Chapter 17 & 18 Waves

Mechanical Waves• Disturbance in matter that carries energy

from one place to another

• Medium: what a wave travels through– Can be a solid, liquid, or gas

• Created when source of energy causes vibration to travel through a medium

Transverse Waves• Causes medium to

vibrate at right angles to direction of wave

• Travels left to right

• Crest: highest point above rest position

• Rest: before wave

• Trough: lowest point below rest position

Longitudinal Waves

• Vibration of the medium is parallel to direction of wave

• Vibration is a back and forth motion• Ex: slinky, P-wave (earthquakes)

• Rarefaction: particles are more spread out

• Compression: particles in a medium are spaced close together

Surface Waves• Travels along a

surface separating two media

• Carries energy from left to right

• Ex: ocean waves

Properties of Mechanical Waves• Frequency and period– Periodic motion: repeats at regular time

intervals– Period: time for one complete cycle– Frequency: number of complete cycles in a

given temperature• Measured in hertz (Hz): cycles/second• Equals frequency of vibrating source

producing wave

Wavelength

• Distance b/w point on 1 wave and same point on next wave cycle

• Increasing frequency decreases wavelength

Wave Speed

• Speed (m/s) = wavelength (m) x frequency (Hz)

• Speed can change if in a new medium

• Wavelength is inversely proportional to frequency

– Longer the wavelength, shorter the frequency

• Shorter the wavelength, longer the frequency

1. The musical note A above middle C has a frequency of 440 Hz. If the speed of sound is

known to be 350 m/s, what is the wavelength of this note?

2. A certain FM radio station broadcasts electromagnetic waves at a frequency of 9.05 × 107 Hz. These radio waves travel at a speed of 3.00 × 108 m/s. What is the wavelength of these radio waves?

3. A wave with a frequency of 60.0 Hz travels through vulcanized rubber with a wavelength of 0.90 m. What is the speed of this wave?

4. A wave with a frequency of 60.0 Hz travels through steel with a wavelength of 85.5 m. What is the speed of this wave?

Amplitude

• Maximum displacement of the medium from its rest position

• More the energy wave has, the greater its amplitude

Wave Behavior• Reflection: wave

bounces off a surface it cannot pass through

–Does not change speed or frequency of a wave

–Can flip wave upside down

• Refraction: bending of a wave as it enters new medium at an angle

–1 side moves more slowly than the other

–Changes wave direction

• Diffraction: bending of wave as it moves around an obstacle or passes through a narrow opening

– Increase if wavelength is large compared to size of opening/obstacle

• Interference: 2 or more waves overlap and combine together

–2 types:

•Constructive: increases amplitude

–2 or more waves combine to make wave with a larger displacement

•Destructive: reduces amplitude

–Smaller displacement

Standing Waves

• Appears to stay in one place (does not seem to move)

• Node: point that has no displacement from the rest position– Destructive interference b/w incoming and reflected

waves

• Antinode: point where a crest or trough occurs midway b/w 2 nodes

• Only forms if ½ or a multiple of half a wavelength fits exactly into length of a vibrating cord

Sound and Hearing

• Properties of sound waves:– Longitudinal waves– Speed: varies in different media• Travels faster in solids > liquids > gases

– Intensity: rate at which a wave’s energy flows through a given area• Depends on wave’s amplitude and distance

from sound source• Measured in decibels (tenfold increase with

every 10-decibel increase)

• Loudness: physical response to intensity of sound

– Increases as intensity increases

• Frequency: depends on how fast the source of the sound is vibrating

• Pitch: frequency of a sound as you perceive it

– Depends on wave’s frequency

• High high pitch

• Low low pitch

Ultrasound

• Sound at frequencies higher than most people hear

• Used in sonar and imaging– Sonar: determines distance to an object under

water• Name means “sound navigation and ranging”

– Imaging used in medical techniques

Doppler Effect• Change in sound frequency caused by

motion of the sound source, motion of listener, or both

–Higher as it gets closer

–Lower as it moves farther away

Electromagnetic Waves• Transverse waves of

changing electric and magnetic fields (regenerate each other)

• Carry energy from place to place

• Electric field: exerts electric forces on charged particles

• Wavelength and frequency: all waves vary

–Speed = wavelength x frequency

–Wavelength is inversely proportional to its frequency

Electromagnetic Spectrum

Types of waves that make up the spectrum:• Infrared, Radio, UV, X-Rays, and Gamma

rays

Behavior of Light• Transparent: transmits

light (can pass through)

• Translucent: scatters light (can see through material but objects not clear)

• Opaque: absorbs or reflects all light that strikes it (none passes through)

• When light strikes a new medium, it can be reflected, absorbed, or transmitted

–Reflection

• Smooth surface: clear, sharp image

•Rough surface: blurred or no image

• Transmitted light

–Refraction: bending light wave can cause a mirage (false or distorted image)

Colors• White light: made up of all colors of visible

spectrum

• Prism separates colors (dispersion)– Shorter wavelengths refract more than longer

wavelengths

• Color of objects is color of light that reaches your eye

–Depends on what object is made out of and color of light that strikes the object