PS-7.1    Illustrate ways that the energy of waves is transferred by interaction with matter (including transverse and longitudinal /compressional waves).
Key Concepts
Wave:  transverse, longitudinal/compressional
Energy transfer
Medium 
    All waves really do only one thing, they carry energy from one place to another. Waves can be either mechanical or electromagnetic. Mechanical waves are those waves that need some form of matter to travel through. Mechanical waves can be transverse waves, like water waves, or longitudinal compression waves like sound waves.
    The matter that a mechanical wave travels through is called the medium. The medium can be anything, air, water, wood, rock...anything! Sound waves are an example of a mechanical wave. Sound waves are mechanical waves caused by a any vibrating object. For example, when a tuning fork vibrates, the back and forth vibrations of the fork set the surrounding air molecules vibrating back and forth. When molecules vibrate in this way they transfer their energy to adjacent air molecules, then these molecules in turn transfer their energy to molecules adjacent to them and so on. It is important to remember that mechanical energy waves, like sound waves, are three dimensional, that is, they can move outward from a source in all directions. Here is an animation of a sound wave generated by a tuning fork. Notice that the tine of the tuning fork moves back and forth. In sound waves the particles move back and forth, parallel to the direction of the wave. The back and forth vibrating motion of the object only displaces the particles of the medium over a short distance. The particles simply vibrate back and forth about a resting point. After a sound wave passes, the molecules remain in approximately the area where they were before the wave passed. This animation shows the motion of a single particle ( molecule) of the medium as a sound wave moves energy through it.   
       
By clicking here you can see an animation of a longitudinal compression wave as it moves through a medium. Note that the particles are only displaced a short distance on either side of their original location, they simple move back and forth.
Transverse waves are waves in which the vibrations are at right angles to the direction of energy transfer. In other words, the direction of the vibration "transverses" the direction of energy transfer. Like longitudinal waves, the particles of the medium are only displaced slightly from their original rest position. Waves produced by wiggling a rope or string are transverse waves.
 
A Slinky spring is a great way to visualize the difference between longitudinal and transverse waves. Here is a short video clip of a Slinky being used to illustrate a transverse wave and then a longitudinal compression wave. Note that when the transverse wave is created the hand motion ( vibration) transverses the direction that the wave energy moves.  When the longitudinal or compression wave is created the hand motion ( vibration) is parallel to the direction of the wave energy. Here is a side by side comparison of a transverse wave and a longitudinal wave on a Slinky spring.
Light waves
    Light waves (or other electromagnetic radiation) is energy that can be transmitted without a medium. Electromagnetic energy is generated by electrons. Electrons have both a magnetic field and an electric field. When electrons vibrate these fields an electromagnetic wave is produced. Electromagnetic waves move best in a vacuum. Most electromagnetic waves travel at a speed of 2.99 x 108 m/s. That's about 186 000miles per second. At this speed it takes light about 8 1/2 min. to travel from the Sun to the Earth. Light waves travel in straight lines in all directions from the source of the light.
Click here to watch a Powerpoint® presentation on waves.