Wave InterferenceWave Interference

PhysicsPhysicsCh 12. Sec. 4Ch 12. Sec. 4

- When two waves come together they can - When two waves come together they can occupy the same spaceoccupy the same space Superposition – multiple waves Superposition – multiple waves occupying the same spaceoccupying the same space

- Mechanical and Mechanical and electromagnetic electromagnetic waves can do thiswaves can do this

Wave InteractionsWave Interactions

Interferance – Interferance – combination of 2 or more wavescombination of 2 or more waves usually produces one waveusually produces one wave

(bigger if in phase)(bigger if in phase)(smaller if out of phase)(smaller if out of phase)

Types of Interference Types of Interference

1. Constructive Interference – individual 1. Constructive Interference – individual displacements on the same side of displacements on the same side of equilibrium add together to form a equilibrium add together to form a resultant wave (in phase)resultant wave (in phase)

- If two pulses move toward each other and - If two pulses move toward each other and are on the same side of equilibrium their are on the same side of equilibrium their amplitudes add together to form a larger amplitudes add together to form a larger wave wave

Wave InteractionsWave Interactions

Constructive Constructive interference – interference – when waves when waves combine to combine to create a bigger create a bigger wave wave

sum of the sum of the amplitudes of the amplitudes of the waveswaves

- They move through and past each other- They move through and past each other- Superposition principle – method of summing - Superposition principle – method of summing

the waves to get a resultant wavethe waves to get a resultant wave

Wave InteractionsWave Interactions

2. Destructive interference – when waves 2. Destructive interference – when waves combine to form a smaller or no wavecombine to form a smaller or no wave some may completely cancel outsome may completely cancel out difference between the amplitudes of difference between the amplitudes of the wavesthe waves

2. Destructive interference – individual 2. Destructive interference – individual displacements on opposite sides of displacements on opposite sides of equilibrium are added together to form a equilibrium are added together to form a resultant wave (out of phase)resultant wave (out of phase)

ReflectionReflection- When a wave strikes a boundary it - When a wave strikes a boundary it

bounces (reflects) off of that boundarybounces (reflects) off of that boundary

1. Free boundary – pulse is reflected 1. Free boundary – pulse is reflected identical to the pulseidentical to the pulse

The rope / spring / The rope / spring / medium is free to medium is free to move up and move up and down at the down at the boundaryboundary

2. Fixed boundary – pulse is reflected but is 2. Fixed boundary – pulse is reflected but is invertedinverted

The rope / spring / The rope / spring / medium is tightly medium is tightly secured at the secured at the boundary and is boundary and is not free to move not free to move up or downup or down

Standing wavesStanding waves - Wave pattern that appears to stand still - Wave pattern that appears to stand still

from waves traveling in opposite directionsfrom waves traveling in opposite directions Waves at same frequencyWaves at same frequency Creates spots of zero Creates spots of zero

displacement displacement

- Node – point on a wave that stands still due to - Node – point on a wave that stands still due to total destructive interference always occurringtotal destructive interference always occurring

- Antinode – point on a standing wave halfway Antinode – point on a standing wave halfway between two nodes (point of largest between two nodes (point of largest displacement)displacement)

*Only certain frequencies produce standing waves*Only certain frequencies produce standing waves

Wave BehaviorsWave Behaviors

A. Refraction - bending of waves as they pass A. Refraction - bending of waves as they pass from one medium to another from one medium to another

Wave BehaviorsWave Behaviors

B. Diffraction – bending of waves around an B. Diffraction – bending of waves around an edge edge

Wave InteractionsWave Interactions- Wave speed depends on the - Wave speed depends on the

mediummediumex. Sound ex. Sound moves fastest through moves fastest through

solidssolids moves moderately fast moves moderately fast

through liquidsthrough liquids moves slowest through moves slowest through

air/gases (340 m/s)air/gases (340 m/s)

- due to movement of - due to movement of molecules in the mediummolecules in the medium

Wave InteractionsWave InteractionsLight travels at a constant speedLight travels at a constant speed3x 103x 1088 m/s = 300,000,000 m/s (186,000 miles/sec.) m/s = 300,000,000 m/s (186,000 miles/sec.)- symbolized as “c”- symbolized as “c”