By the end of this section, you will be able to do the following:
- Define amplitude, frequency, period, wavelength, and velocity of a wave
- Relate wave frequency, period, wavelength, and velocity
- Solve problems involving wave properties
The learning objectives in this section will help your students master the following standards:
- (7) Science concepts. The student knows the characteristics and behavior of waves. The student is expected to:
- (B) investigate and analyze the characteristics of waves, including velocity, frequency, amplitude, and wavelength, and calculate using the relationship between wave speed, frequency, and wavelength;
- (D) investigate the behaviors of waves, including reflection, refraction, diffraction, interference, resonance, and the Doppler effect.
Section Key Terms
[BL][OL][AL] Review amplitude, period, and frequency for simple harmonic motion.
In the chapter on motion in two dimensions, we defined the following variables to describe harmonic motion:
- Amplitude—maximum displacement from the equilibrium position of an object oscillating around such equilibrium position
- Frequency—number of events per unit of time
- Period—time it takes to complete one oscillation
For waves, these variables have the same basic meaning. However, it is helpful to word the definitions in a more specific way that applies directly to waves:
- Amplitude—distance between the resting position and the maximum displacement of the wave
- Frequency—number of waves passing by a specific point per second
- Period—time it takes for one wave cycle to complete
In addition to amplitude, frequency, and period, their wavelength and wave velocity also characterize waves. The wavelength is the distance between adjacent identical parts of a wave, parallel to the direction of propagation. The wave velocity is the speed at which the disturbance moves.
Wave velocity is sometimes also called the propagation velocity or propagation speed because the disturbance propagates from one location to another.
Consider the periodic water wave in Figure 13.7. Its wavelength is the distance from crest to crest or from trough to trough. The wavelength can also be thought of as the distance a wave has traveled after one complete cycle—or one period. The time for one complete up-and-down motion is the simple water wave’s period T. In the figure, the wave itself moves to the right with a wave velocity vw. Its amplitude X is the distance between the resting position and the maximum displacement—either the crest or the trough—of the wave. It is important to note that this movement of the wave is actually the disturbance moving to the right, not the water itself; otherwise, the bird would move to the right. Instead, the seagull bobs up and down in place as waves pass underneath, traveling a total distance of 2X in one cycle. However, as mentioned in the text feature on surfing, actual ocean waves are more complex than this simplified example.
Amplitude, Period, Frequency, and Wavelength of Periodic Waves
This video is a continuation of the video “Introduction to Waves” from the "Types of Waves" section. It discusses the properties of a periodic wave: amplitude, period, frequency, wavelength, and wave velocity.
The crest of a wave is sometimes also called the peak.
The Relationship between Wave Frequency, Period, Wavelength, and Velocity
Since wave frequency is the number of waves per second, and the period is essentially the number of seconds per wave, the relationship between frequency and period is
just as in the case of harmonic motion of an object. We can see from this relationship that a higher frequency means a shorter period. Recall that the unit for frequency is hertz (Hz), and that 1 Hz is one cycle—or one wave—per second.
The speed of propagation vw is the distance the wave travels in a given time, which is one wavelength in a time of one period. In equation form, it is written as
From this relationship, we see that in a medium where vw is constant, the higher the frequency, the smaller the wavelength. See Figure 13.8.
[BL] For sound, a higher frequency corresponds to a higher pitch while a lower frequency corresponds to a lower pitch. Amplitude corresponds to the loudness of the sound.
[BL][OL] Since sound at all frequencies has the same speed in air, a change in frequency means a change in wavelength.
[Figure Support] The same speaker is capable of reproducing both high- and low-frequency sounds. However, high frequencies have shorter wavelengths and are hence best reproduced by a speaker with a small, hard, and tight cone (tweeter), whereas lower frequencies are best reproduced by a large and soft cone (woofer).
These fundamental relationships hold true for all types of waves. As an example, for water waves, vw is the speed of a surface wave; for sound, vw is the speed of sound; and for visible light, vw is the speed of light. The amplitude X is completely independent of the speed of propagation vw and depends only on the amount of energy in the wave.
Waves in a Bowl
In this lab, you will take measurements to determine how the amplitude and the period of waves are affected by the transfer of energy from a cork dropped into the water. The cork initially has some potential energy when it is held above the water—the greater the height, the higher the potential energy. When it is dropped, such potential energy is converted to kinetic energy as the cork falls. When the cork hits the water, that energy travels through the water in waves.
- Large bowl or basin
- Cork (or ping pong ball)
- Measuring tape
- Fill a large bowl or basin with water and wait for the water to settle so there are no ripples.
- Gently drop a cork into the middle of the bowl.
- Estimate the wavelength and the period of oscillation of the water wave that propagates away from the cork. You can estimate the period by counting the number of ripples from the center to the edge of the bowl while your partner times it. This information, combined with the bowl measurement, will give you the wavelength when the correct formula is used.
- Remove the cork from the bowl and wait for the water to settle again.
- Gently drop the cork at a height that is different from the first drop.
- Repeat Steps 3 to 5 to collect a second and third set of data, dropping the cork from different heights and recording the resulting wavelengths and periods.
- Interpret your results.
- No, only the amplitude is affected.
- Yes, the wavelength is affected.
Students can measure the bowl beforehand to help them make a better estimation of the wavelength.
Wave on a String
In this animation, watch how a string vibrates in slow motion by choosing the Slow Motion setting. Select the No End and Manual options, and wiggle the end of the string to make waves yourself. Then switch to the Oscillate setting to generate waves automatically. Adjust the frequency and the amplitude of the oscillations to see what happens. Then experiment with adjusting the damping and the tension.
Which of the settings—amplitude, frequency, damping, or tension—changes the amplitude of the wave as it propagates? What does it do to the amplitude?
- Frequency; it decreases the amplitude of the wave as it propagates.
- Frequency; it increases the amplitude of the wave as it propagates.
- Damping; it decreases the amplitude of the wave as it propagates.
- Damping; it increases the amplitude of the wave as it propagates.
Solving Wave Problems
Calculate the Velocity of Wave Propagation: Gull in the Ocean
Calculate the wave velocity of the ocean wave in the previous figure if the distance between wave crests is 10.0 m and the time for a seagull to bob up and down is 5.00 s.
The values for the wavelength and the period are given and we are asked to find Therefore, we can use to find the wave velocity.
This slow speed seems reasonable for an ocean wave. Note that in the figure, the wave moves to the right at this speed, which is different from the varying speed at which the seagull bobs up and down.
Calculate the Period and the Wave Velocity of a Toy Spring
The woman in Figure 13.3 creates two waves every second by shaking the toy spring up and down. (a)What is the period of each wave? (b) If each wave travels 0.9 meters after one complete wave cycle, what is the velocity of wave propagation?
To find the period, we solve for , given the value of the frequency
Since one definition of wavelength is the distance a wave has traveled after one complete cycle—or one period—the values for the wavelength as well as the frequency are given. Therefore, we can use to find the wave velocity.
We could have also used the equation to solve for the wave velocity since we already know the value of the period from our calculation in part (a), and we would come up with the same answer.
The frequency of a wave is 10 Hz. What is its period?
- The period of the wave is 100 s.
- The period of the wave is 10 s.
- The period of the wave is 0.01 s.
- The period of the wave is 0.1 s.
What is the velocity of a wave whose wavelength is 2 m and whose frequency is 5 Hz?
- 20 m/s
- 2.5 m/s
- 0.4 m/s
- 10 m/s
Check Your Understanding
Use these questions to assess students’ achievement of the section’s Learning Objectives. If students are struggling with a specific objective, these questions will help identify such objective and direct them to the relevant content.
What is the amplitude of a wave?
- A quarter of the total height of the wave
- Half of the total height of the wave
- Two times the total height of the wave
- Four times the total height of the wave
- The wavelength is the distance between adjacent identical parts of a wave, parallel to the direction of propagation.
- The wavelength is the distance between adjacent identical parts of a wave, perpendicular to the direction of propagation.
- The wavelength is the distance between a crest and the adjacent trough of a wave, parallel to the direction of propagation.
- The wavelength is the distance between a crest and the adjacent trough of a wave, perpendicular to the direction of propagation.
When is the wavelength directly proportional to the period of a wave?
- When the velocity of the wave is halved
- When the velocity of the wave is constant
- When the velocity of the wave is doubled
- When the velocity of the wave is tripled