Waves

Georgia Perimeter College

Objectives

  1. Demonstrate an understanding of the differences between electromagnetic and mechanical waves and the way in which they are transmitted.
  2. Demonstrate an understanding of the differences between transverse and longitudinal waves.
  3. Demonstrate an understanding of the properties of waves.
  4. Diagram and label the parts of a wave.   
  5. Demonstrate an understanding of the ways in which waves interact with each other or with an object in the environment.
This section addresses, in whole or in part, the following Georgia GPS standard(s):
  • S1P1. Students will investigate light and sound.
  • S4P2. Students will demonstrate how sound is produced by vibrating objects and how sound can be varied by changing the rate of vibration.
  • S8P4. Students will explore the wave nature of sound and electromagnetic radiation. a. Identify the characteristics of electromagnetic and mechanical waves.
    b. Describe how the behavior of light waves is manipulated causing reflection, refraction, diffraction, and absorption.
    d. Describe how the behavior of waves is affected by medium (such as air, water, solids).
    f. Diagram the parts of the wave and explain how the parts are affected by changes in
    amplitude and pitch.

This section addresses, in whole or in part, the following Benchmarks for Scientific Literacy:
  • Vibrations in materials set up wavelike disturbances that spread away from the source. Sound and earthquake waves are examples. These and other waves move at different speeds in different materials.
  • Waves can superpose on one another, bend around corners, reflect off surfaces, be absorbed by materials they enter, and change direction when entering a new material. All these effects vary with wavelength. The energy of waves (like any form of energy) can be changed into other forms of energy.

This section addresses, in whole or in part, the following National Science Education Standards:
  • Energy is a property of many substances and is associated with heat, light, electricity, mechanical motion, sound, nuclei, and the nature of a chemical. Energy is transferred in many ways.
  • Light interacts with matter by transmission (including refraction), absorption, or scattering (including reflection). To see an object, light from that object--emitted by or scattered from it--must enter the eye.
  • Waves, including sound and seismic waves, waves on water, and light waves, have energy and can transfer energy when they interact with matter.

Waves

A wave is any disturbance that transmits energy through matter or space.

Sound is a type of energy that requires waves traveling through matter.
The material or substance through which a wave may travel is called the medium. The medium for a wave can be any of the common states of matter: solid, liquid, or gas. Sound waves require a medium.

The medium does not move with the energy.

Sound waves travel by vibration of particles.
If there are no particles, there will be no sound.

Waves that require a medium are called mechanical waves.
(In addition to sound waves, ocean waves and seismic waves require a medium.
Therefore ocean waves and seismic waves are mechanical waves.)

Waves that do not require a medium are called electromagnetic waves (or E-M waves). Electromagnetic waves can travel through solids, liquids, and gases, but they travel fastest through empty space.

Waves are classified based on the direction in which the particles of the medium vibrate compared with the direction in which the waves travel. There are three classifications of waves based on this criterion.

  1. Transverse waves are waves in which the particles of the medium vibrate with an up and down motion. Particles in a transverse wave move perpendicular to the direction that the wave is traveling. The crest is the highest point of a transverse wave. The trough is the lowest point of a transverse wave. Although electromagnetic waves do not require a medium, they are considered transverse waves.


  2. (When studying seismic waves associated with earthquakes, these are the S-waves.)


    Animation used with permission from The Tech Museum of Innovation, San Jose, CA.
     

  3. Longitudinal waves are waves in which the particles of the medium vibrate back and forth along the path that the wave travels. A compression is a section of longitudinal wave where the particles are crowded together. A rarefaction is a section of the wave where particles are less crowded than normal. Sound waves are longitudinal waves.

  4. Animation used with permission from The Tech Museum of Innovation, San Jose, CA.

    (When studying seismic waves associated with earthquakes, these are the P-waves.)

     

  5. When waves occur at or near the boundary between two media, a transverse wave and a longitudinal wave can combine to form a surface wave.

    An example of a surface wave is a type of seismic wave formed as a result of an earthquake.


Properties of Waves

Waves have important properties that determine how they transmit energy.

The amplitude of a wave is the maximum distance the wave vibrates from its rest position. The rest position of a wave is where the particles of a medium stay when there are no disturbances. The larger the amplitude, the greater is the energy of the wave.


From NASA.

Wavelength is the distance between two adjacent crests or compressions in a wave. Therefore wavelength is the distance from any point on a wave to the corresponding point on the next wave.


Image courtesy of NASA.

Frequency is the number of waves produced in a given amount of time. Frequency can be measured by counting either the number of crests or the number of troughs that pass a point in a certain amount of time. Frequency is expressed in hertz (Hz). Higher frequency, just like higher amplitude, means more energy.

Wave speed is the speed at which a wave travels. The speed of a wave depends on the medium in which the wave is traveling. Sound waves travel fastest in solids, next fastest in liquids, and slowest in gases. Wave speed can be calculated by multiplying the wavelength (represented with the Greek letter lambda) times the frequency of the wave.

 


Determining Wavelength

From NASA.

If you study waves, you will find that wavelength and frequency are related by an equation

Speed of the wave = Frequency x Wavelength

When you choose your radio channel, you select the Frequency. The default value is 90.0 MHz, or 90,000,000 waves per second - for an FM station. When you choose an AM station, the default frequency is 100.0 kHz, or 100,000 waves per second.

For radio waves or light waves, the Speed of the wave is the speed of light. In the United States, we often think of that as 186,000 miles per second. That's as fast as anything can go - according to Einstein's theory of relativity - which is based on Maxwell's equations for electricity and magnetism. In other countries (or when scientists use this equation), they think of the speed of light as 299,792,458 meters per second. The speed of light is the same whether we use miles per second or meters per second - only the number is different.

We use the equation to find the Wavelength - we just rearrange it so that the Wavelength is on the left

Wavelength = Speed of the wave / Frequency

Because the numbers are so big, doing this calculation by hand is a lot of work. If you were going to do it that way, you would have to solve the division problem

299,792,458 / 90,000,000

See http://eosweb.larc.nasa.gov/EDDOCS/wavelength.html for an online calculator to convert radio frequencies to wavelength in feet or meters.


Wave Interactions

Waves that meet each other or an object in the environment may interact. There are several types of interactions that waves may have.

Reflection occurs when a wave bounces back after striking a barrier. Reflected sound waves are called echoes; reflected light waves allow us to see objects.

Refraction is the bending of a wave as it passes at an angle from one medium to another. One common example of refraction of light waves is the broken pencil effect that can be observed when a pencil is placed in a glass of water. The pencil seems to be "broken" at the surface of the water as the light waves go from the air to the water.

Diffraction is the bending of waves around a barrier or through an opening. The amount of diffraction a wave experiences depends on two factors: the wavelength of the wave and the size of the barrier or opening the wave encounters. Sound travels around corners because it has relatively larger wavelengths than light. We can hear sounds around corners. We can't see around corners because light has a very small wavelength.

Interference is the result of two or more waves overlapping. Waves can meet, share the same space, and pass through each other. There are two types of interference:

  1. Constructive
  2. Destructive

Constructive interference has the net effect of increasing amplitude. When the crests of one wave overlap the crests of another wave, the result is addition of the two waves to create a wave with greater amplitude.


Constructive interference.  From NASA.

 


Image courtesy of NASA.

Destructive interference decreases amplitude. Destructive interference occurs when the crests of one wave and the troughs of another wave overlap. The result is to create a wave with smaller amplitude.


Destructive interference.  From NASA.


Image courtesy of NASA.

Interference can create standing waves. A standing wave is a wave that forms a stationary pattern in which portions of the wave are at the rest position due to total destructive interference and other portions have large amplitude due to constructive interference.

Resonance is another example of interaction of waves. Resonance occurs when an object vibrating at or near the natural frequency of a second object causes the second object to vibrate. Resonant frequencies are the frequencies at which standing waves are produced.

An example of the destructive results of resonance occurred in July 1940 when a bridge over the Tacoma Narrows in Washington was destroyed when a strong wind caused the bridge to start vibrating and continued until the bridge collapsed.
Video.


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Content provided by Ms. Susan Brooks, Renfroe Middle School
Some content from NASAExplores.com website http://www.nasaexplores.com/show_912_student_st.php?id=021224105329 which is now inactive.
Sound icon from NASA
Some content from http://eosweb.larc.nasa.gov/EDDOCS/wavelength.html and http://www.nasa.gov/audience/foreducators/postsecondary/features/f_interference_prt.htm

Page created by Pamela J.W. Gore
Georgia Perimeter College,
Clarkston, GA

Page created November 23 - December 2, 2006
Modified May 28, 2007
Modified April 9, 2009