Chapter 18 The

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Chapter 18


The
Electromagnetic
Spectrum and Light

Jennie L. Borders

Section 18.1


Electromagnetic
Waves


Electromagnetic Waves

are
transverse

waves
consisting of changing
electric fields

and
changing
magnetic fields
.


An
electric field

in a region of space exerts
electric forces on
charged particles
.


A
magnetic field

in a region of space produces
magnetic forces.

Electromagnetic Waves


Electromagnetic waves are produced when an
electric charge vibrates or accelerates
.


This is a
transverse

wave because the particles
are vibrating
perpendicular

to the direction of
the
wave
.


Electromagnetic Waves


Electromagnetic waves

can travel through a
vacuum, or empty space
, as well as through
matter.


The
transfer of energy

by electromagnetic waves
traveling through matter or across space is called
electromagnetic radiation
.

Speed of Electromagnetic Waves


The
speed of light

in a vacuum is
3.0 x 10
8

m/s
.


In a
vacuum
, all electromagnetic waves travel at
the same
speed
.


Electromagnetic waves vary in
wavelength and
frequency
.

Electromagnetic Wave Speed


Formula for speed of a wave



v =
l
x
u



For electromagnetic waves, v is always the speed
of light which is represented by a c.

c =
l
x
u

c = speed of light (3.0 x 10
8

m/s)

l

= wavelength (m)

u

= frequency (Hz)

Sample Problem


A radio station broadcasts a radio wave with a
wavelength of 3.0m. What is the frequency of
the wave?


c =
l

x
u




u

= c/
l


c = 3.0 x 10
8

m/s

u

= 3.0 x 10
8

m/s / 3.0m


l

= 3.0m




u

= 1.0 x 10
8

Hz


u

= ?

Practice Problems


The radio waves of a particular AM radio station
vibrate 680,000 times per second. What is the
wavelength of the wave?



A global positioning satellite transmits a radio
wave with a wavelength of 19cm. What is the
frequency of the radio wave?

c =
l
x
u

l

= c/
u

l

= 3.0 x 10
8

m/s / 680,000 1/s = 440m

19cm = 0.19m

c =
l

x
u

u

= c/
l


u

= 3.0 x 10
8

m/s / 0.19m = 1.6 x 10
9

Hz

Wave or Particle?


Electromagnetic radiation

behaves sometimes
like a
wave

and sometimes like a
particle
.


Evidence for a
wave

includes the fact that light
can produce
constructive and destructive
interference
.


Wave or Particle?


Evidence for a
particle

includes the fact that
light causes the
photoelectric effect
.


The emission of
electrons

from a
metal

caused
by light striking the metal is called the
photoelectric effect
.


Photons


In 1905,
Albert Einstein

proposed that light, and
all electromagnetic radiation, consists of
packets
of energy called photons
.


Each
photon’s energy

is proportional to the
frequency

of the light.

Intensity


Photons

travel outward from a light source in all
directions
.


The
intensity

of light
decreases

as photons travel
farther

from the source.

Section 18.1 Assessment


How fast does light travel in a vacuum?


What makes electromagnetic waves different
from one another?


What happens to the intensity of light as
photons move away from the light source?


What is the wavelength of an AM radio wave in
a vacuum if its frequency is 810 kHz?


810 kHz = 810,000Hz

c =
l
x
u

l

= c/
u


l

= 3.0 x 108 m/s / 810,000 1/s = 370m

Section 18.2


The Electromagnetic
Spectrum


A scientist observed that when light is passed
through a
prism

the temperature of the light is
lower

at the blue end and
higher

toward the red
end.


He also concluded that there was
invisible
radiation

beyond the visible spectrum.

Electromagnetic Spectrum


The full range of frequencies of electromagnetic
radiation is called the
electromagnetic spectrum
.


The electromagnetic spectrum includes
radio
waves, infrared rays, visible light, ultraviolet rays,
X
-
rays, and gamma rays
.

Radio Waves


Radio waves

are used in radio and television
technologies, as well as in
microwave ovens and
radar
.


In
AM

radio, the stations have signals with
varying
amplitudes
.


In
FM

radio, the stations have signals with
varying
frequencies
.

Radio Waves


The
shortest

radio waves are called
microwaves
.


Microwaves also carry
cell phone

conversations.

Infrared Rays


Infrared rays

are used as a source of
heat

and to
discover areas of heat differences.


Thermograms

are color
-
coded pictures that
show variations in
temperature
.

Visible Light

Ultraviolet Rays


Ultraviolet radiation

has higher frequencies than
violet

light.


Ultraviolet rays have
applications

in health and
medicine, and in agriculture.


X
-
Rays


X
-
rays

have high energy and can penetrate
matter that
light

cannot.


X
-
rays are used in
medicine
, industry, and
transportation to make
pictures

of the inside of
solid objects.

Gamma Rays


Gamma rays

have the
shortest

wavelengths in
the electromagnetic spectrum, about 0.005nm or
less.


Gamma rays are used in the medical field to kill
cancer cells

and make pictures of the
brain
, and
in industrial situations as an inspection tool.

Section 18.2 Assessment


List the kinds of waves included in the
electromagnetic spectrum, from longest to
shortest wavelength.


How are AM radio waves different from FM
radio waves?


What type of electromagnetic waves are
microwaves?


Section 18.3


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 to
pass through it
.


A
translucent

material
scatters

light.


An
opaque

material either
absorbs or reflects

all
of the light that strikes it.

Interactions of Light


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 surface, and reflect in
many
different directions
.

Refraction


A
mirage

is a false or
distorted

image.

Polarization


Light with waves that vibrate in only
one plane

is
polarized light
.

Scattering


Scattering

means that light is
redirected

as it
passes through a
medium
.

Section 18.3 Assessment


Explain the differences among opaque,
transparent, and translucent materials.


List and explain three things that can happen to
a light wave when it enters a new medium.


What is the difference between diffuse reflection
and regular reflection?