Transcript Chapter 18 The Electromagnetic Spectrum and Light

Chapter 18 The
Electromagnetic
Spectrum and Light
18.1 Electromagnetic Waves
18.2 The Electromagnetic Spectrum
18.3 Behavior of Light
18.1 Electromagnetic
Waves
 What are Electromagnetic Waves?
 Def.-transverse waves consisting of
changing electric fields and changing
magnetic fields
 Differ from mechanical waves in how
they are produced and how they travel
18.1 Electromagnetic
Waves
 What are Electromagnetic Waves?: How They
Are Produced
 Produced by constantly changing fields
 Electric field-a field in a region of space that
exerts electric forces on charged particles
 Electric fields are produced by electrically
charged particles and by changing magnetic
fields
18.1 Electromagnetic
Waves
 What are Electromagnetic Waves?: How They
Are Produced
 Magnetic Field-a field in a region of space that
exerts magnetic forces
 Magnetic fields are produced by magnets,
changing electric fields, and by vibrating
charges
 Key Concept: Electromagnetic waves are
produced when an electric charge vibrates or
accelerates.
18.1 Electromagnetic
Waves
 What are Electromagnetic Waves?: How They
Travel
 Changing electric fields create changing
magnetic fields and vice versa.
 They regenerate each other; as they
regenerate, their energy travels in the form of a
wave
 Electromagnetic waves do not need a medium
(mechanical waves do)
Figure 2
Electromagnetic Waves
18.1 Electromagnetic
Waves
 What are Electromagnetic Waves?: How They
Travel
 Key Concept: Electromagnetic waves can
travel through a vacuum, or empty space, as
well as through matter.
 Electromagnetic radiation-the transfer of
energy by electromagnetic waves traveling
through matter or across a space
18.1 Electromagnetic
Waves
 The Speed of Electromagnetic Waves:
Michelson’s Experiment and The Speed
of Light
 Light travels faster than sound (lightening
occurs before thunder)
 Albert Michelson (1852-1931)-physicistcalculated the speed of light
18.1 Electromagnetic
Waves
 The Speed of Electromagnetic Waves: Michelson’s
Experiment
 8-sided rotating mirror on one mountain; 1 mirror on
another mountain opposite of 1st mountain
 Shined light at one face of the rotating mirror; light was
reflected to stationary mirror on opposite mountain.
 Knowing how fast mirror rotated and how far the light
traveled from mountain to mountain and back
again=speed of light calculated
18.1 Electromagnetic
Waves
 The Speed of Electromagnetic Waves:
Michelson’s Experiment and The Speed
of Light
 Light and all electromagnetic waves
travel at the same speed when in a
vacuum regardless of observer’s motion
 Key Concept: The speed of light in a
vacuum, c, is 3.00 X 108 m/s
18.1 Electromagnetic
Waves
 Wavelength and Frequency
 Key Concept: Electromagnetic waves
vary in wavelength and frequency.
 Wavelength is inversely proportional to
the frequency b/c the speed of
electromagnetic waves in a vacuum is
constant.
 Speed=wavelength x frequency
Section 18.1
18.1 Electromagnetic
Waves
 Wave or Particle?
 Electromagnetic radiation can also
behave as a stream of particles.
 Newton was first to propose a particle
explanation.
 Evidence: light travels in a straight line
and it casts as shadow
18.1 Electromagnetic
Waves
 Wave or Particle?
 Is light a wave or a particle? It is both.
 Key Concept: Electromagnetic radiation
behaves sometimes like a wave and
sometimes like a stream of particles.
18.1 Electromagnetic
Waves
 Wave or Particle?: Evidence for the Wave
Model
 Thomas Young-physicist-showed that light
behaves like a wave.
 Passed beam of light through a single slit then
a double slit (bright and dark bands)
 Bands showed that light had produced an
interference pattern (bright=constructive;
dark=destructive)
Figure 5
Interference
(Wave Model of
Light)
18.1 Electromagnetic
Waves
 Wave or Particle? Evidence for the Particle
Model
 (Einstein) Blue light causes electrons to be
emitted; red light does not
 Photoelectric effect-the emission of electrons
from a metal caused by light striking the metal
 Einstein proposed that light and all
electromagnetic radiation consists of packets
of energy (photons)
18.1 Electromagnetic
Waves
 Wave or Particle?: Evidence for the Particle
Model
 The greater the frequency of and
electromagnetic wave, the more energy each
of its photons has.
 Blue light=higher frequency (photons have
more energy so electrons emitted); red
light=lower frequency (photons have less
energy so no emission of electrons)
Figure 6
Photoelectric Effect (Particle Model
of Light)
18.1 Electromagnetic
Waves
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 Intensity
The closer to light you are, the brighter it is.
The further away you are, the dimmer the light.
Photons from light travel outward in all
directions.
In a small area (near the light) light is more
intense.
18.1 Electromagnetic
Waves
 Intensity
 Def.-the rate at which a wave’s energy
flows through a given unit (ie. brightness)
 Key Concept: The intensity of light
decreases as photons travel farther from
the source.
18.2 The Electromagnetic
Spectrum
 The Waves of the Spectrum
 William Herschel (1800)-used a prism to
separate the wavelengths present in sunlight
 Colors: red, orange, yellow, green, blue, and
violet
 Each color’s temperature is different; temp. is
higher in area beyond red color band (no color
present); there is invisible radiation beyond the
red end of the color band (infrared radiation)
18.2 The Electromagnetic
Spectrum
 The Waves of the Spectrum
 Electromagnetic spectrum-the full range
of frequencies of electromagnetic
radiation
 Key Concept: The electromagnetic
spectrum includes radio waves, infrared
rays, visible light, ultraviolet rays, X-rays,
and gamma rays.
Figure 9
The Electromagnetic Spectrum
18.2 The Electromagnetic
Spectrum
 Radio Waves
 Have the longest wavelengths in the
spectrum; have lowest frequency
 Key Concept: Radio waves are used in
radio and television technologies, as well
as in microwave ovens and radar.
18.2 The Electromagnetic
Spectrum
 Radio Waves: Radio
 Two ways radio stations code and
transmit information on radio waves
 Both are based on a wave of constant
frequency and amplitude. Pg. 541
 Amplified modulation (AM)-the amplitude
of the wave is varied; frequency remains
the same
18.2 The Electromagnetic
Spectrum
 Radio Waves: Radio
 Frequency modulation (FM)-the
frequency of the wave is varied; the
amplitude remains the same
 AM signals travel farther than FM signals
18.2 The Electromagnetic
Spectrum
 Radio Waves: Television
 Radio waves carry signals for TV
programming.
 The process is like transmitting radio
signals BUT radio waves carry
information for pictures as well as for
sound
18.2 The Electromagnetic
Spectrum
 Radio Waves: Microwaves
 Def.-the shortest-wavelength radio waves
 Cook and reheat food (water or fat
molecules absorb microwaves=increase
in thermal energy of the molecules)
 Also carry cell phone conversations.
18.2 The Electromagnetic
Spectrum
 Radio Waves: Radar
 Radar technology uses a radio
transmitter to send out short bursts of
radio waves
 Waves reflect off objects encountered,
bounce back to where they came from
(picked up and interpreted by radio
receiver)
18.2 The Electromagnetic
Spectrum
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 Infrared Rays
Have higher frequencies than radio waves;
lower frequencies than red light
Key Concept: Infrared rays are used as a
source of heat and to discover areas of heat
differences.
Can’t be seen; can be felt (warmth)
Thermograms-color-coded pictures that show
variations in temperature
18.2 The Electromagnetic
Spectrum
 Visible Light
 The visible part of the electromagnetic
spectrum
 Key Concept: People use visible light to
see, to help keep them safe, and to
communicate with one another.
Figure 13
18.2 The Electromagnetic
Spectrum
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 Ultraviolet Rays
Ultraviolet radiation has higher frequencies
than violet light.
Key Concept: Ultraviolet rays have
applications in health and medicine, and in
agriculture.
Help skin produce vitamin D; kill
microorganisms; help plants grow
Excessive exposure: sunburn, wrinkles, and
eventually skin cancer
18.2 The Electromagnetic
Spectrum
 X-Rays
 Have very short wavelengths; have higher
frequencies than UV rays
 Key Concept: X-rays are used in medicine,
industry, and transportation to make pictures of
the inside of solid objects.
 Teeth and bones absorb x-rays (white); tissue
(dark); too much exposure can kill or damage
living tissue
18.2 The Electromagnetic
Spectrum
 Gamma Rays
 Have the shortest wavelengths in the
electromagnetic spectrum; have the
highest frequencies
 Have the most energy and greatest
penetrating ability of all the
electromagnetic waves
18.2 The Electromagnetic
Spectrum
 Gamma Rays
 Overexposure to gamma rays can be
deadly.
 Key Concept: 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.
18.3 Behavior of Light
 Light and Materials
 Nothing is visible without light.
 How light behaves when it strikes an object
depends on many factors (this includes the
material the object is made of)
 Key Concept: Materials can be transparent,
translucent, or opaque.
18.3 Behavior of Light
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 Light and Materials
Transparent-material transmits light (allows
most of the light that strikes it to pass through)
Ex. Windows, water
Translucent-material scatters light (can see
through the material, but objects seen through
it don’t look clear or distinct)
Ex. Frosted glass, some soaps
18.3 Behavior of Light
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 Light and Materials
Most materials are opaque.
Opaque-material either absorbs or reflects all
of the light that strikes it
Does not allow any light to pass through
Ex. Wood, metal
18.3 Behavior of Light
 Interactions of Light
 When light encounters matter, some or all of
the energy in the light can be transferred to the
matter.
 Light can affect matter; matter can affect light.
 Key Concept: 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.
18.3 Behavior of Light
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 Interactions of Light: Reflection
Image-a copy of an object formed by reflected
(or refracted) waves of light **[image in a
mirror]
When light reflects from a smooth
surface=clear, sharp image
When light reflects from a rough
surface=blurred image or no image
Ex. Calm water vs. “moving” water
18.3 Behavior of Light
 Interactions of Light: Reflection
 Regular reflection- occurs when parallel light
waves strike a surface and reflect all in the
same direction
 Diffuse reflection-occurs when parallel light
waves strikes a rough, uneven surface, and
reflect in many different directions
18.3 Behavior of Light
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 Interactions of Light: Refraction
A light wave can refract, or bend, when it passes at an
angle from one medium into another.
Ex. Underwater objects appear closer and larger;
Objects appear to break at the surface of water
Can cause a mirage-a false or distorted image
Mirage-occurs because light travels faster in hot air
than in cooler denser air; refracts more as it enters
hotter and hotter air=light follows curved path to the
ground; light looks like it was reflected from water
18.3 Behavior of Light
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 Interactions of Light: Polarization
Polarized light-light with waves that vibrate in only one
plane
Polarizing filters transmit this type of light
Unpolarized light vibrates in all directions.
Vertical polarizing filter-stops light vibrating on a
horizontal plane (polarized sunglasses block
horizontal polarized light from the sun *glare b/c
horizontal light reflects more strongly*)
Horizontal polarizing filter-blocks waves vibrating on a
vertical plane
Figure 20
Polarization
18.3 Behavior of Light
 Interactions of Light: Scattering
 Earth’s atmosphere has many molecules and
other tiny particles= sunlight can be scattered
 Def.-light is redirected as it passes through a
medium
 Small particles in the air scatter shorterwavelength blue light more than light of longer
wavelengths.
18.3 Behavior of Light
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 Interactions of Light: Scattering
By the time sunlight reaches your eyes, most of the
blue and some green and yellow have been scattered.
What’s left that we see? Longer wavelengths of light,
orange and red (sunset/sunrise)
Why is the sky blue? The sun scatters blue light in all
directions more than other colors of light.
Sky appears blue although air is colorless.