READ: Waves and Matter

1. Speed of Waves

Assume that you move one end of a rope up and down just once to generate a wave in the rope. How long will take the wave to travel down the rope to the other end? It depends on the speed of the wave.

The Speed of a Wave

Wave speed is the distance a wave travels in a given amount of time, such as the number of meters it travels per second. Wave speed (and speed in general) can be represented by the equation:

Speed = 

Wave Speed, Wavelength, and Wave Frequency

Wave speed is related to both wavelength and wave frequency. Wavelength is the distance between two corresponding points on adjacent waves. Wave frequency is the number of waves that pass a fixed point in a given amount of time. This equation shows how the three factors are related:

Speed = Wavelength x Wave Frequency

In this equation, wavelength is measured in meters and frequency is measured in hertz (Hz), or number of waves per second. Therefore, wave speed is given in meters per second, which is the SI unit for speed.

Q: If you increase the wavelength of a wave, does the speed of the wave increase as well?

A: Increasing the wavelength of a wave doesn’t change its speed. That’s because when wavelength increases, wave frequency decreases. As a result, the product of wavelength and wave frequency is still the same speed. At the following URL, you can see what happens to the wavelength when the frequency of a wave increases.

The Medium Matters

The speed of most waves depends on the medium, or the matter through which the waves are traveling. Generally, waves travel fastest through solids and slowest through gases. That’s because particles are closest together in solids and farthest apart in gases. When particles are farther apart, it takes longer for the energy of the disturbance to pass from particle to particle through the medium. At the following URL, you can watch an animation showing what happens when a wave passes from one medium to another.

CK-12 Foundation, Physical Science. http://creativecommons.org/licenses/by-nc-sa/3.0/

2. Waves and Matter

Did you ever hear an echo of your own voice? An echo occurs when sound waves bounce back from a surface that they can’t pass through. The girl pictured here is trying to create an echo by shouting toward a large building. When the sound waves strike the wall of the building, most of them bounce back toward the girl, and she hears an echo of her voice. An echo is just one example of how waves interact with matter.

How Waves Interact with Matter

Waves interact with matter in several ways. The interactions occur when waves pass from one medium to another. The types of interactions are reflection, refraction, and diffraction. Each type of interaction is described in detail below. You can see animations of the three types at this URL:http://www.acoustics.salford.ac.uk/schools/teacher/lesson3/flash/whiteboardcomplete.swf

Reflection

An echo is an example of wave reflection. Reflection occurs when waves bounce back from a surface they cannot pass through. Reflection can happen with any type of waves, not just sound waves. For example, light waves can also be reflected. In fact, that’s how we see most objects. Light from a light source, such as the sun or a light bulb, shines on the object and some of the light is reflected. When the reflected light enters our eyes, we can see the object.

Reflected waves have the same speed and frequency as the original waves before they were reflected. However, the direction of the reflected waves is different. When waves strike an obstacle head on, the reflected waves bounce straight back in the direction they came from. When waves strike an obstacle at any other angle, they bounce back at the same angle but in a different direction. This is illustrated in diagram below. In this diagram, waves strike a wall at an angle, called the angle of incidence. The waves are reflected at the same angle, called the angle of reflection, but in a different direction. Notice that both angles are measured relative to a line that is perpendicular to the wall.

Refraction

Refraction is another way that waves interact with matter. Refraction occurs when waves bend as they enter a new medium at an angle. You can see an example of refraction in the picture below. Light bends when it passes from air to water or from water to air. The bending of the light traveling from the fish to the man’s eyes causes the fish to appear to be in a different place from where it actually is.

Waves bend as they enter a new medium because they start traveling at a different speed in the new medium. For example, light travels more slowly in water than in air. This causes it to refract when it passes from air to water or from water to air.

Q: Where would the fish appear to be if the man looked down at it from straight above its actual location?

A: The fish would appear to be where it actually is because refraction occurs only when waves (in this case light waves from the fish) enter a new medium at an angle other than 90 °.

Diffraction

Did you ever notice that you can hear sounds around the corners of buildings even though you can’t see around them? The figure  below shows why this happens. As you can see from the figure, sound waves spread out and travel around obstacles. This is called diffraction. It also occurs when waves pass through an opening in an obstacle. All waves may be diffracted, but it is more pronounced in some types of waves than others. For example, sound waves bend around corners much more than light does. That’s why you can hear but not see around corners.  

For a given type of waves, such as sound waves, how much the waves diffract depends on the size of the obstacle (or opening in the obstacle) and the wavelength of the waves. The figure below shows how the amount of diffraction is affected by the size of the opening in a barrier. Note that the wavelength of the wave is the distance between the vertical lines.

3. Summary and Vocabulary

Summary

• Wave speed is the distance a wave travels in a given amount of time, such as the number of meters it travels per second.
• The speed of most waves depends on the medium, or the matter through which they are traveling. Generally, waves travel fastest through solids and slowest through gases.
• Three ways that waves may interact with matter are reflection, refraction, and diffraction.
• Reflection occurs when waves bounce back from a surface that they cannot pass through.
• Refraction occurs when waves bend as they enter a new medium at an angle and start traveling at a different speed.
• Diffraction occurs when waves spread out as they travel around obstacles or through openings in obstacles.

Vocabulary

• wave speed: How far a wave travels in a given amount of time; calculated as wavelength multiplied by wave frequency.
• diffraction: Bending of a wave around an obstacle or through an opening in an obstacle.
• reflection: Bouncing back of waves from a barrier they cannot pass through.
• refraction: Bending of waves as they enter a new medium at an angle and change speed.

CK-12 Foundation, Physical Science. http://creativecommons.org/licenses/by-nc-sa/3.0/