Interference of Waves

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 playedin 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