Waves

What are waves?• Wave: a disturbance that transfers energy from place

to place. (Energy from a wave of water can lift a boat.)

• Medium: –the state of matter (solid, liquid, or gas) through which a wave can travel. Waves that travel through matter (air, water) are called Mechanical Waves. The density of the medium can affect how fast the wave can travel.

• Waves are created when a source of energy causes a medium to vibrate. The vibration is a back and forth or up and down motion of the matter.

Types of Waves: • Waves are classified according to how they move. Transverse Waves - (means across)•  moves the medium at right angles (perpendicular) to the

direction in which the waves are traveling.• As the wave moves in one direction, the particles of the

medium move across the direction of the wave.Crest (high part of the wave)

normal wave direction

trough (low part of the wave)

Transverse Wave

Longitudinal Waves (also called Compressional Waves)

• wave that moves particles of the medium parallel to the direction of the wave. (coils of a spring move back and forth in the same direction as the wave travels)

rarefactions-(part where coil is spread out)   

compressions-part where coil is closed

As compressions and rarefactions travel along the spring, coils moves forward, then back to their resting position

Longitudinal Wave

SURFACE WAVES• A surface wave is a combination of transverse

and longitudinal waves.• occur at the surface between 2 mediums.• As they pass through water, the water moves up

and down and back and forth. This combination makes a water particle move in a circle

Air(gas)

Water (liquid)

Properties of WavesAmplitude

• maximum distance the particles of the medium move away from their rest positions.

• a measure of how much a particle in the medium moves when it is disturbed by the wave.

• amplitude of a water wave is the maximum distance a water particle moves above or below the surface level of calm water.

• Amplitude is a direct measure of the amount of energy of the wave.

Longitudinal Waves that have high amplitude, have crowded compressions and spread out rarefactions.

Transverse Waves that have high amplitude, have high crests and low troughs.

Wavelength is the distance between two corresponding parts of a wave.

Wavelength

Wavelength

Frequency

• the number of complete waves or cycles that pass a given point in a certain amount of time.

• can also be described as the number of vibrations/second

• measured in units called Hertz (Hz)

• a wave or vibration that occurs every second has a frequency of 1Hz.

Wave Speed• The speed of a wave is how far the wave travels in one

unit of time.• Wave Speed = Wavelength (x) Frequency• V = λ • f

SAMPLE PROBLEM: • The speed of a wave on a rope is 50 cm/s and its wavelength is 10cm. What is its frequency?

f = V

λf = 50 cm/s

10 cm f = 5/s or 5 Hz

V

λ fx:

Let’s review the types of waves and the four

wave properties.

1. Wavelength (lambda)– distance from a point on a wave to the same corresponding point on the next wave.

2. Frequency (f)- number of waves that pass a point in one second (expressed in Hz).

Wavelength has an inverse relationship to wave frequency.

3. Wave velocity (speed)

depends on the type of wave and medium.

1) Sound is faster in more dense media and in higher temps.

2) Light is slower in more dense media, but faster in a vacuum.

4. Amplitude – its size is related to the amount of energy carried by the wave.

1) Transverse - how high above or how low below the Normal (crest & troughs).

2) Longitudinal- how dense the medium is at the compressions & rarefactions.

Interaction of Waves• 1. Reflection• When a wave hits a surface through which it cannot

pass, it bounces back. • The incoming wave bounces back as an outgoing wave.

• The angle of incidence is equal to the angle of reflection

Wave Interactions

Reflection - the bouncing back of a wave.

1) Sound echoes

2) Light images in mirrors 3) Law of reflection i = r

• 2. Refraction• When a wave moves from one medium to

another medium at an angle, it changes speed as it enters the second medium, which causes it to bend.

• (The bending is caused by one side of the wave entering the medium before the other.)

Refraction - the bending of a wave caused by a change in speed as the wave moves from one medium to another.

The girl sees the

boy’s foot closer to

the surface than it

actually is.

If the boy looks down at his feet, will they seem closer to him than

they really are?

No! He is looking straight down and not at an angle. There is no

refraction for him.

• 3. Diffraction• When a wave passes a barrier or moves

through a hole in a barrier, it bends and spreads out.

Diffraction - the bending of a wave around the edge of an object.

1) Water waves bending around islands

2) Water waves passing through a slit and spreading out

Diffraction

Diffraction depends on the size of the obstacle or opening compared to the wavelength of the wave.

Less occurs if wavelength is smaller than the object.

More occurs if wavelength is larger than the object.

• 4. Interference• when 2 or more waves meet or hit each other.• Constructive - 2 waves combine to form a wave

with higher amplitude• Destructive -2 waves combine to form a wave

with lower amplitude

Interference - two or more waves overlapping to form a new wave.

Constructive (in phase)

Sound waves that interfere constructively are louder.

Destructive (out of phase)

Sound waves that interfere destructively are not as loud.

• 5. Standing Waves• Standing waves are waves that appear to stand

still. They are produced when 2 waves are interfering with each other.

• Waves are traveling in both directions.• Node - a point established in a standing wave

where 2 waves collide to produce an amplitude of zero

• Antinode - a point established in a standing wave where 2 waves combine to form an amplitude greater than zero. (max. energy)

Standing Wave

Node

Antinode

WallIncoming wave

Reflected wave

Standing Wave

Standing wave - a wave pattern that occurs when two waves equal in wavelength and frequency meet from opposite directions and continuously interfere with each other.

nodeantinode

• Resonance • This happens when the vibration traveling

through an object matches the object’s normal frequency.

• If an object is not very flexible, resonance can shatter an object.

Resonance - the ability of an object to vibrate by absorbing energy at its natural frequency.

Tacoma Narrows Bridge Collapse