1. Interference of Waves
Section 9.1

2. Key Terms Interference Principle of Superposition
Constructive Interference
Destructive Interference

3. Wave Interference
Remember that waves are the result of particle vibrations.
Particles in a medium are connected by forces.
When a single wave passes a particle, it moves in an oval path.
Allows the wave to travel in a certain direction.
When waves meet, a new wave is generated in a process called interference.
The speeds of the combined wave cancel out and the particles move up and down.

4. Wave Interference Cont’d
The waves pass through each other with no loss of energy.
The direction and energy of each wave are preserved.
Once they have passed, their characteristics are unchanged.

5. Principle of Superposition
When two waves meet, the forces of their particles are added together.
If the two waves are in phase, the resulting amplitude will be greater than either of the two individual amplitudes.
If the two waves are out of phase, the resulting amplitude will be less than either of the two individual amplitudes.
Principle of Superposition

6. Constructive Interference
The process of forming a wave with a larger amplitude when two or more waves combine.

7. Constructive Interference

8. Destructive Interference
The process of forming a wave with a smaller amplitude when two or more waves combine

9. Destructive Interference

10. Technology Using Interference of Waves
Noise-cancelling headphones generate a wave that is completely out of phase with the sound waves in the environment.
When the wave is played in the headphones, the destructive interference of the waves, cancels the ambient noise.

11. Applying the Principle of Superposition

12. Practice Problems

13. Summary
The process of generating a new wave when two or more waves meet is called interference.
Vibrating particles in a medium react to the sum of all forces on them. Their motion is caused by the sum total of forces on them.
The principle of superposition states that, when two waves meet, the resulting amplitude is the sum of the individual amplitudes.
Constructive interference occurs when two waves combine and the amplitude of the resulting wave is greater than the amplitudes of all the individual waves.
Destructive interference occurs when two waves combine and the amplitude of the resulting wave is less than at least one of the original amplitudes.
Humans can design technologies to take advantage of wave properties. An example of such a technology is noise-cancelling headphones.

14. Homework Principle of Superposition Worksheet Page 419
Questions 1 – 3
Chapter 8 Review Questions
Page 408
1-18
Page 409
19-21, 23-27, 29-43