Transcript Ch. 18 Powerpoint

Chapter 18 – The
Electromagnetic
Spectrum and Light
Jennie L. Borders
Section 18.1 – Electromagnetic
Waves
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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
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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
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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
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The speed of light in a vacuum is 3.0 x 108 m/s.
In a vacuum, all electromagnetic waves travel at
the same speed.
Electromagnetic waves vary in wavelength and
frequency.
Electromagnetic Wave Speed
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Formula for speed of a wave –
v=lxu
For electromagnetic waves, v is always the speed
of light which is represented by a c.
c=lxu
c = speed of light (3.0 x 108 m/s)
l = wavelength (m)
u = frequency (Hz)
Sample Problem
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A radio station broadcasts a radio wave with a
wavelength of 3.0m. What is the frequency of
the wave?
c=lxu
u = c/l
c = 3.0 x 108 m/s u = 3.0 x 108 m/s / 3.0m
l = 3.0m
u = 1.0 x 108 Hz
u=?
Practice Problems
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The radio waves of a particular AM radio station
vibrate 680,000 times per second. What is the
wavelength of the wave?
c=lxu
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l = c/u
l = 3.0 x 108 m/s / 680,000 1/s = 440m
A global positioning satellite transmits a radio
wave with a wavelength of 19cm. What is the
frequency of the radio wave?
c=lxu
u = c/l
19cm = 0.19m
u = 3.0 x 108 m/s / 0.19m = 1.6 x 109 Hz
Wave or Particle?
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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?
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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
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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
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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
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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=lxu
l = c/u
l = 3.0 x 108 m/s / 810,000 1/s = 370m
Section 18.2 – The Electromagnetic
Spectrum
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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
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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
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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
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The shortest radio waves are called microwaves.
Microwaves also carry cell phone conversations.
Infrared Rays
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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
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Ultraviolet radiation has higher frequencies than
violet light.
Ultraviolet rays have applications in health and
medicine, and in agriculture.
X-Rays
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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
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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
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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
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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
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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
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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
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A mirage is a false or distorted image.
Polarization
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Light with waves that vibrate in only one plane
is polarized light.
Scattering
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Scattering means that light is redirected as it
passes through a medium.
Section 18.3 Assessment
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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?