Light: The Cosmic Messenger -...
Transcript of Light: The Cosmic Messenger -...
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Chapter 5
Light: The Cosmic Messenger
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5.1 Basic Properties of Light and Matter
Our goals for learning:
• What is light?
• What is matter?
• How do light and matter interact?
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What is light?
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Light is an
electromagnetic wave.
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Wavelength and Frequency
wavelength frequency = speed of light = constant
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The Electromagnetic Spectrum
Electromagnetic Spectrum
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Particles of Light
• Particles of light are called photons.
• Each photon has a wavelength and a
frequency.
• The energy of a photon depends on its
frequency.
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Wavelength, Frequency, and Energy
l f = c
l = wavelength, f = frequency
c = 3.00 108 m/s = speed of light
E = h f = photon energy
h = 6.626 10-34 joule s = photon energy
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Thought Question
The higher the photon energy,
A. the longer its wavelength.
B. the shorter its wavelength.
C. Energy is independent of wavelength.
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Thought Question
The higher the photon energy,
A. the longer its wavelength.
B. the shorter its wavelength.
C. Energy is independent of wavelength.
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What is matter?
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Atomic Terminology
• Atomic Number = # of protons in nucleus
• Atomic Mass Number = # of protons + # of neutrons
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Atomic Terminology
• Isotope: same # of
protons but different
# of neutrons (4He, 3He)
• Molecules:
consist of two or
more atoms (H2O,
CO2)
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How do light and matter interact?
• Emission
• Absorption
• Transmission
— Transparent objects transmit light.
— Opaque objects block (absorb) light.
• Reflection or scattering
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Reflection and Scattering
Mirror reflects
light in a
particular
direction.
Movie screen scatters light in all
directions.
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Interactions of Light with Matter
Interactions between light and matter determine the
appearance of everything around us.
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Thought Question
Why is a rose red?
A. The rose absorbs red light.
B. The rose transmits red light.
C. The rose emits red light.
D. The rose reflects red light.
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Thought Question
Why is a rose red?
A. The rose absorbs red light.
B. The rose transmits red light.
C. The rose emits red light.
D. The rose reflects red light.
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What have we learned? • What is light?
— Light is a form of energy.
— Light comes in many colors that combine to form white light.
— Light is an electromagnetic wave that also comes in individual “pieces” called photons. Each photon has a precise wavelength, frequency, and energy.
— Forms of light are radio waves, microwaves, infrared, visible light, ultraviolet, X rays, and gamma rays.
• What is matter?
— Ordinary matter is made of atoms, which are made of protons, neutrons, and electrons.
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What have we learned?
• How do light and matter interact?
—Matter can emit light, absorb light, transmit light, and reflect (or scatter) light.
— Interactions between light and matter determine the appearance of everything we see.
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5.2 Learning from Light
Our goals for learning:
• What are the three basic types of spectra?
• How does light tell us what things are made of?
• How does light tell us the temperatures of planets and stars?
• How does light tell us the speed of a distant object?
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Continuous Spectrum
Emission Line Spectrum Absorption Line Spectrum
What are the three basic types of
spectra?
Spectra of astrophysical objects are usually combinations of
these three basic types.
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Continuous Spectrum
• The spectrum of a common (incandescent) light
bulb spans all visible wavelengths, without
interruption.
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Emission Line Spectrum
• A thin or low-density cloud of gas emits light only at
specific wavelengths that depend on its composition and
temperature, producing a spectrum with bright emission
lines.
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Absorption Line Spectrum
• A cloud of gas between us and a light bulb can absorb light
of specific wavelengths, leaving dark absorption lines in
the spectrum.
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How does light tell us what
things are made of?
Spectrum of the Sun
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Chemical Fingerprints
• Each type of atom
has a unique set of
energy levels.
• Each transition
corresponds to a
unique photon
energy, frequency,
and wavelength.
Energy levels of hydrogen
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Chemical Fingerprints
• Downward transitions produce a unique pattern of emission lines.
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Production of Emission Lines
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Chemical Fingerprints
• Because those atoms can absorb photons with those same energies, upward transitions produce a pattern of absorption lines at the same wavelengths.
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Production of Absorption Lines
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Chemical Fingerprints
• Each type of atom has a unique spectral fingerprint.
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Chemical Fingerprints
• Observing the fingerprints in a spectrum tells us
which kinds of atoms are present.
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Example: Solar Spectrum
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Thought Question
Which letter(s) label(s) absorption lines?
A B C D E
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Thought Question
Which letter(s) label(s) absorption lines?
A B C D E
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Thought Question
A B C D E
Which letter(s) label(s) the peak (greatest
intensity) of infrared light?
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A B C D E
Thought Question
Which letter(s) label(s) the peak (greatest
intensity) of infrared light?
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Thought Question
Which letter(s) label(s) emission lines?
A B C D E
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Thought Question
Which letter(s) label(s) emission lines?
A B C D E
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How does light tell us the
temperatures of planets and stars?
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Thermal Radiation
• Nearly all large or dense objects emit thermal radiation, including stars, planets, and you.
• An object’s thermal radiation spectrum depends on only one property: its temperature.
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Properties of Thermal Radiation
1. Hotter objects emit more light at all frequencies per
unit area.
2. Hotter objects emit photons with a higher average
energy.
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Thought Question
Which is hottest?
A. A blue star
B. A red star
C. A planet that emits only infrared light
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Thought Question
Which is hotter?
A. A blue star
B. A red star
C. A planet that emits only infrared light
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Thought Question
Why don’t we glow in the dark?
A. People do not emit any kind of light.
B. People only emit light that is invisible to our eyes.
C. People are too small to emit enough light for us to see.
D. People do not contain enough radioactive material.
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Thought Question
Why don’t we glow in the dark?
A. People do not emit any kind of light.
B. People only emit light that is invisible to our eyes.
C. People are too small to emit enough light for us to see.
D. People do not contain enough radioactive material.
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Interpreting an Actual Spectrum
• By carefully studying the features in a spectrum, we can learn a great deal about the object that created it.
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What is this object?
Reflected sunlight:
Continuous spectrum of
visible light is like the
Sun’s except that some of
the blue light has been
absorbed—the object
must look red.
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What is this object?
Thermal radiation:
Infrared spectrum peaks
at a wavelength
corresponding to a
temperature of 225 K.
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What is this object?
Carbon dioxide:
Absorption lines are the
fingerprint of CO2 in the
atmosphere.
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What is this object?
Ultraviolet emission lines:
Indicate a hot upper
atmosphere
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What is this object?
Mars!
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How does light tell us the speed
of a distant object?
The Doppler Effect
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Same for
light
The Doppler Effect for Visible Light
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Measuring the Shift
• We generally measure the Doppler effect from shifts in
the wavelengths of spectral lines.
Stationary
Moving Away
Away Faster
Moving Toward
Toward Faster
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The amount of blue or red shift tells us an
object’s speed toward or away from us.
The Doppler Shift of an Emission Line Spectrum
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Doppler shift tells us ONLY about the part of an
object’s motion toward or away from us.
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Thought Question
A. It is moving away from me.
B. It is moving toward me.
C. It has unusually long spectral lines.
I measure a line in the lab at 500.7 nm. The
same line in a star has wavelength 502.8 nm.
What can I say about this star?
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Thought Question
A. It is moving away from me.
B. It is moving toward me.
C. It has unusually long spectral lines.
I measure a line in the lab at 500.7 nm. The
same line in a star has wavelength 502.8 nm.
What can I say about this star?
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Measuring
Redshift
Doppler Shift of Absorption Lines
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Measuring
Velocity
Determining the Velocity of a Gas Cloud
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What have we learned?
• What are the three basic types of spectra?
—Continuous spectrum, emission line spectrum, absorption line spectrum
• How does light tell us what things are made of?
—Each atom has a unique fingerprint.
—We can determine which atoms something is made of by looking for their fingerprints in the spectrum.
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What have we learned?
• How does light tell us the temperatures of
planets and stars?
—Nearly all large or dense objects emit a
continuous spectrum that depends on
temperature.
—The spectrum of that thermal radiation tells
us the object’s temperature.
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What have we learned?
• How does light tell us the speed of a distant object?
—The Doppler effect tells us how fast an object is
moving toward or away from us.
• Blueshift: objects moving toward us
• Redshift: objects moving away from us
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5.3 Collecting Light with Telescopes
Our goals for learning:
• How do telescopes help us learn about the
universe?
• Why do we put telescopes into space?
• How is technology revolutionizing
astronomy?
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How do telescopes help us learn
about the universe?
• Telescopes collect more light than our eyes
light-collecting area
• Telescopes can see more detail than our eyes
angular resolution
• Telescopes/instruments can detect light that is
invisible to our eyes (e.g., infrared, ultraviolet)
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Bigger is better
1. Larger light-collecting area
2. Better angular resolution
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Bigger is better
Light Collecting Area of a Reflector
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Angular Resolution
• The minimum angular separation that the telescope
can distinguish
Angular Resolution Explained Using Approaching Car Lights
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Angular Resolution: Smaller Is Better
Effect of Mirror Size on Angular Resolution
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Basic Telescope Design
• Refracting: lenses
Refracting telescope Yerkes 1-m refractor
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Basic Telescope Design • Reflecting: mirrors
• Most research telescopes today are reflecting
Reflecting telescope Gemini North 8-m
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Keck I and Keck II
Mauna Kea, Hawaii
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Mauna Kea, Hawaii
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Different designs for different wavelengths of light
Radio telescope (Arecibo, Puerto Rico)
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X-ray Telescope: “grazing incidence” optics
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Want to buy your own telescope?
• Buy binoculars first (e.g., 7 35) — you get much more for the same amount of money.
• Ignore magnification (sales pitch!).
• Notice: aperture size, optical quality, portability.
• Consumer research: Astronomy, Sky & Telescope, Mercury magazines; astronomy clubs.
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Why do we put telescopes into
space?
It is NOT because they are closer
to the stars!
Recall our 1-to-10 billion scale:
• Sun size of grapefruit
• Earth size of a tip of a
ballpoint pen, 15 m from
Sun
• Nearest stars 4000 km away
• Hubble orbit
microscopically above tip of
a ballpoint pen-size Earth
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Observing problems due to Earth’s atmosphere
1. Light pollution
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Star viewed with
ground-based telescope
2. Turbulence causes twinkling blurs images
View from Hubble
Space Telescope
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3. Atmosphere absorbs most of EM spectrum, including
all UV and X ray and most infrared.
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Telescopes in space solve all three problems.
• Location/technology can help overcome
light pollution and turbulence.
• Nothing short of going to space can solve the
problem of atmospheric absorption of light.
Chandra X-Ray
Observatory
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Adaptive optics
• Rapid changes in mirror shape compensate for atmospheric
turbulence.
How is technology revolutionizing
astronomy?
Without adaptive optics With adaptive optics
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Adaptive Optics
Jupiter’s moon Io observed with the Keck telescope
without adaptive optics with adaptive optics
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Interferometry
• This technique allows two or more small telescopes to
work together to obtain the angular resolution of a
larger telescope.
Very Large Array (VLA), New Mexico
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The Moon would be a great spot for an observatory (but at
what price?).
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What have we learned?
• How do telescopes help us learn about the universe?
—We can see fainter objects and more detail than we can see by eye. Specialized telescopes allow us to learn more than we could from visible light alone.
• Why do we put telescopes into space?
—They are above Earth’s atmosphere and therefore not subject to light pollution, atmospheric distortion, or atmospheric absorption of light.
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What have we learned?
• How is technology revolutionizing astronomy?
—Technology greatly expands the capabilities of telescopes.
—Adaptive optics can overcome the distorting effects of Earth’s atmosphere.
— Interferometry allows us to link many telescopes so that they act like a much larger telescope.