Chapter 12 Section 4

Wave Interference

It’s impossible for two material objects to occupy the same space at the same time.
 Waves on the other hand are able to occupy the same space at the same time.

Superposition
 Superposition – The ability for a wave to occupy the same space at the same time.
 Superposition is possible for all forms of waves:
 Sound
 Water
 Light

Constructive Interference
 Constructive Interference – Interference in which individual displacements on the same side of the equilibrium position are added together to form a resultant wave.
 When the two waves meet, they form a larger amplitude wave. Once they pass through one another, they return to their original amplitude.

Constructive Interference

Destructive Interference
 Destructive Interference – Interference in which individual displacements on opposite sides of the equilibrium position are added together to form a resultant wave.
 When the two waves meet, they form a smaller amplitude wave or even cancel out. Once they pass through one another, they return to their original amplitude.

Destructive Interference

Constructive vs. Destructive
Interference
 Superposition of Waves Animation
 http://www.acs.psu.edu/drussell/Demos/sup erposition/superposition.html

Interferences In Waves
 The method of summing the displacements of waves is known as the
Superposition Principle.
 This principle only holds true for small waves
 Superposition principle is valid for both transverse and longitudinal waves.

Wave Reflection
 At a free boundary, waves are reflected back to the source and the amplitude is unchanged.
 At a fixed boundary, waves are reflected and inverted back to the source with the amplitude unchanged but upside down.

Reflection Diagram

Standing Waves
 Standing Wave – Wave pattern that results when two waves of the same frequency, wavelength, and amplitude travel in opposite directions and interfere.
 Node – Point in a standing wave that always undergoes complete destructive interference and therefore is stationary.
 Antinode – Point in a standing wave, halfway between two nodes, at which the largest amplitude occurs.

Standing Waves Explained
 Standing Waves are formed when both ends of a string, rope or spring, are fixed at both ends and the waves reflect on both ends.
 The incident and reflected waves combine according to superposition principle.
 As long as the frequency of the waves remains constant a standing wave will form.

Standing Wave Diagram

Doppler Effect?
 Your standing on a street and some driving by honks the horn, you will notice the pitch of the horn change.
 The pitch gets higher as the car comes closer.
 Once the car passes, the pitch will drop.
 The pitch depends on the frequency of the wave.

Doppler Effect Diagram

Doppler Effect Animation
 http://www.astro.ubc.ca/~scharein/a311/
Sim.html#Doppler

Doppler Effect of Light
 Doppler effect can happen for light waves as well.

Doppler Effect
 Doppler Effect – Frequency shift that is the result of relative motion between the course and an observer.
 When an object moves, it effects the frequency of the wave that is being emitted.
 Doppler effect occurs for all types of waves.