A. Wave -A Disturbance Involving the Transfer of Energy from Place to Place

Waves

1.  Characteristics of Waves

a.  Wave -a disturbance involving the transfer of energy from place to place

b.  Mechanical waves must travel through a material called a medium

i.  Gases like air

ii.  Liquids like water

iii.  Solids like rock

c.  Waves and Energy

i.  Mechanical waves form when a source of energy causes a medium to vibrate

1.  Vibration- repeated back and forth or up and down motion of a medium

d.  Types of Mechanical Waves

i.  Transverse waves- vibration occurs perpendicular to (at right angles to) the direction that the wave travels.

1.  Forms high and low points along a rope

a.  Crest-the top of the wave (high point)

b.  Trough-the bottom of the wave (low point)

ii.  Longitudinal - vibrates the medium in the same direction that the wave travels.

1.  Stretching a slinky out and watching the movement go back and forth.

a.  Compression- the area of coils that move closer together as energy travels along the slinky

b.  Rarefaction- the area of coils that are spread out before or after energy travels along the slinky.

iii.  Surface Waves- a combination of transverse and longitudinal waves.

1.  The energy travels at right angles causing an up and down motion and a back and forth at the same time.

a.  Causes particles to travel in a circular motion.

2.  Properties of Waves

a.  Amplitude

i.  The maximum distance a medium vibrates from its resting position. The more energy a wave has, the higher its amplitude. The height of a wave’s crest depends on its amplitude

ii. 

1.  Transverse waves-amplitude is the maximum distance the medium moves up or down from its resting position

2.  Longitudinal wave- amplitude is a measure of how compressed or rarefied the medium becomes

a.  Very compressed=high amplitude

3.  Surface waves- amplitude depends on how far the water particles move above or below the surface when the water is calm

a.  The more energy a wave has, the higher its amplitude

i.  The height of a wave’s crest depends on its amplitude

b.  Wavelength-

i.  Transverse wave- wavelength =the distance from crest to crest or trough to trough

ii.  Longitudinal wave-wavelength= the distance between compressions

c.  Frequency

i.  The number of waves that pass a given point in a certain amount of time

1.  Measured in hertz

a.  1 wave per second 1/s = 1Hz

b.  2 waves per second 2/s = 2Hz

d.  Speed

i.  How far the wave travels in a given amount of time

a.  ex 300 meters per second 300 m/s

2.  Different types of wave travel at different rates of speed

a.  Light is much faster than sound

e.  How are they all related?

i.  Speed = Wavelength x Frequency

1.  Rule applies as long as the temperature and pressure of the medium do not change.

ii.  Frequency = Speed/Wavelength

iii.  Wavelength = Speed/Frequency

3.  Interactions of Waves

a.  What changes the direction of waves

i.  Reflection

1.  Any part of a wave that cannot pass through a surface it hits bounces back

a.  Reflections in a mirror

b.  Sound of echos

2.  Law of Reflection- The angle of incidence is equal to the angle of of reflection (all waves follow this law)

a.  Incoming wave- a wave moving toward the surface at an angle.

b.  Angle of Incidence- the angle between the incoming wave and the normal.

c.  Normal- a line perpendicular to the surface at the point where the reflection occurs

d.  Angle of reflection- the angle between the reflected wave and the normal

e.  Reflected wave- A wave bouncing off the surface at an angle.

ii.  Refraction

1.  The bending of a wave due to a change in speed.

a.  Changes in the speed of a wave causes the wave to change direction.

i.  When a wave enters a new medium at an angle, the speed of the front of the wave slows down, while the speed at the back of the wave remains the same.

1.  This causes the wave to bend.

a.  Bending happens because different parts of a wave travel at different speeds.

b.  Refraction doesn’t always happen when a wave enters a new medium

i.  Won’t happen if the wave hits at a 90* angle (right angle)

ii.  Won’t happen if the speed of the wave in the new medium is the same as the speed of the wave in the old medium

iii.  Diffraction

1.  Waves bend around barriers or pass through openings.

a.  They

b.  bends and spreads out

b.  Wave interference

i.  Interference is the interaction when waves meet and occupy the same space at the same time.

1.  Two types

a.  Constructive Interference- when two waves combine to form a wave with a larger amplitude than either of the original waves.

i.  Create higher crests and lower troughs

ii.  “Builds/ makes things bigger

1.  Rogue waves on the ocean

b.  Destructive Interference- when the crest of one wave interferes with the trough of another wave

i.  Creates a lower crest or a higher trough

ii.  Cancel each other out if the crest and trough that are interacting have the same amplitude.

c. Standing Waves

i. A wave that appears to stand in one place even though it is 2 waves interfering as they pass through each other.

1.  Incoming wave is reflected at the right frequency and the wave looks like it's standing still

a.  Destructive interference among the incoming and reflected

wave produces “nodes” or points of zero amplitude.

i.  Nodes appear to be directly on the “rest position”

ii.  Appear to be evenly spaced along the wave

b.  Constructive interference among the incoming and reflected wave produces “antinodes” or points of maximum amplitude.

i.  Evenly spaced antinodes are always half way between 2 nodes.

Ii. Resonance

FYI All objects have a natural frequency.

1.  Resonance-an increase in the amplitude of a vibration that occurs when external vibrations match an object’s natural frequency.

a.  Resonance causes an object’s natural vibrations to amplify--get bigger.

i.  Galloping Gertie’s natural frequency was the matched by the storm winds which were resonating with the same frequency.

1.  This caused the waves to increase in amplitude--and the bridge swayed so much it collapsed.