WAVES:

A way to transmit energy

.

Waves are defined as a periodic disturbance that carries energy from one place to another.

A periodic disturbance is one that happens over and over again at

regular intervals.

Like a bouncing ball, the motion repeats over and

over again.

There are two kinds of waves you should recognize.

Longitudinal or compression waves

and transverse waves.

Longitudinal waves, like sound waves, are mechanical waves because they must have a material to travel through.

The material they travel through is called the “medium”.

Air

The medium can be any material.

Water

WoodRock

Anything!

Sometimes longitudinal waves are referred to as compression waves because they are caused by a vibrating object compressing and rarefying the atoms or molecules of the medium.

Vibrating object

Alternating areas of compression and rarefaction in the medium.

•By vibrating back and forth, the vibrating object alternates between compressing and rarefying the molecules of the medium.

Areas of compression are high pressure areas where the molecules are squeezed closer together by a vibrating object.

Areas of rarefaction are low pressure areas where the vibrating object pulls the molecules further apart.

In longitudinal waves the molecules of the medium vibrate along with the vibrating object. Molecules of the medium vibrate back and forth on each side of a fixed resting point, transferring their kinetic energy to adjacent molecules. In this way the energy is transferred, but the molecules remain in place.

Longitudinal waves get their name because the vibration that puts energy into the wave vibrates in the same plane in which the energy moves.

In a transverse wave the particle displacement is perpendicular to the

direction of the energy transfer.

The particles do not move along with the wave, they simply vibrate up and down about their individual rest

positions as the wave passes by. Pick a single particle and watch its motion.

Transverse Waves

The wavelength of a wave is the distance between successive vibrations. Wavelength is measured in meters. Here is how wavelength is measured on a transverse wave.

This diagram shows how wavelength can be measured on a longitudinal wave. It is measured from compression to compression.

The Amplitude of a wave determines how much energy a wave is carrying. This can easily be seen in a transverse wave where the wave height is the amplitude. Obviously, the higher the wave crest the higher the amplitude, and the greater the wave energy.

In longitudinal waves amplitude is a measure of the degree of compression. The more compressed the compressions are the greater the amplitude.

Low amplitude ( Less compression)

High amplitude ( greater compression)

Wave velocity is a measure of how fast the energy is being transferred. Wave velocity is measured in meters per second (m/s).

Wave frequency, for a transverse wave, is a measure of how many wave crests pass a point in one second. Wave frequency is measured in Hertz. 1Hertz is equal to 1wave per second. If 5 waves pass the smiley in one second, then the wave frequency is 5Hz or 5 waves per second.

Longitudinal wave frequency is how many compressions pass a point in one second.

Wave period, for transverse and longitudinal waves, is the time it takes for one complete wave ( one wavelength) to pass a point. If it takes .3seconds for a single wave to pass the smiley then the wave period is .3s. Wave period is measured in seconds.

The relationship between wave frequency, wave length, and wave velocity is expressed in a very simple equation, V=f λ. In this equation V is velocity measured in m/s, f is frequency measured in Hz, and λ is wavelength, measured in meters. λ is the Greek letter Lambda. Shown below is an example problem using this equation.EXAMPLE 1: What is the velocity of a longitudinal wave with a frequency of 2000Hz and a wavelength of .17m?

Solution

Three ways to use the wave equation.

Hmmm… they look like they are the inverse of each other!

What is the frequency of a wave with a period of .17s

Just find the inverse of .17s

Two More Equations

Wave period is measured in seconds per wave.

Wave frequency is measured in waves per second…

The answer is 5.9Hz

What is the Period of a wave with a frequency of .046Hz?

Just find the inverse of .046Hz

The answer is 21.7s

The source of a wave can be stationary, such as a bell, a light bulb, or even a ripple spreading outward in a pond of water, or it can be moving. When the source of a wave is moving the waves are compressed closer together in the direction of motion and spread further apart behind the source. This changes the frequency of the wave.

Moving source

Stationary source

THE DOPPLER EFFECT:

an apparent change in frequency caused by the motion of the

wave source

Joe hears a low

frequency

Moe hears a higher

frequency

THE DOPPLER EFFECT AS IT APPLIES TO SOUND WAVES

Here is a situation in which you will recognize the Doppler effect. The the driver of a car keeps a hand on the horn constantly as he approaches and then passes an observer. Listen to the high pitch as the car approaches and the low pitch after the car passes the observer.

DOPPLER EFFECT

An observer on the Earth sees light shifted towards the red ( lower frequency) end of the spectrum. This is referred to as the RED

SHIFT.

THE DOPPLER EFFECT AS IT APPLIES TO LIGHT WAVES

Low frequency long wavelength

end of the spectrum

To be continued…