copyright © 2012 pearson education, inc. chapter 5 light: the cosmic messenger
TRANSCRIPT
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Chapter 5Light: The Cosmic Messenger
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What is light?
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Light is an electromagnetic wave.
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Anatomy of a 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
f = c
= 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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Continuous Spectrum
Emission Line SpectrumAbsorption 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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Introduction to Spectroscopy
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Three Types of Spectra
Illustrating Kirchhoff's Laws
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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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Production of Emission Lines
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Chemical Fingerprints
• Each type of atom has a unique spectral fingerprint.
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Composition of a Mystery Gas
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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 Radiation1. 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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Wien’s Law
Wien’s Law
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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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The Doppler Effect
Hearing the Doppler Effect as a Car Passes
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Explaining the Doppler Effect
Understanding the Cause of 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
The Doppler Shift of an Emission Line Spectrum
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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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Measuring Velocity
Determining the Velocity of a Cold Cloud of Hydrogen Gas
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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?).