Characteristics of Waves
Section 1: What are waves?
It was a long swim, but now you’re resting on the swimming raft in the lake. You hear the water lapping gently against the raft as the sun warms your skin. Suddenly a motorboat zooms by. A few seconds later, you’re bobbing wildly up and down as the boats waves hit the raft. Although the speedboat didn’t touch the raft, its energy caused waves in the water. Then the waves moved the raft – and you!
You can see and feel the water waves when you’re on a swimming raft. But did you know that many kids of waves affect you every day? Sound is a wave. Sunlight is a different kind of wave. Light, sound and water waves may seem very different, but they are all waves. What is a wave?
You can see and feel the water waves when you’re on a swimming raft. But did you know that many kids of waves affect you every day? Sound is a wave. Sunlight is a different kind of wave. Light, sound and water waves may seem very different, but they are all waves. What is a wave?
Waves & Energy
Figure 1: Motion of a Medium
Waves travel through water, but they do not carry the water. Notice the particles of the water moves in a circular motion while the wave moves from left to right.
Waves travel through water, but they do not carry the water. Notice the particles of the water moves in a circular motion while the wave moves from left to right.
A wave is a disturbance that transfers energy from place to place. In science, energy is defined as the ability to do work. To understand waves, think about the swimming raft. A wave that disturbs the surface of the water also will disturb the raft. The wave’s energy lifts the heavy raft as the wave’s passes under it. But the disturbance caused by the wave is temporary. After the wave passes, the water is calm again and the raft stops bobbing.
What carries waves?
Most kinds of waves need something to travel through. Sound waves travel through air. Water waves travel along the surface of the water. A wave can even travel along a rope. The material through which a wave travels is called a medium. Gases (such as air), liquids (such as water), and solids (such as rope) all act as mediums. Waves that require a medium through which to travel are called mechanical waves. But not all waves require a medium to travel through. Light from the sun, for example, can carry energy through empty space. If light could not travel through empty space, you could not even see the sun! Waves that can travel without medium are called electromagnetic waves. You will learn more about electromagnetic waves in a later chapter.
How do waves transfer energy?
Although mechanical waves travel through a medium, they do not carry the medium with them. Look at the particles in figure 1. When a wave travels through the medium, the particles move up and down. But the particles do not travel with the wave. After the wave passes, the particles in the water return to where they started.
Why doesn’t the medium travel along with the wave? All mediums are made of tiny particles. When a wave enters a medium, it transfers energy to the medium’s particles. The particles bump into each other, passing the waves’ energy along. To understand this, think about how food is passed at a table. You had the food for the next person, who passes it to the next person, and so on. The food is transferred, but the people don’t move. The food is like the wave’s energy, and the people are like particles in a medium.
Why doesn’t the medium travel along with the wave? All mediums are made of tiny particles. When a wave enters a medium, it transfers energy to the medium’s particles. The particles bump into each other, passing the waves’ energy along. To understand this, think about how food is passed at a table. You had the food for the next person, who passes it to the next person, and so on. The food is transferred, but the people don’t move. The food is like the wave’s energy, and the people are like particles in a medium.
What causes waves?
Energy always is required to make a wave. Mechanical waves are produced when a source of energy causes a medium to vibrate. A vibration is a repeated back-and-forth or up-and-down motion. When a vibration moves through a medium, a wave results.
Moving objects have energy. A moving object can transfer energy to a medium producing waves. For example, you can make waves by dipping your finger in water. Your finger has energy because it is moving. When your finger touches the water, it transfers energy to the water and makes waves. In the same way, a motorboat slicing through cam water transfers energy to the water and makes waves.
Moving objects have energy. A moving object can transfer energy to a medium producing waves. For example, you can make waves by dipping your finger in water. Your finger has energy because it is moving. When your finger touches the water, it transfers energy to the water and makes waves. In the same way, a motorboat slicing through cam water transfers energy to the water and makes waves.
Types of Waves
Waves move through mediums in different ways. Mechanical waves are classified by how they move. There are two types of mechanical waves: transverse waves and longitudinal waves.
Transverse Waves
Figure 2: Transverse Waves
A transverse wave moves the rupe up and down in a direction perpendicular to the direction in which the wave travels.
A transverse wave moves the rupe up and down in a direction perpendicular to the direction in which the wave travels.
When you make a wave on a rope, the wave moves from one end of the rope to the other. But the rope itself moves up and down or from side to side, at right angles to the direction in which the wave travels. Waves that move the medium at right angles to the direction in which the waves travel are called transverse waves. Transverse means “across.” As a transverse wave moves, the particles of the medium move across, or at a right angle to, the direction of the wave.
In Figure 2, you can see that the particles on the wave are first at a low point of the wave, then it is at a high point. The high part of a transverse wave is called a crest, and the low part is called a trough (trawf).
In Figure 2, you can see that the particles on the wave are first at a low point of the wave, then it is at a high point. The high part of a transverse wave is called a crest, and the low part is called a trough (trawf).
Longitudinal Waves
Figure 3: Longitudinal Waves
A longitudinal wave moves the coils of a spring toy back and forth in a direction parallel to the direction the wave travels.
A longitudinal wave moves the coils of a spring toy back and forth in a direction parallel to the direction the wave travels.
Figure 3 shows a different kind of wave. If you stretch out a slinky and push and pull one end, you can produce a longitudinal wave. Longitudinal waves (lawn juh too dug nul) move the medium parallel to the direction in which the waves travel. The coils in the spring move back and forth parallel to the wave motion.
Notice in Figure 3 that in some parts of the wave, the particles are close together. In other parts of the wave, the particles are more spread out. The parts where the coils are close together are called compressions (kum presh unz). The parts where the coils are spread out, or rarified, are called rarefactions (rair uh fak shunz).
As compressions and rarefactions travel along the spring toy, each coil moves forward and then back. The energy travels from one end of the spring to the other, creating a wave. After the wave passes, each coil returns to the position where it started.
Sound is also a longitudinal wave. In air, sound waves cause air particles to move back and forth. In areas where the particles are pushed together, compressions form. In between the compressions, particles are spread out. These are rarefactions.
Notice in Figure 3 that in some parts of the wave, the particles are close together. In other parts of the wave, the particles are more spread out. The parts where the coils are close together are called compressions (kum presh unz). The parts where the coils are spread out, or rarified, are called rarefactions (rair uh fak shunz).
As compressions and rarefactions travel along the spring toy, each coil moves forward and then back. The energy travels from one end of the spring to the other, creating a wave. After the wave passes, each coil returns to the position where it started.
Sound is also a longitudinal wave. In air, sound waves cause air particles to move back and forth. In areas where the particles are pushed together, compressions form. In between the compressions, particles are spread out. These are rarefactions.
Representing Types of Waves
Figure 4 Representing Waves
The compressions of a longitudinal wave correspond to the crests of a transverse wave. The troughs correspond to the rarefactions.
The compressions of a longitudinal wave correspond to the crests of a transverse wave. The troughs correspond to the rarefactions.
You can use diagrams to represent transverse and longitudinal waves. Transverse waves like those on a rope are easy to draw. You can draw a transverse wave as shown in Figure 4. Think of the horizontal line as the position of the rope before it is disturbed. This position I called the rest position. As the wave passes, the rope moves above or below the rest position. Remember that the crests are the highest points of the wave and the troughs are the lowest points of the wave.
To draw longitudinal waves, think of the compressions in the spring as being similar to the crest of a transverse wave. The rarefactions in the spring toy are like the trough of a transverse wave. By treating compressions as crests and rarefactions as trough, you can draw longitudinal waves in the same way as transverse waves.
To draw longitudinal waves, think of the compressions in the spring as being similar to the crest of a transverse wave. The rarefactions in the spring toy are like the trough of a transverse wave. By treating compressions as crests and rarefactions as trough, you can draw longitudinal waves in the same way as transverse waves.
Section 2: Properties of Waves
One of the most elegant and graceful Olympic sports is rhythmic gymnastics. A ribbon dancer flicks a stick attached to a ribbon, making waves that travel down the ribbon. Some of the waves are longer, while others are shorter. The rate at which the gymnast flicks her hands affects both the length and shape of the waves in the ribbon.
This is just one of many different kinds of waves. Waves can carry a little energy or a lot. They can be short or long. They can be rare or frequent. They can travel fast or slow. All waves, however, share certain properties. The basic properties of waves are amplitude, wavelength, frequency, and speed.
This is just one of many different kinds of waves. Waves can carry a little energy or a lot. They can be short or long. They can be rare or frequent. They can travel fast or slow. All waves, however, share certain properties. The basic properties of waves are amplitude, wavelength, frequency, and speed.
Amplitude
Some crests are very high, while others are very low. The distance the medium rises depends on the amplitude of the wave. Amplitude is the maximum distance that the particles of the medium carrying the wave move away from their rest positions. For example, the amplitude of a wave is the maximum distance a water particle moves above or below the surface level of calm water. You can increase the amplitude of a wave in a rope by moving your hand up and down a greater distance. To do this, you have to use more energy. This energy is transferred to the rope. Thus, the more energy a wave has, the greater its amplitude.
Amplitude of Transverse Waves
Figure 5 Amplitude, Wavelength and Frequency
The basic properties of all waves include amplitude, wavelength and frequency.
The basic properties of all waves include amplitude, wavelength and frequency.
As shown in Figure 5, the amplitude of a transverse wave is the maximum distance the medium moves up or down from its rest position. You can find the amplitude of a transverse wave by measuring the distance from the rest position to a crest or to a trough.
Amplitude of Longitudinal Waves
The amplitude of a longitudinal wave is a measure of how compressed or rarefied the medium becomes. A high-energy wave causes more compression and rarefaction than a low-energy wave. When the compressions are dense, it means that the wave’s amplitude is large.
Wavelength
Figure 6 Wavelength
A wavelength is the distance between two corrosponding parts of a wave.
A wave travels a certain distance before it starts to repeat. The distance between two corresponding parts of a wave is its wavelength. You can find the wavelength of a transverse wave by measuring the distance from crest to crest, as shown in Figure 6. Or you could measure from trough to trough. The wavelength of a longitudinal wave is the distance between compressions.
Frequency
Wave frequency is the number of complete waves that pass a given point in a certain amount of time. For example, if you make waves on a rope so that one wave passes by every second the frequency is 1 wave per second. How can you increase the frequency? Simply move your hand up and down more quickly, perhaps two or three times per second. To decrease the frequency, move your hand up and down more slowly.
Frequency is measured in units called Hertz (Hz). A wave that occurs every second has a frequency of 1 Hz. If two waves pass you every second, then the frequency of the wave is 2 waves per second or 2 Hz. The hertz was named after Heinrich Hertz, the German scientist who discovered radio waves.
Frequency is measured in units called Hertz (Hz). A wave that occurs every second has a frequency of 1 Hz. If two waves pass you every second, then the frequency of the wave is 2 waves per second or 2 Hz. The hertz was named after Heinrich Hertz, the German scientist who discovered radio waves.
Figure 7: Wave Machine
On the Phet program above, do the following:
Click on "oscillate" then mess with the controls below to see how amplitude, frequency and wavelength are interconnected with each other.
On the Phet program above, do the following:
Click on "oscillate" then mess with the controls below to see how amplitude, frequency and wavelength are interconnected with each other.