Transcript Waves Sound and Light [CH 17-18]

Waves, Sound and Light
Chapter 17 & 18
Mechanical Waves
• A mechanical wave is created when a source
of energy causes a vibration to travel through
a medium.
–A mechanical wave is a disturbance in matter that carries
energy from one place to another.
• The material through which a wave travels is called a medium.
• Mechanical waves require a medium to travel through. Solids,
liquids, and gases all can act as mediums.
• A vibration is a repeating back-and-forth motion.
Mechanical Waves
• There are three types of mechanical waves
– Transverse wave - a wave that causes the medium
to vibrate at right angles to the direction in which
the wave travels
– Longitudinal wave – a wave in which the vibration
of the medium is parallel to the direction the wave
travels.
– Surface wave - a wave that travels along a surface
separating two media.
Mechanical Waves
• Transverse waves
• The highest point of the wave is the crest.
• The lowest point of the wave is the trough.
• A single point on the rope vibrates up and down between
a crest and trough.
Mechanical Waves
•A transverse wave is a wave that causes the
medium to vibrate at right angles to the
direction in which the wave travels.
•The wave carries energy from left to right, in a
direction perpendicular to the up-and-down
motion of the rope.
Mechanical Waves
• Longitudinal Waves
• An area where the particles in a medium are spaced close
together is called a compression.
• An area where the particles in a medium are spread out is
called a rarefaction.
A.
B.
A compression starts
to move along the
spring.
A rarefaction follows
the compression
along the spring.
Properties of Mechanical Waves
Any motion that repeats at regular intervals is called
periodic motion.
• Frequency ( f )– the number of complete cycles in a
given time
– Frequency is measured in cycles per second or hertz (Hz)
– f = 1/T
• Period (T)– the time required for one complete cycle
• Wavelength (λ) – the distance between a point on one
wave and the same point on the next cycle of the wave
Note: Increasing the frequency of a wave decreases its
wavelength – they are inverses of each other
Properties of Mechanical Waves
• Wave Speed
–When the wavelength is in meters, and the frequency is in
hertz, the units for speed are meters per second.
–The speed of a wave is also calculated by dividing its
wavelength by its period.
Note: v = λf = λ/T
Problem
One end of a rope is vibrated to produce a
wave with a wavelength of 0.25 meter. The
frequency of the wave is 3.0 hertz. What is the
speed of the wave?
Problem
A wave on a rope has a wavelength of 2.0 m
and a frequency of 2.0 Hz. What is the speed
of the wave?
Problem
A motorboat is tied to a dock with its motor
running. The spinning propeller makes a
surface wave in the water with a frequency of
4 Hz and a wavelength of 0.1 m. What is the
speed of the wave?
Problem
What is the speed of a wave in a spring if it
has a wavelength of 10 cm and a period of
0.2 s?
Problem
What is the wavelength of an earthquake
wave if it has a speed of 5 km/s and a
frequency of 10 Hz?
Properties of Mechanical Waves
• The speed of a wave can change if it enters a
new medium, or if variables such as pressure
and temperature change.
• For many kinds of waves, the speed of the
waves is roughly constant for a range of
different frequencies.
• The wave with the lower frequency has a
longer wavelength.
Properties of Mechanical Waves
• Amplitude
– The amplitude of a wave is the maximum
displacement of the medium from its rest
position.
– The more energy the wave has, the greater the
wave’s amplitude
Behavior of Waves
• Reflection occurs when a wave bounces off a
surface that it cannot pass through
– Does not change the speed of the wave or
frequency, but the wave can be flipped upside
down
• Refraction is the bending of a wave as it enters
a new medium at an angle.
– The speed of the wave changes so one side of the
wave moves slower than the other side
Behavior of Waves
• Diffraction is the bending of a wave as it
moves around an obstacle or passes through a
narrow opening
• A wave diffracts more if its wavelength is large
compared to the size of an opening or
obstacle
Behavior of Waves
• Interference occurs when two or more waves
overlap and combine together
• Two types of interference:
– Constructive = larger displacement
– Destructive = smaller displacement
Behavior of waves
• Standing wave
– A wave that appears to stay in one place – doesn’t
move through a medium
Note: A standing wave forms only if half a
wavelength or a multiple of half a wavelength
fits exactly into the length of a vibrating cord.
Behavior of Waves
Standing Waves
•A node is a point on a standing wave that has
no displacement from the rest position. At the
nodes, there is complete destructive
interference between the incoming and
reflected waves.
•An antinode is a point where a crest or trough
occurs midway between two nodes.
Properties of Sound Waves
Sound Waves –
longitudinal waves
(undergo compression
and rarefaction)
• Speed – the speed of a
sound wave varies in
different media.
– Effected by density,
temperature, and
pressure of the media
Properties of Sound Waves
Intensity is the rate at which a wave’s energy flows
through a given area.
• Sound intensity depends on both the wave’s amplitude
and the distance from the sound source.
• The decibel (dB) is a unit that compares the intensity of
different sounds.
For every 10-decibel increase, the
sound intensity increases tenfold.
•A 0-decibel sound can just
barely be heard.
•A 20-decibel sound has 100
times more energy per second
than a 0-decibel sound.
•A 30-decibel sound delivers
1000 times more energy per
second than a 0-decibel sound.
Properties of Sound Waves
Loudness is a physical response to the intensity of
sound, modified by physical factors.
• The loudness depends on sound intensity.
• Loudness also depends on factors such as the health of
your ears and how your brain interprets sound waves.
Properties of Sound Waves
• Frequency and Pitch
The frequency of a sound wave depends on how fast
the source of the sound is vibrating.
The air in the tubing of brass instruments forms a
standing wave. Longer tubing makes a standing wave
with a longer wavelength and a lower frequency.
Pitch is the frequency of a sound as you perceive it.
• High-frequency sounds have a high pitch, and lowfrequency sounds have a low pitch.
• Pitch also depends on other factors such as your
age and the health of your ears.
Properties of Sound Waves
Ultrasound
Most people hear sounds between 20 hertz and 20,000
hertz.
• Infrasound is sound at frequencies lower than most people
can hear.
• Ultrasound is sound at frequencies higher than most
people hear.
Properties of Sound Waves
The Doppler Effect
• The Doppler effect is a change in sound
frequency caused by motion of the sound
source, motion of the listener, or both.
Electromagnetic (EM)Waves
Electromagnetic waves are transverse waves consisting of
changing electric fields and changing magnetic fields.
• Like mechanical waves, electromagnetic waves carry energy from
place to place.
• Electromagnetic waves differ from mechanical waves in how they are
produced and how they travel.
Electromagnetic waves are produced by constantly changing
electric fields and magnetic fields.
• An electric field in a region of space exerts electric forces on charged
particles. Electric fields are produced by electrically charged particles
and by changing magnetic fields.
• A magnetic field in a region of space produces magnetic forces.
Magnetic fields are produced by magnets, by changing electric fields,
and by vibrating charges.
Electromagnetic (EM)Waves
Electromagnetic waves are transverse waves
because the fields are at right angles to the
direction in which the wave travels.
Electromagnetic (EM)Waves
Changing electric fields produce changing magnetic
fields, and changing magnetic fields produce changing
electric fields, so the fields regenerate each other.
• Electromagnetic waves do not need a medium.
• The transfer of energy by electromagnetic waves traveling
through matter or across space is called electromagnetic
radiation.
Note: speed of light in a vacuum (c) = 3x108m/s
Electromagnetic (EM)Waves
The speed of an electromagnetic wave is the product of
its wavelength and its frequency.
• The speed of electromagnetic waves in a vacuum is
constant, so the wavelength is inversely proportional to
the frequency.
• As the wavelength increases, the frequency decreases.
Problem
A radio station broadcasts a radio wave with a
wavelength of 3.0 meters. What is the
frequency of the wave?
Problem
A global positioning satellite transmits a radio
wave with a wavelength of 19 cm. What is the
frequency of the radio wave? (Hint: Convert
the wavelength to meters before calculating
the frequency.)
Problem
The radio waves of a particular AM radio
station vibrate 680,000 times per second.
What is the wavelength of the wave?
Problem
Radio waves that vibrate 160,000,000 times
per second are used on some train lines for
communications. If radio waves that vibrate
half as many times per second were used
instead, how would the wavelength change?
Electromagnetic (EM) Waves
Wave-Particle Duality
Interference pattern
appears on screen.
Card with two
slits
• Electromagnetic radiation
with one
behaves as both a wave Card
slit
Light source
and a particle
– Evidence of particles –
shadows and
photoelectric effect
• Light is made of packets of
energy called photons
– Evidence of a wave –
double slit experiment
Bright bands
show
constructive
interference.
Light from
single slit
produces
coherent light
at second card.
Dark bands
show
destructive
interference.
Electromagnetic (EM) Waves
Intensity is the rate at which a wave’s energy flows
through a given unit of area. A wave model also
explains how intensity decreases.
• As waves travel away from the source, they pass through a
larger and larger area.
• The total energy does not change, so the wave’s intensity
decreases.
Electromagnetic Spectrum
The full range of frequencies of electromagnetic
radiation is called the electromagnetic spectrum.
• Visible light is the only part of the electromagnetic
spectrum that you can see, but it is just a small part.
• Each kind of wave is characterized by a range of
wavelengths and frequencies. All of these waves have
many useful applications.
The electromagnetic spectrum consists of
radio waves, infrared rays, visible light,
ultraviolet rays, X-rays, and gamma rays.
Electromagnetic Spectrum
Electromagnetic Spectrum
Radio waves have the longest wavelengths in
the electromagnetic spectrum. Wavelengths
range from 1 millimeter to as much as
thousands of kilometers or longer.
Radio waves also have the lowest frequencies in
the spectrum—300,000 megahertz (MHz) or
less.
Electromagnetic Spectrum
The shortest-wavelength radio waves are called microwaves.
Microwave wavelengths are from about 1 m to about 1 mm.
• Frequencies vary from about 300 MHz to about 300,000 MHz.
• Microwaves cook and reheat food. Microwaves also carry cell phone
conversations. The process works much like a radio broadcast.
Radar
The word radar is an acronym for radio detection and ranging.
Radar technology uses a radio transmitter to send out short
bursts of radio waves.
• The waves reflect off the objects they encounter and bounce back
toward where they came from.
• The returning waves are then picked up by a radio receiver.
Electromagnetic Spectrum
Infrared rays have higher frequencies than radio
waves and lower frequencies than red light.
Infrared wavelengths vary from about 1 millimeter
to about 750 nanometers (10–9 meter).
Your skin senses infrared radiation as warmth.
Restaurants use infrared lamps to keep foods warm.
Warmer objects give off more infrared radiation
than cooler objects.
Electromagnetic Spectrum
The visible part of the
electromagnetic
spectrum is light that
the human eye can
see.
Each wavelength in
the visible spectrum
corresponds to a
specific frequency and
has a particular color.
Electromagnetic Spectrum
Ultraviolet rays vary from about 400 nm to about 4 nm.
• Some exposure to ultraviolet rays helps your skin produce
vitamin D, which helps the body absorb calcium from
foods.
• Excessive exposure can cause sunburn, wrinkles, skin
cancer, and eye damage.
• Ultraviolet rays are used to kill microorganisms. In winter,
plant nurseries use ultraviolet lights to help plants grow.
Electromagnetic Spectrum
X-Rays
X-rays have very short wavelengths, from about
12 nm to about 0.005 nm.
X-rays have high energy and can penetrate
matter that light cannot.
Too much exposure to X-rays can kill or damage
living tissue.
Electromagnetic Spectrum
Gamma rays have the shortest
wavelengths in the
electromagnetic spectrum,
about 0.005 nm or less.
They have the highest
frequencies, the most energy,
and the greatest penetrating
ability of all the
electromagnetic waves.
Exposure to tiny amounts of
gamma rays is tolerable, but
overexposure can be deadly.
Electromagnetic Spectrum
There are two ways that signals are encoded for radio.
• In amplitude modulation, the amplitude of the wave is
varied. The frequency remains the same. AM radio stations
broadcast by amplitude modulation.
• In frequency modulation, the frequency of the wave is
varied. The amplitude remains the same. FM stations
broadcast by frequency modulation.
Behavior of Light
Materials can be transparent, translucent, or opaque
• A transparent material transmits light, which means it
allows most of the light that strikes it to pass through
it.
• A translucent material scatters light. If you can see
through a material, but the objects you see through it
do not look clear or distinct, then the material is
translucent.
• An opaque material either absorbs or reflects all of the
light that strikes it. Most materials are opaque.
Behavior of Light
When light strikes a new medium, the light can be
reflected, absorbed, or transmitted.
Reflection
•An image is a copy of an object formed by reflected (or
refracted) waves of light.
•Regular reflection occurs when parallel light waves strike a
surface and reflect all in the same direction.
•Diffuse reflection occurs when parallel light waves strike a
rough, uneven surface and reflect in many different directions
Behavior of Light
When light is transmitted it can be refracted, polarized, or
scattered
Refraction
•A light wave can refract, or bend, when it passes at an angle
from one medium into another.
•Refraction makes underwater objects appear closer and
larger than they really are.
•Refraction can also make an object appear to break at the
surface of the water.
•Refraction can also sometimes cause a mirage, a false or
distorted image
Behavior of Light
Polarization
• Light with waves that vibrate in only one plane
is polarized light. Light reflecting from a
nonmetallic flat surface, such as a window or
the surface of a lake, can become polarized
Behavior of Light
In scattering, light is redirected as it passes
through a medium.
Color
Sunlight is made up of all the colors of the visible spectrum.
A prism separates white light into a visible spectrum.
• When red light, with its longer wavelength, enters a glass prism,
it slows down the least of all the colors.
• Red light is bent the least.
• Violet light is bent the most.
The process in which white light separates into
colors is called dispersion. A rainbow forms when
droplets of water in the air act like prisms.
Color
An object’s color is the color of light that reaches your eye when
you
look at the object.
Primary colors are three specific colors that can be combined in
varying amounts to create all possible colors.
The primary colors of light are red, green, and blue
Each secondary color of light is a combination of two primary
colors.
The secondary colors of light are cyan, yellow, and magenta.
•If you add a primary color to the proper secondary color, you will
get white light.
•Two colors of light that combine to form white light are
complementary colors of light.
•A complementary color pair is a combination of one primary
color and one secondary color
Color
The primary colors of pigments are cyan,
yellow, and magenta.
A pigment is a material that absorbs some colors of light and reflects other colors.
• Paints, inks, photographs, and dyes get their colors from pigments.
• Color printers and photocopiers use three colors–cyan, magenta, and yellow–
plus black.
• You can mix varying amounts of these primary pigment colors to make almost
any other color.
Any two colors of pigments that combine to
. make black pigment are complementary colors
of pigments.
•Cyan and magenta combine to form blue.
•Cyan and yellow combine to form green.
•Yellow and magenta combine to form
red.
•The secondary colors of pigments are red,
green, and blue