What are they? Where have you seen them? What do they do?

 Wave: a disturbance that transfers energy from place to place  Remember, in science, energy is the ability to do work  Raft example on p. 510: wave’s energy lifts raft as wave passes under it

 Most waves need something to travel through: called a medium  May be gas (air), liquid (water), or solid (rope)  Mechanical waves: waves that require a medium through which to travel  Electromagnetic waves: waves that can travel without a medium  Ex: sunlight travels through empty space

 Wave enters medium, which is made of tiny particles  Transfers energy to medium’s particles  Particles bump into each other and pass along wave’s energy  After wave has passed, medium is unchanged – it doesn’t move with the wave  **think of food being passed around a table

 Energy – always required to make a wave  Mechanical waves are produced when a source of energy causes a medium to vibrate  Vibration: repeated back-and-forth or up-and- down motion  Wave results when a vibration moves through a medium  Usually, a moving object has energy and will transfer that energy to a medium

 2 types of mechanical waves – classified by how they move:  Transverse waves  Longitudinal waves

 Waves move the medium at right angles to the direction that the waves travel  Transverse = “across”  As wave moves left to right, medium moves up and down

 High point of wave: crest  Low point of wave: trough  Middle (resting position): equilibrium  Distance from one crest to the next: wavelength  Distance from equilibrium to crest or trough: amplitude

 Wave that moves the medium parallel to the direction that the waves travel  Ex: in a spring, the coils move back and forth parallel to the wave motion  After wave passes, each coil returns to its starting point  Example: sound (air particles move back and forth parallel to distance sound travels)

 Compressions: parts where coils are close together  Rarefactions: parts where coils are spread out  Wavelength: distance from one compression to the next

 Waves can vary greatly  short or long  lots of energy or a little  rare of frequent  fast or slow

 Amplitude  Wavelength  Frequency  Speed

 Maximum distance that the medium moves away from its resting position  Distance from rest to the crest or trough  Ex: water  Transverse waves: medium moves up and down  Longitudinal waves: measured by how compressed or rarefied medium becomes

 Distance between 2 corresponding parts of a wave  Can be measured as distance between 2 crests or between 2 troughs

 Number of complete waves that pass a point in a certain amount of time  Measured in Hertz (Hz)  Hertz = waves that pass per second  Ex: 3 Hz = 3 waves per second

 Distance traveled over time (same as when we talked about motion)  Speed = Wavelength x Frequency  Can rearrange formula to solve for other factors

 When a wave hits a surface that it cannot pass through, so it bounces back  Ex: mirror reflects light so you can see yourself  Ex: when you shout in an empty gym, your voice echoes back

 All waves obey the law of reflection: angle of incidence = angle of reflection

 When a wave enters a new medium at an angle, one side of the wave changes speed before the other side  The change in speed causes the wave to bend  Refraction: bending of waves due to a chance in speed

 Only occurs when wave enters new medium at an angle  One side enters first, changes speed  Other side continues moving at original speed  Wave bends

 Skateboarding – one wheel in the grass  Reaching for an object underwater

 When a waves bends around a barrier or through an opening in a barrier  Wave bends and spreads out

 Interference: the interactions of 2 waves when they meet  Can be constructive or destructive – what do you think the difference is?

 Waves combine to make a wave with a larger amplitude  Constructive = works together

 Waves combine to make a wave with a smaller amplitude  Destructive = destroy

 In both cases, waves will continue in their own directions as if they had not met

 A wave might travel through a medium then be reflected back (think tying a rope to a doorknob)  Reflected wave and incoming wave will interfere  If waves have the right frequency, they may appear to stand still  Called a standing wave – looks like no wave at all but is really 2 waves passing through each other

 Nodes: points with amplitude of 0 (zero)  Caused by destructive interference  Evenly spaced  Antinodes: points of maximum amplitude  Caused by constructive interference  Maximum energy  Halfway between nodes

 Most objects have a natural frequency of vibrations  Standing waves occur when objects vibrate at their natural frequency  If nearby objects vibrate at that same frequency, it causes resonance  Resonance: increase in amplitude of a vibration that occurs when external vibrations match an object’s natural frequency

 Can be good or bad  Good: musical instruments use resonance to produce stronger, clearer sounds  Bad: earthquake in Mexico City caused more damage because seismic waves had same natural frequency as clay layer of ground