Ch. 25: Waves

A WAVE is a rhythmic disturbance that carries energy through matter or space

General Wave Characteristics

Mechanical Waves Not all waves travel through a medium. Mechanical waves only transfer energy through matter. There are two types: Transverse Longitudinal (or compressional) Wave motion transfers energy without transferring matter Click here for a visual of mechanical waves (at bottom of page)

Transverse Waves: matter in the medium moves perpendicular to direction the wave travels

Longitudinal (Compressional) Waves: matter in the medium moves parallel to the direction the wave travels

Combination Waves: Ocean Water An ocean wave is a combination of transverse and longitudinal waves. The individual particles move in ellipses as the wave disturbance moves toward the shore.

Concept Check What is a wave? What are the two types of mechanical waves?

Wave Properties There are four wave properties: Amplitude Wavelength Frequency Wave Velocity

Amplitude is a measure of the amount of energy in a wave In Transverse Waves… In Compressional Waves… look at how dense the compressions are. The more dense, the higher the amplitude

Frequency is the number of wavelengths that pass a fixed point each second f = waves/second SI unit = Hertz (Hz) As Frequency , Wavelength Is this a high- or low-frequency wave?

Wavelength is the distance between a point on one wave and the identical point on the next wave

Calculating Frequency F = 1/T Remember T… Called “period” The time it takes for one wave to pass a given point Measured in seconds What is the frequency of ocean waves that wash up on the shore every 2.5 seconds?

Velocity = wavelength x frequency Wave Velocity is how “fast” the wave moves Velocity = wavelength x frequency v = λf v = velocity (m/s) λ = wavelength (m) f = frequency (Hz)

Example A sound wave travels from a speaker to your ear. If it has a frequency of 12,000 Hz and a wavelength of 2 cm, what is its velocity?

List and describe the four Concept Check List and describe the four wave properties.

How do waves differ from one another? 1) Energy, 2) Velocity, 3) Appearance TRANSVERSE VS. COMPRESSIONAL

Some other terms to know… Period Time for one wave to pass a given point (seconds) Inverse of frequency Node The point where the medium is never displaced On the “normal” line Antinode – The point of maximum displacement Occurs at the wave’s crest

Standing Waves ANTINODES NODES

Wave Behavior All waves of all types exhibit these four behaviors: Reflection Refraction Diffraction Interference

The “bouncing back” of a wave Reflection The “bouncing back” of a wave

The Law of Reflection says… Angle of Incidence = Angle of Reflection

Refraction The change in wave direction at the boundary between two different media Different materials allow waves to travel at different speeds Angle of Refraction Amount of bending Depends on how much the material changes the wave’s speed

Diffraction The bending of a wave around a barrier or a small opening It’s the same process for sound AND light

Interference When two or more waves (blue and red) exist in the same medium, each wave moves as though the other were absent When they meet, they interfere with one another and make a new wave (green). Visualization of Interference

Constructive or Destructive? Constructive Interference – 2 waves combine to form a new wave with a larger amplitude. Destructive Interference – 2 waves combine to produce a wave with a smaller amplitude.

Wave Formulas f = 1/T v = λ f f = frequency (Hz) T = period (s) v = velocity (m/s) λ = wavelength (m) f = frequency (Hz)