a straight line. However, passing a pure-wavelength beam through vertical slits with a size close to the wavelength of the beam reveals the wave character of light, as the beam spreads out horizontally into a pattern of bright and dark regions caused by systematic constructive and destructive interference.
Rather than spreading out, a ray would continue traveling straight ahead after passing through slits. The most certain indication of a wave is interference. This wave characteristic is most prominent when the wave interacts with an object that is not large compared with the wavelength. Interference is observed for water waves, sound waves, light waves, and as we will see in Special Relativity for matter waves, such as electrons scattered from a crystal.
Figure 1.
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