Ch. 11 Waves

What is a wave? Write down what you think a wave is.

Wave Is a disturbance that transmits energy through matter or space.

Ex’s: water ripple from something dropped in it, sound , earthquake, etc. All of these waves travel through something.

Medium The matter through which a wave travels. Waves travel at different speeds in different medium.

Animation courtesy of Dr. Dan Russell, Kettering University Mechanical Wave A wave that requires a medium through which to travel. Water Waves Wave “Pulse” People Wave Animation courtesy of Dr. Dan Russell, Kettering University

Light doesn’t require a medium to travel Light doesn’t require a medium to travel. Also, radio waves for radio or television.

Electromagnetic Wave A wave caused by a disturbance in electric and magnetic fields and that does not require a medium; also called light waves.

Because waves do work, they carry energy and transfer energy Because waves do work, they carry energy and transfer energy. Waves spread out in a sphere and the energy is spread out as it travels.

Waves are related to vibrations. Most waves are caused by vibrating objects. Vibrations involve a transfer of energy. A wave can pass through a series of vibrating objects.

Transverse Wave A wave that causes the particles of the medium to vibrate perpendicularly to the direction of the wave. Ex: Think of a ‘wave’ at a sporting event, also high.

Longitudinal Wave A wave that causes the particles of the medium to vibrate parallel to the direction the wave travels. Ex: sound waves, like a pulse.

Surface waves occur at a boundary between two different medium. Particles move both perpendicularly and parallel to the wave direction. Causing a circular motion. The ball actually stays in the same place.

Wave Properties Ideal shape for a transverse wave is a sine curve. Hence, the term “sine waves”.

Parts of a Wave crest : wavelength 3 equilibrium A: amplitude 2 4 6 x(m) -3 trough y(m)

The highest point of a transverse wave. Crests The highest point of a transverse wave.

The lowest point of a transverse wave. Trough The lowest point of a transverse wave.

Amplitude (A) The greatest distance that particles in a medium move from their normal position when a wave passes.

Wavelength (l) The distance between any two successive identical parts of a wave.

Period (T) The time required for one full wavelength to pass a certain point. Units: seconds

Frequency (f) The number of vibrations that occur in a 1 second time interval.

Frequency-Period Equation

Units: hertz (Hz) or 1/sec

Ex: The wave crests pass a buoy ever three seconds Ex: The wave crests pass a buoy ever three seconds. What is the frequency? G: T = 3 sec U: f = ? E: f = 1/T S: f = 1/3 S: f = 0.333 Hz

Ex: Star 104 operates at 103.7 MHz, what is the period of this wave?

Light comes is in the range called visible light Light comes is in the range called visible light. It’s in a wide range of frequencies and wavelengths. From red to violet.

Electromagnetic waves exist at other frequencies.

Wave Speed The speed at which a wave passes through a medium.

Speed = distance/time

v f l

G: v = 346 m/s, f = 440 Hz U: l = ? E: v = l f  l = v/f Ex: The frequency of an oboe’s A is 440 Hz. What is the period of this note? What is the wavelength? Assume a speed of sound in air of 346 m/s. G: v = 346 m/s, f = 440 Hz U: l = ? E: v = l f  l = v/f S: l = 346 / 440 S: l =

Ex: The speed of a wave is 334 m/s, if you yell to a friend who is 150 m away, how long does it take before he hears your voice?

The speed of the wave depends on the medium. Sounds travel through air at about 340 m/s. The speed changes with temperature. Sound travels 3 – 4 times faster in water and 15-20 times faster in rock or metal.

Kinetic Theory explains difference in wave speed. Waves moves fastest through solids, then liquids, and slowest through gases.

Doppler Effect An observed change in the frequency of a wave when the source or the observer is moving.

Pitch is determined by the frequency of the sound waves.

Doppler Effect The Doppler Effect is the raising or lowering of the perceived pitch of a sound based on the relative motion of observer and source of the sound. When a car blowing its horn races toward you, the sound of its horn appears higher in pitch, since the wavelength has been effectively shortened by the motion of the car relative to you. The opposite happens when the car races away.

Doppler Effect Stationary source Moving source http://www.kettering.edu/~drussell/Demos/doppler/mach1.mpg Animations courtesy of Dr. Dan Russell, Kettering University Supersonic source http://www.lon-capa.org/~mmp/applist/doppler/d.htm

The Doppler effect has many practical applications: weather radar, speed radar, medical diagnostics, astronomical measurements. At left, a Doppler radar shows the hook echo characteristic of tornado formation. At right, a medical technician is using a Doppler blood flow meter.

11.3 Wave Interactions

Reflection The bouncing of a wave as it meets a surface or boundary.

Reflection Types Fixed-end reflection: The wave reflects with inverted phase. Open-end reflection: The wave reflects with the same phase Animation courtesy of Dr. Dan Russell, Kettering University

Diffraction The bending of a wave as it passes an edge or an opening.

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

Animation courtesy of Dr. Dan Russell, Kettering University Refraction of waves Refracted waves may change speed and wavelength. Refraction is almost always accompanied by some reflection. Refracted waves do not change frequency. Animation courtesy of Dr. Dan Russell, Kettering University

Interference The combination of two or more waves that exist in the same place at the same time.

Constructive Interference Any interference in which the waves combine so that the resulting wave is bigger than the original waves.

Constructive Interference crests aligned with crest waves are “in phase”

Constructive Interference

Destructive Interference Any interference in which waves combine so that the resulting wave is smaller than the largest of the original waves.

Destructive Interference crests aligned with troughs waves are “out of phase”

Destructive Interference

Standing Waves A wave form caused by interference that appears not to move along the medium and that shows some regions of no vibration (nodes) and other regions of maximum vibration (antinodes).

Standing waves exist: - whenever a multiple of half-wavelengths will fit exactly in the length of the string. - With nodes always existing at both ends.