Wave Interactions

1. Wave Interactions http://www.physicsclassroom.com/mmedia/waves/ltm.cfm

2. Boundary • Where 2 media meet • Where wave conditions change • Reflection, transmission, or both can occur

3. Waves at Boundaries A wave travels from one medium to another (i.e. air into water) • Waves enter the new medium upright • How much energy is transmitted into the new medium depends on how different the media are. • The greater the difference in the media the less transmission • More of the energy is reflected (returns to the original medium)

4. Waves at Boundaries When the wave goes from low density to high density the reflected wave is flipped (In this case it would return on the bottom)

6. Waves at Boundaries

7. Free/Open/Loose Boundary Boundary that can move • Boundary moves in response to the wave • When wave travels from: • More dense to less dense • Wave remains upright upon reflection • Less dense to more dense • Wave flips upon reflection

8. Fixed Boundary Boundary that does not move • Most energy is reflected • Wave flips upon reflection (crest becomes a trough) Boundary Conditions

9. Free Boundary Boundary that does move • Most energy is reflected • Wave maintains orientation upon reflection (crest stays a crest) Boundary Conditions

10. Absorption In the real world • some of the energy is absorbed by the medium in the form of heat (usually as a result of friction) • Some of the wave amplitude is lost as a result

11. Interference Principle of Superposition -when the particles of a medium are affected by multiple waves at the same time -the oscillation of each particle is the combination of the oscillation that would occur as a result of each individual wave

12. Types of Interference Constructive Interference: When multiple waves add up to create a larger amplitude

13. Types of Interference Destructive Interference: When multiple waves add up to create a smaller amplitude

14. Interference Constructive Interference Destructive Interference

15. Standing Waves • occur when a wave is confined between two boundaries • Wave is reflected back and forth • As the frequency changes the standing wave pattern changes • low frequency, long wavelength • high frequency, short wavelength

16. Standing Waves Standing Waves applet

17. Standing Waves Interference occurs between the wave and its reflection • Where destructive interference occurs, nodes form • Node – particle remains at equilibrium • Where maximum constructive interference occurs, antinodes form • Antinode – particle is farthest from equilibrium

18. Diffraction Bending of waves around a boundary - no change in medium Huygen’s Principle • Each molecule that is part of a wave front emits its own wave front • Individual wave fronts are circular • When many circular wave fronts overlap only the combined affect is seen • Very far from an individual source, the combined wave front appears straight

19. Diffraction • When a straight wave front hits a boundary the edges of the individual wave fronts show

20. Diffraction The amount of bending that is noticeable depends on the wavelength and the size of the opening Wavelength ≥ slit size → more bending Wavelength < slit size → less bending

21. Diffraction and Interference When there are multiple openings the diffracted waves will overlap and interfere Constructive interference occurs when wave fronts are in phase i.e. where crests happen at the same time and place (where lines overlap on the diagram) Destructive interference occurs when wave fronts are out of phase i.e. where a crest and a trough happen at the same time and place (where lines overlap spaces on the diagram)

22. Diffraction and Interference • Where constructive interference occurs an increase in amplitude occurs (brighter area on diagram) • Wave form is magnified • Where destructive interference occurs a decrease in amplitude occurs (grey areas on diagram) • Wave form is cancelled out

23. What Depends on What • Frequency: depends on source • Amplitude: depends on energy • Speed: depends on medium