Waves!

First a little vocab…• WAVE:

– A periodic disturbance in a solid, liquid or gas

• MEDIUM:– Matter or space through which waves

travel

I. Waves transfer energy

• Some waves carry more energy than others– Energy spreads out as a wave travels

• Ex- sound waves

Ex- sound waves…• Sound waves – travel as spherical waves

from a speaker• Wave front – each circular wave of energy• As they travel out, the wave front gets

bigger – each front has the same amount of energy. As the wave front gets bigger, the energy is spread out over a bigger area

•Why don’t you want to sit close to a loud speaker?•When you are close to the speaker (or close to light) more energy is transferred to your ears (or eyes)

II. Waves can be classified by what they move through

a. Mechanical- Requires a medium

- ex: ripples on a pond, sound waves

b. Electromagnetic-Does not require a medium

– Ex- light waves, radio waves, ultraviolet waves…

– Youtube: http://www.youtube.com/watch?v=b0JQt4u6-XI

2. Waves can also be classified by how they move

a. Transverse waves – travel perpendicular to the direction of the wave (drawn “up and down”)

b. Longitudinal waves (aka compression waves )

travel parallel to the direction of the wave (drawn “back and forth”)

Longitudinal (top) compared to transverse (bottom)…

Surface Waves: Have BOTH

• Occur at the border between two mediums…

• Water waves: Particles at the surface move parallel and perpendicular (in circles)

• The ball up and down and side to side

Check your pictures…

• Youtube video:

• http://www.youtube.com/watch?v=e5tGaTIW3A8

Anatomy of a Transverse Wave

Crest:Trough:

Anatomy of a Longitudinal Wave

Compression:Rarefaction:

How do we describe waves?

• Wavelength• Amplitude• Period• Frequency

Wavelength ()

– The distance between two identical parts of a wave• The distance btwn 2 crests for a TRANSVERSE

wave or two compressions for a LONGITUDINAL wave. – The symbol for wavelength is ________

Draw 2 waves – one with a long wavelength (5cm) and one with a short wavelength (2cm)

Amplitude– Distance from the

middle of the wave to the crest

• (or half the distance between the crest and the trough)

– Larger amplitudes carry more energy

Draw 2 waves – one with an amplitude of 5 mm and one with an amplitude of 2 mm

Amplitude of 2

FrequencyThe number of cycles/wavelengths/

vibrations per unit of time-Measured in Hertz (named after Heinrich Hertz – 1888 – 1st demonstrated the presence of electromag. waves)

– Represented by “ f ” • (Equal to vibrations per second (1Hz =

1 vibration/sec)• You can hear 20 Hz to 20,000 Hz)

Remember that each wave carries energy…• Higher frequency = more energy and

shorter wavelength…The more wavelengths per second, the more energy.

1. Draw 2 waves – one with a high frequency and one with a low frequency:

Wave speed – how fast a wave moves

• Draw on Notes Sheet and fill in blanks

Wave speed depends on medium In a gas, particles are farther apart so it

takes longer for them to find each other and bump into each other - sending the wave

• In liquids, particles are closer together so waves travel faster

• In solids, particles are very close together so waves travel the fastest

Ex- sound waves are mechanicalCan sound waves travel in air?

Yes! However, they travel even faster in ___?___ than

in air or waterExample: hitting a metal railing and listening

(you will hear two bangs)Why?The first sound comes through the steel rail itself

and reaches you shortly before the second sound, which travels through the air.

https://www.youtube.com/watch?v=GkNJvZINSEY(sol, liq, gas at 2:22)

More practice/Thinking ahead…

1.Will the type of medium affect the speed of Electromagnetic Waves?

2. What about Electromagnetic Waves?

Will the type of medium affect their speed?

Yes! Electromagnetic waves actually travel

faster in gases,slower in liquids,

and slowest in solids

More on this later!

Doppler Effect: an observed change in the frequency of a wave when the source or

observer is moving A) Pitch of a sound (how high or low it is;

shrillness of the sound) is determined by the frequency at which the sound waves strike the eardrum1) When an object that is emitting sound waves (i.e. ambulance) is at rest, the sound waves spread out in circles and anyone who is standing near the object will hear the same pitch

2) When an object that is emitting sound waves (i.e. ambulance) is moving however, the waves are closer together in the direction of the motion. So if one person is standing in the same direction as the motion, the pitch will seem higher than if a person was standing in the opposite direction

Doppler Effect– Ambulance example:

• When the ambulance moves toward you, the sound waves are compressed because the ambulance moves a short distance – so the waves hit your eardrum at a higher frequency – higher pitch

• Then, when the ambulance moves away from you, the distance between the wave fronts is more than before, so the frequency is lower – pitch is lower

– Doppler effect can also happen in light and other types of waves

LIGHT ENERGY

(electromagnetic waves)

Frequency of Light Spectrum (example)

• Wide range of frequencies and wavelengths• Your eyes can detect 4.3 x 1014 Hz to 7.5 x 1014 Hz (visible

light range)• Electromagnetic waves exist at frequencies we can’t see

Reflection and Refraction of Light

What is light?

• Energy!• Where does it come from?

– The SUN– Makes its own energy by NUCLEAR

fusion!

How do you see something?• Light from the sun appears white,

however when shone through a prism, it will separate into all the colors.

• Objects have color because they reflect certain wavelengths of light

Why does a plant appear to be green?• It absorbs the other

wavelengths and reflects the green light (550 nm)

• Light really is not “green” – your brain interprets light at certain wavelengths as certain colors

• Petals of a red rose look red because they absorb wavelengths and then reflect the red light

Black is not a “wavelength of light” – it is the absence of light!

All the colors are ASORBED!

How does light reflect?• Light reflects at an angle• Rough surfaces - light that strikes the surface

will be reflected at all angles– Called DIFFUSE REFLECTION

• Smooth surfaces – light that strikes a smooth surface like a mirror will all reflect in a single direction

Ray Diagrams: used to trace the path of LIGHT!

Refraction of Light• What happens to

light waves when they travel?

• Through empty space – travel in straight line

• Refraction – how light bends when passing from one medium to another

• Light changes direction because the speed of light is different in different mediums

How do objects appear if light is refracted?

• Objects appear to be in a different position

• The path of the light is changed as it goes from one medium to another

• However the brain still sees it as though the light traveled in a straight line

• So, the position of the object (cat or fish) looks different than where it really is

Pencil Demo:

Diffraction • Diffraction is the BENDING of waves

around an edge.– Ex- Hearing sound waves from around a corner– When waves pass the edge of an object or

pass through an OPENING, they spread out as if a new wave were created there.

– Diffraction is the reason a SHADOW never has perfectly sharp edges.

Wave Interference

When several waves are in the same location , the waves combine to produce a single new wave that is different from the original wave. This behavior is called SUPERPOSITION

1) Constructive Interference: when two waves interact and the resultant wave has a larger amplitude (Part A, Figure below)

2) Destructive Interference: when two waves interact and the resultant wave has a smaller amplitude (Part B, Figure below)

INTERFERENCE IN LIGHT WAVES: (transverse electromagnetic)

• Create colorful displays• Ex- soap bubbles- pg 476

– You see many colors because the light is bouncing off different parts of the bubble (creating differing wavelengths)

– These waves interfere constructively in some places and destructively in others.

– These different frequencies are interpreted by your eyes to create many different colors .

INTERFERENCE IN LIGHT WAVES: (transverse)

INTERFERENCE IN SOUND WAVES: (longitudinal mechanical)

• If two waves are “out of sync” they cause “beats” of louder noise (when they are in line and have constructive interference) and then quiet (when they are opposite and destructive interference occurs)

• http://library.thinkquest.org/19537/Physics5.html

STANDING WAVES: • Form when a wave is reflected at the

boundary of a medium. – Interference of the original wave with the

reflected wave causes the medium to vibrate in a stationary pattern that resembles a series of loops.

– The wave appears to be standing still, but it is not!!! (actually moving in both directions)

• fire demo video– https

://www.youtube.com/watch?v=2IbxQpSz7nU&NR=1

• http://paws.kettering.edu/~drussell/Demos/superposition/superposition.html

Thermal Energy & Energy Transfer

Energy can be transferred in 3 ways…

1. Conduction2. Convection3. Radiation

ConductionConduction: the transfer of

energy through direct contact of a material– occurs within a material or

between materials that are touching

What phase of matter do you think conduction travels best in?

SLOWER in gases is than in liquids and solids because the particles collide less often in a gas

Conduction occurs fastest in SOLIDS!!!

Thermal conductor: transfer of heat energy through a material

a) Examples: -Wire rack in hot oven- Tile floor

Thermal Insulator: material that is a poor conductor of heat and therefore reduces energy transfer

a) Examples: -Wooden spoon

- Air- Styrofoam

Materials can be Conductors or Insulators:

Convection: the movement of matter due to different temperatures having different densities!

Convection

Convection current: the vertical movement of air currents due to temperature variation

Convection currents are important in many natural cycles, such as ocean currents, weather systems, and movements of hot rock in Earth’s interior.

Convection causes currents!• Energy flows from HIGH to LOW (hot to cold)• Example: Oven

– Temp is higher at bottom of the oven– Air at bottom heats up & is then less dense

=HOT air= RISES– The air that was at the top, cools off & is then

more dense= COLD air= SINKS

Radiation: the energy that is transferred as electromagnetic waves, such as visible light & infrared waves

Travel: Requires NO medium!!

Examples: - Sun- Standing by grill & feel the heat- Heat lamp in restaurant

Radiation

Heat Transfer Recap!

THERMAL EQUILIBRIUM

• Thermal (heat) energy is continually being emitted & absorbed from objects.

• How much energy is absorbed vs emitted will determine if that object will change temperature.

• When the amount of energy absorbed is equal to the amount emitted, this is called THERMAL EQUILIBRIUM.

.

Which of these are at equilibrium?

Different surfaces interact differently with waves• Dark vs. Light

• What do we mean by absorbed by the surface? There are some instances where all the light hitting an object is captured or absorbed by the object. The light interacts with the surface and is not allowed to escape. So we see no light bouncing off the object and the object is said to be black.

• Rough vs Smooth- • Rough- light scatters• Diffuse reflection is the reflection of

light from a rough surface. The many different angles that incident light rays encounter at the surface cause reflection in many directions.

• Smooth- reflects in one direction

• Transparent vs Opaque– An object is said to

be transparent when light passes through it without being dispersed, or scattered

–  Opaque materials do not allow transmission of light waves. In other words, we can't see through an opaque object. We can only see the surface because the light waves are re-emitted from the surface back to our eyes.

• Translucent medium• translucent means 'shining through'

-- i.e, some light passes through, • translucent allows light through but

not an image (e.g. frosted glass).

Light cannot pass through ALL objects….

Overview

• Something that is transparent lets all light through without distorting the image -- it is as if the transparent object were not even there, visually speaking.

• Something that is translucent lets some but not all light through. Something that is opaque lets no light through.

• http://physics.bu.edu/~duffy/semester1/c20_wave_speed.html

• wave speed different medium• http://www.teachersdomain.org/resource/ess05.sc

i.ess.watcyc.wavemotion/• surface wave• http://www.acoustics.salford.ac.uk/feschools/wave

s/wavetypes.htm• transverse right angles• http://www.acoustics.salford.ac.uk/feschools/wave

s/reflect.htm#reflect• reflection