Ch 17 & 18

You know about waves? What do they look like? What do they do? How fast are they? Where are they? What are the different kinds? What do you call the parts of a wave?

Energy travels in Waves Wave – a disturbance that transfers energy but not mass Usually Needs something to travel through… Medium – any matter that can transport a wave’s energy (particles are displaced to transfer the energy) Can be solid, liquid, or gas Ex: earthquake energy through rocks, tsunami energy through water, voice through air, etc.

Mechanical waves Longitudinal– move like a slinky toy (a.k.a. Compressional wave)

Longitudinal waves Ex. Sound waves Compression – areas of high particle density (particles moved close together) Rarefaction – areas of lowest particle density (particles are spread farther apart)

Transverse – move like a rope, up & down, or like water when something is dropped in it

Transverse waves Ex. Pebble dropped into still water, fans doing the wave in stands Particles move perpendicular to direction of wave

Wave Characteristics Wavelength is the distance between two successive points which are in phase, (points are doing the same thing at the same time) Wavelength, l or λ, measured in meters (m). Wavelength

Rest position – what the wave looks like if there is no movement (equilibrium position) Crest – highest point above equilibrium or rest (“above rest”) Trough - lowest point below equilibrium or rest

Amplitude – the max. amount of displacement of particles from their rest position In transverse wave = distance from rest position to crest or trough In longitudinal wave = diff. in compressions and undisturbed particles Greater amplitude = greater energy Ex. Louder sound, brighter light

Frequency - # of wave cycles the particles go through per second 1 wave cycle = 1 wavelength Frequency measured in Hertz (Hz) Frequency & wavelength are inversely related = when one increases, the other decreases

Period – the time it takes for one complete cycle to pass a specific point Period is measured in time (seconds) Period is inversely related to frequency = as one increases, the other decreases Ex. If a drum is banged 2 times in 1 second… the drum’s period = 0.5 sec (divide 1 sec by # of times it was banged in 1 sec) the drum’s frequency = 2.0 Hz (b/c 2 wave cycles passed in 1 sec) Don’t confuse period & frequency

Velocity = ? Speed = ? You may not see the direction referred to with wave equations but the term “velocity” is still used Ex. Think of a man & small child walking together, the man takes longer steps than the child so the child has to take more steps & quicker steps to keep up… the velocity is the same for both man & child but the frequency & wavelength is different for each V w = ( f )( λ ) Speed = d / t

Electromagnetic Waves Electromagnetic waves are the most significant waves on Earth b/c they deliver light & heat energy that is necessary for us to survive They are transverse waves that have both electric & magnetic components ( ┴ ) Travel through space at the speed of light ( 299,792,458 m/s or ≈ 300,000 km/s or ≈ 186,000 mi/sec)

Electromagnetic waves are classified by f & λ The breakdown of this classification is called the Electromagnetic Spectrum

Shortest em waves are gamma waves If gamma waves have the shortest wavelengths, they will have ________ frequency (Remember λ & f are inversely related) (gamma = extremely high frequency) Gamma waves are smaller than an atom Next are X-rays ( λ =.01 to 10 nm) UV light ( λ = 10 to 100 nm) “Visible light” ( λ = 380 to 750 nm)

“Visible light” = we don’t really see light Matter illuminated by “visible light” Each color is at a different λ Ex. Green λ = 495 to 570 nm depending on what shade of green The longest λ in “visible light” = red Shortest λ = violet ROY G BIV red, orange, yellow, green, blue, indigo, violet (***Indigo is not on EOC “pink sheet” for test.)

After “visible light” Infrared (750 nm to 1 mm) Infrared = thermal heat & night vision The rest are part of radio waves Radar & microwaves FM radio TV signals (0.1 to 10 m) Shortwave radio (Ham radio) AM radio

Remember – Greater f = greater energy (gamma & x-rays can burn tissue quickly) Longer λ = lower f

Sound waves Longitudinal waves Speed of sound is affected by the medium Temperature Pressure Density Elastic rigidity In general, sound travels faster in solids and liquids than in gases CO 2 = 259 m/s O 2 = 316 m/s Air = 331 m/s H 2 O = 1482 m/sPb = 1960 m/s Glass = 5640 m/sSteel = 5960 m/s

When a wave changes media… One of three things can happen Reflection - Part or all of the wave energy can bounce off the medium surface & change direction (90° angle) Refraction – Wave’s direction of travel is bent because the speed changed from the 1 st medium to the 2 nd medium Diffraction – Light waves bend around a barrier or the edge of an opening

Interference When two waves interact…one of two things can happen Constructive interference can occur – two waves combine and energy is increased (ex. Greater amplitude seen here)

Destructive interference can occur – two waves combine and form a smaller wave (can possibly cancel each other out) (ex. Noise cancelling headphones)