Waves, Light, and Sound

Teacher Domain: Waves

http://www.teachersdomain.org/resource/lsps07.sci.phys.energy.waves/

Waves Notes 1) Two Major Types

A. Mechanical - must travel through a medium (water, sound)

B. Electromagnetic - no medium required, can travel through a vacuum (light, UV, radio, gamma rays)

Mechanical waves

Transverse waves - medium travels at a right angle to the direction of the wave

Parts of a Mechanical Transverse Wave Crest – highest point of a wave Trough – lowest point of a wave Wave Height – vertical distance

between the crest and the trough Wavelength – horizontal distance

between two crests or two troughs Amplitude is the distance from the

baseline to the top of the crest or bottom of the trough.

Wave Parts

http://www.teachersdomain.org/resource/lsps07.sci.phys.energy.amplitude/

Mechanical waves

Compressional waves - medium travels in the same direction as the wave.

Frequency - the number of waves to pass a point in a given period of time. Can be calculated as

f = # of waves (cycles) / t

units

f - frequency Hertz (Hz)

(cycles)/ second

t - time second

Wave measurementsPeriod - how long it takes one wave to pass a

point. Can be calculated as

T = 1 / f

units

T - period seconds

f - frequency Hertz (Hz)

(cycles)/ second

Flash card

T =1

f

1

T f

Period Units

T - period sec, min

f - frequency hertz

Flash card

t

# of wavesf =

waves

tf

#of

Frequency Units

t - time sec, min

f - frequency hertz

# of waves cycles

Wavelength - length of one wave cycle (wave cycle is a crest and a trough or one compression and one rarefaction). Can be calculated as

λ = total distance (d) / # of waves

units

λ – wavelength m, mm, nm, pm, μm

d – distance m, mm, nm, pm, μm

Flash card

d

# of waves

d

# ofwaves

Wavelength Units

- wavelength m, cm, nm

d - total distance m, cm, nm

# of waves cycles

Wave velocity – how fast a wave travels. Can be calculated as

v = λ funits

v – velocity m/sec, μm/secλ – wavelength μm, m, mm, nm, pmf – frequency Hertz (Hz) = /sec

NOTE: There are no set units for velocity or wavelength

Flash card

v = f

v

f

Wave velocity Units

- wavelength cm, m, nm

f - frequency hertz

v - velocity cm/sec, m/sec, nm/sec

Brain Pop: Waves

http://glencoe.mcgraw-hill.com/sites/0078802482/student_view0/brainpop_movies.html#

ExampleIf an earthquake’s p-wave travels at 5600

m/sec with a wavelength of 372 m, what is the frequency of the wave?

v = 5600 m/sec f = ?

λ = 372 m v = λ f

f = v / λ

f = 5600 m/sec / 372 m

= 15.05 /sec = 15.05 Hz

WAVE PRACTICE PROBLEMS

It takes 2 seconds for this wave to go this distance.

1. What is the period of this wave?

2. What is the frequency of this wave?

3. What is the wavelength of this wave?

4. What is the speed of this wave?

Teacher Domain Video: What is Sound?http://www.teachersdomain.org/resource/phy03.sci.phys.howmove.collage/

5. 2 waves leave the same point at the same time. They both travel 30 m. The first wave takes 30 seconds, what is its velocity? The second wave takes 180 seconds, what is its velocity?

6. What is the frequency of a 990 m/sec wave that has a wavelength of 30 m?

The speed of a wave depends on the medium and temperature.

Liquids conduct sound better than gases. Solids conduct sound better than liquids. This is due to how close the particles are to each other.

In air, the speed of sound is 344 m/sec at 20 ºC, but only 332 m/sec at 0ºC. This is due to the particles moving faster at higher temperatures.

How high or low the frequency of a sound is, is the pitch of the sound. The average human hearing range is 20 Hz to 20000 Hz.

Sounds higher than 20000 Hz are ultrasonic. Sounds lower than 20 Hz are subsonic or

infrasonic.

Loudness of a sound depends on the person hearing it.

Loudness is related to the intensity of the sound.

The higher the intensity, the louder the sound.

The intensity of a sound is determined by the amount of energy in a wave.

Therefore, the higher the amplitude the higher the intensity.

Intensity level for a sound is measured in decibels (dB).

At 120 dB, a sound causes pain and permanent hearing loss.

Intensities of Some Sounds

Lawn mower 100 dB

Chain Saw 115 dB

Jet taking off 150 dB

Cat purring 25 dB

Sound: Pitch and Intensity

http://www.teachersdomain.org/resource/hew06.sci.phys.energy.sound/

Doppler effect results in higher and lower sound/light intensity as an object move toward/away from us.

Approaching sounds get louder. Retreating sounds get quieter.

Approaching lights get brighter. Retreating lights get dimmer.

Doppler Effect

Teacher Domain: Video Doppler

http://www.teachersdomain.org/resource/phy03.sci.phys.energy.doppler/

Doppler Effect

http://glencoe.mcgraw-hill.com/sites/0078807220/student_view0/chapter15/concepts_in_motion.html

Types of sounds1. White noise – all waves

have the same amplitude but different frequencies – it is calming.

2. Noise – no set pattern or definite pitch – it is unpleasant/irritating.

3. Music – definite pattern using special pitches and sound quality.

THE MAKING OF SOUND

1. resonance – making an object vibrate at its natural frequency

Overtones – when each half of a string vibrates on its own

The number of overtones produced determines the quality of sound.

Each overtone is a multiple of the fundamental frequency• 1st overtone 2 X FF• 2nd overtone 3 X FF• 3rd overtone 4 X FF

6. notes – there are 8 notes in music each with its own frequency

7. The frequency range for a note is an octave.

8. The highest note in an octave is 2X the frequency of the lowest note.

Wave Interferences1. interference – combining of 2 or

more waves to make a new wave

2. constructive – compressions/crest arrives together and combine → greater compression/crest amplitude ↑ → loudness↑

3. destructive – compression arrives with rarefaction → less compression or canceling of the wave amplitude ↓ or 0 → loudness ↓ or 0

http://glencoe.mcgraw-hill.com/sites/0078807220/student_view0/chapter14/concepts_in_motion.html

Reverberation

echoes

Electromagnetic waves Waves that form the electromagnetic spectrum

have characteristics of both waves and particles.

These are transverse waves.

The waves can travel through empty space.

Energy is transferred by radiation.

Teacher Domain: Electromagnetic Waveshttp://www.teachersdomain.org/resource/phy03.sci.phys.energy.emspectrum/

Teacher Domain: Electromagnetic Waveshttp://www.teachersdomain.org/resource/phy03.sci.phys.energy.nasaspectrum/

Produced by electrically charged particles.

This is electromagnetic radiation.

In a vacuum, all waves have the same velocity, even though they have different frequencies and wavelengths.

The radiation has momentum because it is particle-like and travels in bundles called photons.

THE ELECTROMAGNETIC SPECTRUM

A. Radio wavesComposed of AM radio, FM radio, shortwave radio, TV,

radar, and microwaves. Have the longest wavelength and lowest frequency. Sound and images are transmitted by modulation –

varying of amplitudes and frequencies – this process is used by radio, tv, cellular phones, and cordless phones.

Microwaves are used for communication and cooking. These have the highest frequency and energy.

Longer wavelength than visible but shorter wavelength than radio waves.

Used to measure the amount of heat given off by an object.

B. Infrared

Uses for IR radiation Medicine can use it to detect tumors which are areas of

higher heat than the rest of the body Construction – to determine areas of a house that needs

more insulation (areas of greatest heat loss) Security – to set-up alarm systems and trigger alarms. Military – Night vision and heat sensing equipment to find

targets and individuals or shelter

Teacher Domain Video: IR

http://www.teachersdomain.org/resource/ess05.sci.ess.earthsys.infrared/

C. Visible

The light we see. Very small portion of the spectrum.

Broken into 7 bands of color.

Used to cause chemical reactions. Energy source for photosynthesis.

Teacher Domain: Video Light

http://www.teachersdomain.org/resource/lsps07.sci.phys.energy.lightcolor/

D. Ultraviolet

Shorter wavelength than visible.

Our body uses it to make Vitamin D.

Blocked to a degree by the ozone (O3) layer of the atmosphere. Sunglasses and sunscreen can block its rays.

Uses:Destroy bacteria, viruses and sterilize

materials (including water)Detect fluorescence. (stamps at an

amusement park)

Side effects:Overexposure can kill healthy cellsProlonged and frequent exposures and lead

to sagging, dry skin and in some cases skin cancer.

E. X-raysShorter wavelength than UV and higher frequency

with higher energy and greater penetrating power.

Can travel through several types of matter (skin, muscle, suitcases)

Absorbed by dense materials

Contained within lead container

UsesX-ray pictures of bones and body tissue for

diagnosis.X-rays of luggage to reduce searches

Side effects:Loss of hair, fatigue, and nausea due to

prolonged exposureDestruction of body tissueBurns

F. Gamma raysShortest wavelength, highest frequency, most energy of any

wave.

Can penetrate most materials Uses:

Radiation treatment for cancer or destroy a group of cells Side effects:

Major exposure – death Moderate exposure – cancer Prolonged exposure – loss of hair, fatigue and nausea

VISIBLE LIGHT Materials are classified according to how light

passes through them.

Opaque materials block all light and allow no light to pass through. This light can be reflected or absorbed by the material

Transparent materials allow light to pass through and objects can be seen clearly.

Translucent materials allow some light to pass through but objects can not be seen clearly

Colors are perceived by the absorption and reflection of lightBlue objects reflect blue light.White objects reflect all light.Black objects absorb all light.

Colors Reflect and Absorb of Light

Light Colors

The primary colors of light are green, blue and red.

When primary color of light are combined, white light is produced. These are additive colors.http://glencoe.mcgraw-hill.com/sites/0078807220/student_view0/chapter18/concepts_in_motion.html

Pigments are colored materials that absorb some colors and reflect others. 3 primary pigment colors

are magenta (red), cyan (blue), and yellow

When primary color pigments are combined the pigments make black. These are subtractive colors.

Pigment Colors

THE HUMAN EYE AND SIGHT Our eye picks-up dim light and movement in the rod

cells in the back of our eye.

The cone cells pick-up color and brightness. They are specialized to pick-up certain colors. Red cones pick-up red and yellow light. Blue cones pick-up blue and violet light. Green cones pick-up green and yellow light.

Cones and rods pick-up the information of what has stimulated them and pass it to the brain along the optic nerve.

Color BlindnessSome people, mostly males, experience color

blindness. This is an inherited trait usually passed through

the mother (x-chromosome trait). A person will not be able to distinguish one or

more color. Usually reds and greens. Some have trouble with blue.

PROPERTIES OF LIGHTLight like all electromagnetic radiation acts like

a wave when it bounces and bends.

1. Reflection occurs when light strikes an object and bounces off.

Incoming light is the incident beam The bounced off light is the reflected beam Smooth surfaces reflect light in one

direction, giving an image. While rough surfaces reflect light in many directions and do not give an image.

Law of reflection states that the angle of incidence is equal to the angle of reflection.

http://glencoe.mcgraw-hill.com/sites/0078807220/student_view0/chapter17/concepts_in_motion.html

normal

Reflected light

Incoming light

i

r

i = angle of incidence r = angle of reflection

Smooth surface

Refraction occurs when light passes from one medium to another and the angle of the light waves is bent.The bending is

caused by a change in the speed of the light waves.

http://glencoe.mcgraw-hill.com/sites/0078807220/student_view0/chapter18/concepts_in_motion.html

Diffraction occurs when the wave bends to go around a barrier.

Diffraction occurs in sound, water, and electromagnetic waves.

Diffracting grating are a series of slits that can bend light and separate it into its colors.

Interference also occurs in light waves when crests combine – bright bands form.

When a crest and a trough combine – dark bands form.

Additional Facts Scattered light is redirected in all directions

When particles of dust scatter blue light, the sky appears blue.

Yellow light is produced when red and green overlap

Cyan pigment will be used to produce blue and greens. While magenta pigment will be used to produce orange and purples.

Ultraviolet light can be seen by bees but not by humans.

During sunset and sunrise orange light is scattered giving the sky an orange-cast.

Light is absorbed by the retina of the eye.

Mirrors

Plane mirrors have a reflective coating on the back of a flat piece of glass.

Produce virtual images. Images that have no light rays

passing through them and appear to come from behind the mirror.

Concave mirrors curve inward and can reflect in three ways.

1. If the object is placed beyond the focal point of the mirror, an upside down, enlarged, real image will appear.

2. If the object is placed on the focal point, no image is produced.

3. If the object is placed between the focal point and the mirror, an upright, enlarged, virtual image is produced.

Convex mirrors curve outward and produce small, virtual images.

For all mirrors: Optical axis is a straight line drawn through the center

of the mirror Focal point is the place on the optical axis where the

parallel lines of reflection meet. Focal length is the distance from the focal point to the

center of the mirror. Virtual images have no light rays passing through

them. Real images are produced by reflected light rays.

LensesConvex lens are thick in the middle and thin on the

edges These lenses refract light to a focal point and an

image can be projected. These lenses can produce real or virtual, enlarged

or reduced, or upright or inverted images according to the location of the object.

Used to correct farsightness.

Concave lens are thinner in the middle and thicker at the edges.

These lenses refract light toward the edges. These lenses never produce a real image. Image is virtual, upright and reduced. Used to correct nearsightness.

Optical Instruments

Aid the human eye to make observations

1. Telescopes – view faraway objects

2. Refracting telescope Use 2 convex lens Due to size of lenses, images can be

distorted if it sags Lenses are very heavy and costly to make

3. Reflecting telescope Use a concave mirror, plane mirror and a convex lens

4. Binoculars A refracting telescope with 2 lenses for each eye and a

reflecting prism to invert the image so we see an upright image.

NOTE: Not all radiation passes through our atmosphere, therefore, to get a clear image of what is in space we send telescopes above our atmosphere. These telescope pick-up UV, IR, x-rays, and visible light.

5. Microscope 2 convex lens used to magnify small

objects. 1st lens produces an enlarged real

image that the 2nd lens enlarges and changes into a virtual image.

6. Cameras Lenses gather light and project an image on

light-sensitive film. The shutter controls the amount of light that

enters the aperture. The image produced is real and inverted

but smaller than the object.

Wide-angle lensesproduce small image that includes much of

the surroundingshave short focal length

Telephoto lenseshave long focal lengthprotrude from the camera bringing the image

closer to the camera

Polarized lightAll light waves vibrate in one plane.

Some 3D images can be produced using polarized filters that are aligned at 90º angle to each other. Two projectors with filters are used to produce the image on the screen.

Polarizing filters can be used on cameras to reduce glare.

LasersUse coherent light – all crest and troughs are aligned and

travel in the same direction and have the same wavelength.

Use 2 mirrors – one that is partially coated to produce photons of light that have the same energy. When the light has the right wavelength it is allowed to pass through the holes in the coating on the second mirror.

Uses: Read bar codes, read CD’s, surgery, cutting and welding,

surveying and leveling

Optical fibers Total internal reflection occurs when light strikes

the surface between two materials and reflects totally back into the first material.

Optical fibers transmit light from one end of the fiber to the other.Consist of a glass core surrounded by another

layer of glass and encased inside a plastic coating.

The light reflects within the core and can only exit the ends.

Uses:Communication – TV, telephone, computersMedical examinations and surgery

Advantages:One fiber can carry thousands of phone

conversationsSignals don’t leak or interfere with other

fibers.