Or What’s the fine for breaking the sound barrier? Wave Phenomena Or What’s the fine for breaking the sound barrier? Standards: 4f Students know how to identify the phenomena of interference and the Doppler effect and know these are characteristic wave properties.

Wave Interference Occur when two or more waves are at same place at the same time. Total Constructive Interference occurs when waves are in phase and results in larger amplitudes Total destructive interference occurs when waves are out of phase and results in smaller amplitudes Occurs for all waves Questions: Under what conditions does wave interference occur? Draw pictures of two waves which are constructively interfering. Do the same for two waves that are destructively interfering. For what type of waves does interference occur? Activities: Slinky demo showing constructive and destructive interference with first pulses and then standing waves. State that whenever two or more waves are in the same place at the same time interference occurs. Draw the constructive and destructive case on board. Define what is meant by in phase and adding by superposition. Draw triangle and square wave on board and ask what superposition would produce? (House) Discuss the modulation of an AM carrier wave. Wave transparency showing longitudinal and transverse waves adding.

Standing Waves Occurs when two waves with same amplitude frequency and wavelength travel in opposite directions and interfere. Nodes are places of destructive interference Antinodes are places of constructive interference Standing waves are reinforced waves so result in loudest sounds, most motion and brightest light. For string fixed at both ends length=n l/2 where n is 1,2,3 For pipe open at one end length = n l/4 where n is 1,3,5,… For pipe open at both ends length=nl/2 where n =1,3,5… Questions: Under what conditions do standing waves occur? What type of interference occurs at a node? How about an antinode? What part do standing waves play in musical instrument design? Activities: If haven’t already show slinky standing waves and go through the requirements for such a wave. Demo standing wave with shoe string vibrator. Point out nodes and antinodes and discuss what type of interference is going on at each. Turn off lights and demo string vibrator. Again point out nodes and antinodes. Also show how different standing waves can be gotten if string length is changed. Discuss how the same is true of musical instruments whose horn length or string length determine the note that is heard. For physics E derive the formula length = n wavelength /2 for string fixed at both ends. Fundamental frequency = 1st harmonic = lowest and loudest tone produced.

Doppler Effect The apparent shift of frequency and wavelength due to the relative motion of a wave’s source and receiver. When source and receiver move away from one another the wavelength increases and the frequency decreases When source and receiver approach one another the wavelength decreases and the frequency increases Velocity of wave remains the same regardless of motion Applies to all types of waves. Question: Under what conditions does the Doppler effect occur? What happens to the frequency, wavelength and velocity of a wave if its source and receiver approach one another? What if they move away from one another? What type of waves can experience the Doppler effect? Activities: Demonstrate Doppler effect with nerf ball with buzzer in it. Show transparency and go through bopping bug explanation. Emphasize that frequency and wavelength change but the speed of the wave remains the same. The effect is due to the source catching up or moving away from the waves it produces. State all waves can experience this and briefly mention red and blue shift of light. Ask what would be heard if person ran towards oncoming ambulance? Observed Frequency =Emitted frequency (v wave /v wave-v source)

Shock and Bow Waves Bow and shock waves are made of matter that has been forced together. They occur when the speed of a wave source is greater than the wave it produces. Bow waves are 2D while shock waves are 3D Examples: Wake behind boat, Shock wave of supersonic aircraft. Questions: Under what circumstances do shock and bow waves form? In what order do the following occur: plane flies overhead, plane breaks sound barrier, sonic boom is heard? Activites: Show next two examples on transparence and continue bopping bug example. Describe what bug would see behind it and ask if this isn’t what we see behind a fast moving boat? Extend above idea to the shockwave example and then pose question about Jet and sonic boom arrival times. Show Chp 40 Video of FB 111 flying by camera. Show picture of fighter jets flying through condensation clouds from Robinson Disk. If have time ask if a fighter pilot can hear the engines? How about his own voice?

Check Question In what order do the following occur: plane is seen overhead, plane breaks sound barrier, sonic boom is heard?

Check Question Blue light has a higher frequency than red light. If a star is blue shifted, is the star coming towards us or moving away from us?