Transcript 18.1 Electromagnetic Waves Chapter 18: The Electromagnetic Spectrum and Light

Chapter 18: The Electromagnetic
Spectrum and Light
18.1 Electromagnetic
Waves
Electromagnetic waves are
produced when an electric
charge vibrates or accelerates.
Electromagnetic waves can
travel through a vacuum, or
empty space, as well as through
matter.
Electromagnetic waves are
transverse waves consisting of
changing electric fields and changing
magnetic fields.
• Like mechanical waves, they
carry energy from place to place.
• Electromagnetic waves differ
from mechanical waves in how
they are produced and how they
travel.
How They Are Produced
Electromagnetic waves are produced by
constantly changing electric fields and
magnetic fields.
• An electric field exerts electric forces on
charged particles.
• Electric fields are produced by electrically
charged particles and by changing magnetic
fields.
• A magnetic field produces magnetic forces.
• Magnetic fields are produced by magnets, by
changing electric fields, and by vibrating
charges.
Electromagnetic waves are
transverse waves because the fields
are at right angles to the direction in
which the wave travels.
How They Travel
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.
The Speed of Light
All electromagnetic waves
travel at the same speed
when in a vacuum, regardless
of the observer’s motion.
The speed of light in a
vacuum, c, is 3.00 × 108
meters per second.
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.
Electromagnetic radiation
behaves sometimes like a wave
and sometimes like a stream of
particles.
The emission of electrons from a
metal caused by light striking the
metal is called the photoelectric
effect.
In 1905, Albert Einstein (1879–1955)
proposed that light, and all
electromagnetic radiation, consists of
packets of energy.
These packets of electromagnetic
energy are now called photons.
Each photon’s energy is proportional
to the frequency of the light.
Blue light has a higher frequency
than red light, so photons of blue
light have more energy than photons
of red light.
The intensity of light
decreases as photons
travel farther from the
source.
Intensity is the rate at which a
wave’s energy flows through a given
unit of area.
• 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.
Intensity
The closer you are to a surface when you
spray paint it, the smaller the area the
paint covers, and the more intense the
paint color looks.
Assessment Questions
1. How are electromagnetic waves different
from all mechanical waves?
a. Electromagnetic waves don’t carry
energy.
b. Electromagnetic waves are invisible.
c. Electromagnetic waves are longitudinal
waves.
d. Electromagnetic waves can travel through
a vacuum.
Assessment Questions
1. How are electromagnetic waves different
from all mechanical waves?
a. Electromagnetic waves don’t carry
energy.
b. Electromagnetic waves are invisible.
c. Electromagnetic waves are longitudinal
waves.
d. Electromagnetic waves can travel
through a vacuum.
Assessment Questions
2. What is the wavelength of a radio
wave that has a frequency of 1.5 x 106
Hz? (c = 3.0x108 m/s)
a. 45 m
b. 200 m
c. 450 m
d. 2 km
Assessment Questions
2. What is the wavelength of a radio
wave that has a frequency of 1.5 x 106
Hz? (c = 3.0x108 m/s)
a. 45 m
b. 200 m
c. 450 m
d.2 km
wave speed = λf
Assessment Questions
3. The photoelectric effect is evidence
that light behaves like
a. a wave.
b. a particle.
c. both a wave and a particle.
d. neither a wave nor a particle.
Assessment Questions
3. The photoelectric effect is evidence
that light behaves like
a. a wave.
b.a particle.
c. both a wave and a particle.
d. neither a wave nor a particle.
Assessment Questions
4.
As photons travel farther from a light
source, the intensity of light stays the
same.
True
False
Assessment Questions
4.
As photons travel farther from a light
source, the intensity of light stays the
same.
True
False
Chapter 18: The Electromagnetic
Spectrum and Light
18.2 The
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.
Radio waves have the longest
wavelengths in the electromagnetic
spectrum.
Radio waves also have the lowest
frequencies.
Microwaves
The shortest-wavelength radio waves are
called microwaves.
Microwaves cook and reheat food.
Microwaves also carry cell phone
conversations. The process works
much like a radio broadcast.
Radar (radio detection and ranging)
Radar technology uses a radio
transmitter to send out short bursts
of radio waves.
• They reflect off the objects they
encounter and bounce back
toward where they came from.
• Returning waves are then picked
up by a radio receiver.
Infrared rays have higher
frequencies than radio waves and
lower frequencies than red light.
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.
A device called a thermograph uses
infrared sensors to create
thermograms, color-coded pictures
that show variations in temperature.
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.
Ultraviolet rays have shorter
wavelengths and higher frequencies
• 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.
X-rays have very short
wavelengths
X-rays have high energy and can
penetrate matter that light
cannot.
Too much exposure to X-rays can
kill or damage living tissue.
Gamma rays have the shortest
wavelengths in the electromagnetic
spectrum.
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.
• Gamma rays are used in
•
•
radiation therapy to kill
cancer cells without
harming nearby healthy
cells.
Gamma rays are also used
to make pictures of the
human brain, with different
levels of brain activity
represented by different
colors.
Pipelines are checked with
machines that travel on the
inside of a pipe, taking
gamma ray pictures along
the entire length.
Radiation treatment
using radioactive cobalt60.
Assessment Questions
1. Which waves have the longest
wavelength?
a. radio waves
b. infrared rays
c. visible light
d. ultraviolet rays
Assessment Questions
1. Which waves have the longest
wavelength?
a.radio waves
b. infrared rays
c. visible light
d. ultraviolet rays
Assessment Questions
2. What type of electromagnetic radiation
is used to keep prepared foods warm
in a serving area?
a. ultraviolet rays
b. infrared rays
c. X-rays
d. gamma rays
Assessment Questions
2. What type of electromagnetic radiation
is used to keep prepared foods warm
in a serving area?
a. ultraviolet rays
b.infrared rays
c. X-rays
d. gamma rays
Chapter 18: The Electromagnetic
Spectrum and Light
18.3 Behavior of Light
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.
An opaque object does not allow
any light to pass through it.
When light strikes a new
medium, the light can be
reflected, absorbed, or
transmitted. When light is
transmitted, it can be refracted,
polarized, or scattered.
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.
Law of Reflection
• Angle of incidence (angle of
incoming light rays) equals the
angle of reflection (angle of
reflected light rays)
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 sometimes
cause a mirage, a false or distorted
image.
• occurs because light travels
faster in hot air than in cooler,
denser air.
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.
• Horizontally polarized light reflects more
strongly than the rest of the sunlight. This
reflection produces glare.
• Polarized sunglasses have vertically polarized
filters to block the horizontally polarized light.
Scattering
In scattering, light is redirected as it
passes through a medium.
• Most of the particles in the
atmosphere are very small. Small
particles scatter shorter-wavelength
blue light more than light of longer
wavelengths.
Blue light is scattered in all
directions more than other
colors of light, which makes
the sky appear blue.
Assessment Questions
1. How do polarized sunglasses reduce
glare?
a. by scattering light as it passes
through the glasses
b. by providing a smooth surface that
light can reflect off
c. by absorbing all light
d. by blocking horizontally polarized
light
Assessment Questions
1. How do polarized sunglasses reduce
glare?
a. by scattering light as it passes
through the glasses
b. by providing a smooth surface that
light can reflect off
c. by absorbing all light
d.by blocking horizontally
polarized light
Assessment Questions
2.
An opaque material passes light
through but scatters the light so that
objects do not look clear.
True
False
Assessment Questions
2.
An opaque material passes light
through but scatters the light so that
objects do not look clear.
True
False
Chapter 18: The Electromagnetic
Spectrum and Light
18.4 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.
The color of any object
depends on what the
object is made of and on
the color of light that
strikes the object.
An object’s color is the color
of light that reaches your
eye when you look at the
object.
• Sunlight contains all the
colors of the visible
spectrum.
• A red car in sunlight reflects
mostly red light.
• Most of the rest of the light
is absorbed at the surface of
the paint.
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.
Mixing Colors of Light
The three
primary colors
of light are red,
green, and
blue. When
any two
primary colors
combine, a
secondary
color is
formed.
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.
Light Question
• Why would a purple-people eater
appear black under yellow light?
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.
Mixing Pigments
The three
primary colors of
pigments are
cyan, yellow,
and magenta.
When the three
primary colors of
pigments are
combined, the
secondary colors
of pigments are
formed.
Assessment Questions
1. A prism separates white light into the visible
spectrum because
a. longer wavelengths are absorbed more
than shorter wavelengths.
b. shorter wavelengths refract more than
longer wavelengths.
c. shorter wavelengths reflect more than
longer wavelengths.
d. longer wavelengths experience more
interference.
Assessment Questions
1. A prism separates white light into the visible
spectrum because
a. longer wavelengths are absorbed more
than shorter wavelengths.
b. shorter wavelengths refract more
than longer wavelengths.
c. shorter wavelengths reflect more than
longer wavelengths.
d. longer wavelengths experience more
interference.
Assessment Questions
2.
Which of these colors is one of the
primary colors of light?
a. green
b. magenta
c. yellow
d. white
Assessment Questions
2.
Which of these colors is one of the
primary colors of light?
a.green
b. magenta
c. yellow
d. white
Chapter 18: The Electromagnetic
Spectrum and Light
18.5 Sources of Light
Objects that give off their own
light are luminous. The sun is
luminous, as are all light sources.
The light produced when an object
gets hot enough to glow is
incandescent. The filaments in
incandescent light bulbs are made
of a substance called tungsten.
(Old light bulbs)
Incandescent bulbs give off most
of their energy as heat, not light.
http://www.edisontechcenter.org/incandesc
ent.html
In a process called fluorescence, a
material absorbs light at one
wavelength and then emits light at a
longer wavelength.
A phosphor is a solid material that
can emit light by fluorescence.
A fluorescent bulb is a glass tube,
containing mercury vapor, that is
coated with phosphors. (over your
head now)
http://www.edisontechcenter.org/Fluorescent.htm
l
Light in which waves have the same
wavelength, and the crests and troughs are
lined up, is coherent light.
• A laser is a device that generates a
beam of coherent light.
• http://www.engineerguy.com/elements/vide
os/video-laser.htm
Neon lights emit light
when electrons move
through a gas or a
mixture of gases
inside glass tubing.
http://www.edisontechcenter
.org/NeonLamps.html
Sodium-vapor lights contain a small
amount of solid sodium, in a mixture
of neon and argon gases. (street
lights)
The current of electrons knocks
electrons in sodium to higher energy
levels. When the electrons move back
to lower energy levels, the sodium
atoms emit light.
http://www.edisontechcenter.org/SodiumLa
mps.html
Inside a tungsten-halogen
bulb, electrons flow through
a tungsten filament. The
filament gets hot and emits
light. (Automobile headlight bulbs)
http://www.edisontechcenter.org/halogen.html
Assessment Questions
1. The light produced when an object
becomes hot enough to glow is
a. incandescent.
b. fluorescent.
c. phosphorescent.
d. coherent.
Assessment Questions
1. The light produced when an object
becomes hot enough to glow is
a.incandescent.
b. fluorescent.
c. phosphorescent.
d. coherent.
Assessment Questions
2. The most efficient source of lighting
rooms of a building is
a. incandescent light.
b. fluorescent light.
c. sodium-vapor light.
d. tungsten-halogen light.
Assessment Questions
2. The most efficient source of lighting
rooms of a building is
a. incandescent light.
b.fluorescent light.
c. sodium-vapor light.
d. tungsten-halogen light.