For details see book:  Hewitt (Ch.19,20,21).
There is also a short course on the Web on waves   on   sound  and on  light  which you can consult. Credits for some of the graphics belong to this source.
Demos: The basic properties of waves (including sound, light) are illustrated in the animations (credits at websites).

• Pulses and vibrations are sources of waves. Regular vibrations produce regular waves.
• Any point on a transverse wave moves up and down in a repeating pattern

• Examples of waves: waves in water, sound waves, electromagnetic waves (light, radio/TV, X-rays, gamma rays, microwaves).
• Sound waves and matter waves propagate in a medium (gases, liquids, gases), but  there are also waves that can propagate in vacuum (E&M, gravity, quantum waves of particles).
• A wave can be transverse, longitudinal, or a mixture of these.

 
Transverse and Longitudinal Waves
Animation of transverse and longitudinal wave motion.

Transverse waves
Longitudinal waves
• A wave has crests, troughs, amplitude, frequency, period, wavelength, velocity, energy.

 
• The shortest time that a point takes to return to the initial position (one vibration) is called period, T.
• The number of vibrations per second is called frequency and is measured in hertz (Hz).
• Example 5 Hz means 5 vibrations per second.
• AM radio (Kilohertz), FM radio (Megahertz = million hertz). Example USC FM-91.5 means in its antenna electrons are vibrating at a rate of 91,500,000 each second - same thing in your radio receiver.

 

Frequency (Hz) = 1/ Period (s)

speed = frequency x wavelength = wavelength / period

energy is proportional to (amplitude)2


 
• Wave motion is propagation of a disturbance - it is not motion of matter. Energy, information, propagates without any transfer of matter.

• Waves interfere with each other and enhance (constructive interference) or diminish (destructive interference) the disturbance at various points in space at various times.

• Adding transverse waves , see also Interference of two sine waves
• Adding longitudinal waves
• Interference in a ripple tank  , see also this ripple tank
• Single Slit Diffraction Animation of single slit diffraction. Double Slit Wave Interference Animation of interference pattern formed by two slit diffraction.

• The Doppler effect - is the effect perceived by an observer that receives the waves of a moving object (approaching or receding).
• The Doppler shift is a wave phenomenon. A source that emits waves appears to have shorter wavelengths (higher frequency, shift toward blue) if it is approaching an observer, and it appears to have longer wavelengths (lower frequency, shift toward red) if it is receding from an observer.
• here is a simulation of the Doppler effect. Here is another simulation where you can change the speed.
• A demonstration of the Doppler effect in the classroom.
• The light from a star that is moving away from us has its spectroscopic lines shifted toward the red end of the spectrum. This is called the "red shift", it serves as a speedometer for stars. By measuring the red shift for a star we can tell how fast it is moving away from us.
• This figure shows the spectral lines of sodium. If the source of light moves away from the observer these lines shift toward the red end. This is what is observed in starlight coming from other galaxies - so we conclude that those galaxies are moving away from Earth, and we can measure at what speed!!

• Bow waves occur when when the source travels faster than the speed of the wave in a medium (e.g.  boat in lake). See simulation.
• Shock waves. Speed boat - water boom.   Supersonic aircraft or the space shuttle - sonic boom.
• Sound is produced by vibration of matter (vocal chords, piano strings, etc.).
• Vibration of the source stimulates the vibration of surrounding matter and in turn of air molecules (or other matter molecules if in water, etc.).
• The frequency of the air molecules is normally the same as the frequency of the source.
• Pitch of sound is related to the frequency of the sound wave. High (low) pitch corresponds to high (low) frequency.
• The human ear can hear in the range 20 Hz - 20,000 Hz (depending on age). This is called the "sonic" range. The human ear is not sensitive to sounds outside of this range (infrasonic, ultrasonic)
• sound waves are longitudinal waves. Air does not travel, the pulse does.
• similar to  e.g. open door, close door (compression is like crest, rarefaction is like trough)
• tuning fork vibrations propagate in a similar fashion
• loudspeaker - paper cone - air - ear drum - electronic impulses in nerve - brain
• Speed of sound:
• in 0 ^oC air 330 m/s, in 20 ^oC air 340 m/s, this is about one million times slower than light (300,000 km/s). You hear the thunder much later than seeing the light of the thunderbolt.
• in elastic matter the speed of sound is greater (you can hear under water, you can hear tapping on table)
• Sound waves (just like light waves) can be
• reflected from surfaces (echo, reverberations) - and can be absorbed by porous surfaces (design of concert halls!)

 
•  refracted (change direction, speed, wavelength, but NOT FREQUENCY) when moving from one medium to another.

 

• Refraction of Light  Simulation of refraction and change of wavelength and wavespeed. See also this link on  refraction

• A dramatic increase in the amplitude occurs when when an object's natural frequency matches the frequency of forced oscillations - this is called a resonance.
• Pushing a swing.
• The collapse of the Tacoma-Narrows bridge in Washington is an example of resonance.
• Resonance demos in class:  the sympathetic forks,  the breaking of the wine glass.

• When two waves of slightly different frequency are combined a disturbance of fluctuating amplitude results. This is called beats.
• see this animation of beats
• tuning forks of slightly different pitches - (demo in class)
• tuning a piano

 

Reading assignment:
Read Ch. 19,20,21 , Study Next Time questions for ch19, ch20, ch21

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