Chapter 9 Wave Interactions
9.1 Interference of Waves
Wave interference at the Particle Level
➲ Interference
the process of generating a new wave when two or more waves meet.
Wave motion is efficient: in most media, little energy is lost as waves move.
When waves come together, this efficiency continues. When one wave passes in the vicinity of a particle, the
particle moves up and down in an oval path, which allows the wave to move in a specific direction, as shown in
Figure (a).
(a) The basic motion of a vibrating particle in a travelling wave. (b) When two waves meet, the particle motion is more up
and down. The wave characteristics are unchanged after
the waves pass through each other.
When a second wave is also present, the vibration of the particle is modified. The oval motion of the particles
stimulates the next particle in the direction of the wave’s motion to begin vibrating. When two (or more) waves
come together, as shown in Figure (b), the particle moves up and down rather than in an oval path because the
speeds of the combined waves cancel each other out.
The motion of the particle allows the waves to pass through each other. The waves are not modified, so the
amount of energy stays the same. Thus, when two or more waves interact, the particle vibration is such that the
direction and energy of each wave are preserved. After the waves have passed through each other, none of their
characteristics—wavelength, frequency, and amplitude—change.
Constructive and Destructive Interference
When two waves meet, the forces on their particles are added together.
Principle of superposition:
➲ At any point the amplitude of two interfering waves is the sum of the amplitudes of the individual waves.
1
,Constructive interference
➲ The process of forming a wave with a larger amplitude when two or more waves combine.
Figure 3: Constructive interference, two waves pulses approach each other on a rope. The amplitude during
interference in (c) is the sum of the amplitudes of the two waves.
Destructive interference
➲ The process of forming a wave with a smaller amplitude when two or more waves combine.
Figure 4: Destructive interference, when two wave pulses that are out of phase come together, the resulting
amplitude reduced.
2
, 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.
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9.1 Interference of Waves
Wave interference at the Particle Level
➲ Interference
the process of generating a new wave when two or more waves meet.
Wave motion is efficient: in most media, little energy is lost as waves move.
When waves come together, this efficiency continues. When one wave passes in the vicinity of a particle, the
particle moves up and down in an oval path, which allows the wave to move in a specific direction, as shown in
Figure (a).
(a) The basic motion of a vibrating particle in a travelling wave. (b) When two waves meet, the particle motion is more up
and down. The wave characteristics are unchanged after
the waves pass through each other.
When a second wave is also present, the vibration of the particle is modified. The oval motion of the particles
stimulates the next particle in the direction of the wave’s motion to begin vibrating. When two (or more) waves
come together, as shown in Figure (b), the particle moves up and down rather than in an oval path because the
speeds of the combined waves cancel each other out.
The motion of the particle allows the waves to pass through each other. The waves are not modified, so the
amount of energy stays the same. Thus, when two or more waves interact, the particle vibration is such that the
direction and energy of each wave are preserved. After the waves have passed through each other, none of their
characteristics—wavelength, frequency, and amplitude—change.
Constructive and Destructive Interference
When two waves meet, the forces on their particles are added together.
Principle of superposition:
➲ At any point the amplitude of two interfering waves is the sum of the amplitudes of the individual waves.
1
,Constructive interference
➲ The process of forming a wave with a larger amplitude when two or more waves combine.
Figure 3: Constructive interference, two waves pulses approach each other on a rope. The amplitude during
interference in (c) is the sum of the amplitudes of the two waves.
Destructive interference
➲ The process of forming a wave with a smaller amplitude when two or more waves combine.
Figure 4: Destructive interference, when two wave pulses that are out of phase come together, the resulting
amplitude reduced.
2
, 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.
3
