Unit 6 Waves and Light.

Unit 6 Waves and Light

Waves Wave – a repeating disturbance or movement that transfers energy through matter or space

Waves All waves transfer energy without transporting matter from place to place

Waves Example: Throwing a pebble into a puddle of water

Waves The pebble transfers some of its energy to nearby water molecules, causing them to move These molecules then pass the energy along to neighboring water molecules, which, in turn, transfer it to their neighbors

Waves The energy moves farther and farther from the source of the disturbance What you see is energy traveling in the form of a wave on the surface of the water

Waves The waves DO NOT carry the water along with them – they ONLY transfer energy

Waves A wave will travel only as long as it has energy to transfer
This is why the ripples on a puddle eventually die out after traveling for a short distance

Waves All waves are produced by something that vibrates

Waves Q: What are mechanical waves?
A: Waves that require a medium in which to travel.

Waves A medium is the matter that waves travel through
Mediums can be solid, liquid, or gas

Waves Examples of mechanical waves include sound waves, seismic waves, ocean waves, etc…

Waves Q: Describe two types of mechanical waves.
A: Transverse waves and compressional (or longitudinal) waves

Waves Transverse Waves –
matter in the medium moves back and forth at right angles to the direction that the waves travels in

Waves Example: ocean waves

Waves A wave in the ocean moves horizontally, but the water that the wave passes through moves up and down Think of people doing “the wave” at a football game

Waves Compressional Waves –
matter in the medium moves back and forth along the same direction the wave travels Also known as “longitudinal waves”

Waves Example: Sound Waves
When you talk, the air molecules are pushed together by the vibrations The compressions travel through the air to make a wave

Waves Q: Are there waves that don’t require a medium in which to travel? A: Yes.

Waves Electromagnetic waves can travel through a vacuum or empty space, as well as through matter

Waves Carry energy from place to place like mechanical waves
Differ from mechanical waves in how they are produced and how they travel Considered transverse waves

Parts of a Wave Q: What are the parts of a wave?
A: Crests and troughs or compressions and rarefactions.

Parts of a Wave Transverse waves have crests and troughs

Parts of a Wave Crests – high points in the wave Troughs –
low points in the wave

Parts of a Wave

Parts of a Wave Compressional waves have compressions and rarefactions

Parts of a Wave Compression –
region where the medium becomes crowded together, or more dense Rarefaction – the less-dense regions of a compressional wave

Parts of a Wave

Measuring Waves Q: How can we measure waves?
A: We use wavelength, frequency, amplitude, and speed

Measuring Waves Wavelength –
the distance between one point on a wave and the nearest point just like it

Measuring Waves

Measuring Waves With transverse waves, we measure wavelength from crest to crest or trough to trough

Measuring Waves With compressional waves, we measure wavelength from the start of one compression to the start of the next compression, or from the start of one rarefaction to the start of the next rarefaction

Measuring Waves Frequency –
the number of wavelengths that pass a fixed point each second

Measuring Waves Simply count the number of crests or troughs (or compressions or rarefactions) that pass by a given point each second

Measuring Waves The unit for frequency is Hertz (Hz)
One Hz means that one wavelength passes by in one second Hz = 1/s

Measuring Waves As frequency of a wave increases, wavelength decreases

Measuring Waves

Measuring Waves Frequency is always equal to the rate of vibration of the source that creates it

Measuring Waves

Measuring Waves Speed – how fast a wave travels

Measuring Waves Speed of a wave depends on the medium it is traveling through Example: Sound waves generally travel faster in a material when the temperature of the material is greater

Speed (m/s) = Frequency (Hz) x Wavelength (m)
Measuring Waves Calculating Wave Speed: Speed (m/s) = Frequency (Hz) x Wavelength (m) or v = f x 

Measuring Waves Sample Problem
What is the speed of a sound wave that has a wavelength of 2 m and a frequency of Hz?

Measuring Waves Wavelength = 2m Frequency = 170.5 Hz
1. What information are you given? Wavelength = 2m Frequency = Hz

Measuring Waves What is the unknown you are trying to solve for?
Speed (velocity)

Measuring Waves 3) Write an equation that contains both the given quantities and the unknown variable. v = f x 

Measuring Waves V= 170.5Hz x 2m V =341 m/s
4) Substitute in the known quantities and solve the equation. V= 170.5Hz x 2m V =341 m/s

Measuring Waves Practice Problem #1:
Waves in a lake have a wavelength of 6 m apart and pass a person on a raft with a frequency of 0.5 Hz. What is the speed of the waves?

What information are you given?
Wavelength = 6m Frequency = .5 Hz

What is the unknown you are trying to solve for?
Speed (velocity) of wave

Write an equation that contains both the given quantities and the unknown variable.
v = f x 

Substitute in the known quantities and solve the equation.
V= .5 Hz x 6m V= 3 m/s

A buoy bobs up and down in the ocean. The waves have a wavelength of 2.5 m, and they pass the buoy at a speed of 4.0 m/s. What is the frequency of the waves? How much time does it take for one wave to pass under the buoy?

Wavelength = 2.5 m Velocity = 4.0 m/s v = f x  4.0 m/s = f x 2.5m f= 4.0m/s f= 1.6 Hz 2.5m wave passes every .63 seconds 1wave/1.6hz

The musical note A above middle C has a frequency of 440 Hz. If the speed of sound is known to be 350 m/s, what is the wavelength of this note?

Frequency = 440 Hz Velocity = 350 m/s v = f x  350 m/s = 440 Hz x  = 350 m/s 440Hz = .8 m

Measuring Waves Amplitude –
related to the energy transferred by a wave

Measuring Waves The greater the wave’s amplitude, the more energy the wave transfers Amplitude is measured differently for transverse and compressional waves

Measuring Waves Transverse Waves - distance from the crest or trough of the wave to the rest position of the medium Think about the difference between a tall and a short wave when standing in the water? Which has more energy to knock you over?

Measuring Waves

Measuring Waves Compressional Waves - related to how tightly the medium is pushed together at the compressions The denser the medium at the compressions, the higher the amplitude of the wave and the more energy that is transferred

Measuring Waves

Waves Q: What’s in a wave? A: Energy, energy, energy…

ELECTROMAGNETIC WAVES
Q: Remind me again of the basic properties of waves… A: Here’s a short summary on how waves work:

All waves are produced by something that vibrates Waves transmit energy from one place to another

Some types of waves require a medium in which to travel, while other types of waves do not Sound waves require air particles to travel through

Water waves must have water molecules These waves travel because energy is transferred from particle to particle Without matter, these waves could not move

Electromagnetic waves do NOT require a medium to transfer energy – they can travel through space where no matter is present

Q: How do electromagnetic waves transfer energy without matter? A: They use electric and magnetic fields.

Instead of transferring energy from particle to particle, electromagnetic waves travel by transferring energy between vibrating electric and magnetic fields

Magnetic fields exist around all magnets, even if the space around the magnet contains no matter

Think about a paper clip being attracted to a magnet without the two even touching – this occurs because of the magnetic field around the magnet

Electric charges are surrounded by electric fields

This allows the charges to exert forces on each other even when they are far apart

Electric charges are also surrounded by magnetic fields

It’s the motion of electrons that generates the magnetic field An electric current flowing through a wire is surrounded by both an electric field, as well as a magnetic field

A changing magnetic field creates a changing electric field and vice versa When an electric charge vibrates back and forth, the electric field around it changes

Because the electric charge is in motion, it also has a magnetic field around it This magnetic field also changes as the electric charge vibrates

But how does this become an electromagnetic wave? The changing electric field around the charge creates a changing magnetic field, which in turn creates a changing electric field

This process continues, with each creating the other These vibrating electric and magnetic fields travel outward from the moving charge and vibrate at right angles to the direction the wave travels

This makes an electromagnetic wave (which is also a transverse wave)

Q: What are some properties of electromagnetic waves? A: Electromagnetic waves have speed, wavelength, and frequency like mechanical waves.

Wave Speed All electromagnetic waves travel at 300,000 km/s in the vacuum of space This is also known as the speed of light

Nothing travels faster than the speed of light in nature

When an electromagnetic wave travels through matter, this speed changes The speed will depend on the material the wave is passing through

Electromagnetic waves usually travel most slowly through solids and fastest through gases

Wavelength and Frequency Like all waves, electromagnetic waves can be described by their wavelengths and frequencies

Wavelength – Distance from one crest to another

Frequency – number of wavelengths that pass a given point in one second Also can be described as the number of vibrations made by the electric charge in one second

The wavelength and frequency of an electromagnetic wave are related As the frequency increases, the wavelength decreases and vice versa

Electromagnetic waves can behave as waves and as particles These particles are referred to as photons

The amount of energy in a photon is dependent on the frequency of the wave (rather than the amplitude)

The higher the frequency, the greater the energy in the photon

ELECTROMAGNETIC SPECTRUM
Q: Now that I understand electromagnetic waves, what exactly is the electromagnetic spectrum? A: The entire range of electromagnetic wave frequencies.

The electromagnetic spectrum is composed of 7 types of electromagnetic waves that interact with matter very differently

They are separated from one another based on their wavelengths and frequencies

Radio Waves Low-frequency electromagnetic waves with wavelengths longer than 1 mm

Radio waves with wavelengths less than 30 cm are called microwaves

Microwaves are best known for cooking food in microwave ovens Microwaves are also used for communication in cellular phones and satellite signals

Radio waves also have other uses To find the movement and position of objects using radar

To take a picture of the bones and soft tissue on the inside of a patient’s body using MRI

To carry audio signals from radio stations to your radio But remember, you can’t hear a radio wave

The audio signal that the radio wave carries is converted into sound through your radio What you can hear is the sound wave coming from the radio as it moves through the air

Infrared Waves Have higher frequencies than radio waves and lower frequencies than red light

Wavelengths vary from 1 mm to 750 nm 1 nanometer = 10-9 meters

Often used as a source of heat Red lamps in cafeterias keep food warm with infrared radiation

Other uses include: Controlling televisions through a remote control Reading CDs via a computer or game console

Detecting trapped victims or heat loss in a building through a thermogram

Visible Light The range of electromagnetic waves that you can detect with your eyes

Each wavelength in the visible spectrum corresponds to a specific frequency and has a particular color These range from long-wavelength red to short-wavelength violet

If all the colors are present, you see the light as white

Ultraviolet Waves Wavelengths vary from 400 nm to 4 nm

Ultraviolet waves are energetic enough to enter skin cells Overexposure can cause skin damage and cancer

Some ultraviolet waves are helpful Exposure allows the body to create vitamin D, which is needed for bones and teeth to absorb calcium

Also have the ability to kill bacteria on food or medical supplies Can make some materials fluoresce – absorbs the UV rays and reemits the energy as visible light

This is used by CSI technicians when looking for fingerprints or blood at a crime scene

X-Rays Very short wavelengths ranging from 12 nm to nm

Have high energy and can penetrate matter that light cannot

Often used in medicine, industry, and transportation to make pictures of the inside of solid objects

Gamma Rays Have the shortest wavelength in the electromagnetic spectrum at about nm or less

Have the highest frequency and therefore the most energy and greatest penetrating ability of all the electromagnetic waves

Exposure to small amounts of gamma rays are tolerable, but overexposure can be deadly

Used in the medical field to kill cancer cells and make pictures of the brain based on brain activity

Unit 6 Waves and Light.

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