kirchoff’s laws doppler effect & motion...
TRANSCRIPT
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Today
Kirchoff’s Laws
Doppler Effect & Motion
Telescopes
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Kirchoff’s Laws• Hot, dense objects emit a
– continuous spectrum• light of all colors & wavelengths• follows thermal distribution• obeys Wien’s & Steffan-Boltzmann Laws.
• Hot, diffuse gas emits light only at specific wavelengths.– emission line spectrum
• A cool gas obscuring a continuum source will absorb specific wavelengths – absorption line spectrum
e.g., a light bulb
e.g., a neon light
e.g., a star
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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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Kirchoff’s Laws• Hot, dense objects emit a
– continuous spectrum• light of all colors & wavelengths• follows thermal distribution• obeys Wien’s & Steffan-Boltzmann Laws.
• Hot, diffuse gas emits light only at specific wavelengths.– emission line spectrum
• A cool gas obscuring a continuum source will absorb specific wavelengths – absorption line spectrum
e.g., a light bulb
e.g., a neon light
e.g., a star
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How does light tell us what things are made of?
Spectrum of the Sun
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Atomic Terminology
• Atomic Number = # of protons in nucleus • Atomic Mass Number = # of protons + 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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• 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
Chemical Fingerprints
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Energy levels of hydrogen
Possible Electron orbits
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Energy levels of hydrogen
Transitions between orbitsrelease energy (photons)
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• Downward transitions produce a unique pattern of emission lines.
Chemical Fingerprints
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• Atoms can absorb photons with those same energies, so upward transitions produce absorption lines.
Chemical Fingerprints
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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
All the dark regions are absorption lines due to all the elementsin the sun’s atmosphere. The strengths of the lines tell us about the sun’s composition and other physical properties.
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Solar composition• 73% Hydrogen• 25% Helium• 2% everything else
– “metals”
• Other stars similar– H & He most common stuff in the universe– Helium was discovered in the spectrum of the sun
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What is this object? Mars!
Hot upper atmosphere Carbon Dioxide in atmosphere
Reflected Sunlight: Mars is red
Infrared peak wavelength tells us T = 225 K
We can learn an enormous amount from spectra:temperature, density, and composition
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The Doppler Effect
Doppler ball
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• Motion away -> redshift• Motion towards -> blueshift
Doppler Effect for Light
∆λ
λ=
λobs − λem
λem
=v
c
wavelengthshift
wavelength
speed
speedof 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
redshiftblueshift
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Spectrum
wavelength
posi
tion
alon
g sl
it
Doppler shift
��
spectrographslit
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Doppler shift tells us ONLY about the part of an object’s motion toward or away from us
(along our line of sight).
maximum doppler effect
no doppler effect
intermediate case
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Telescopes• Telescopes collect more light than our eyes ⇒
light-collecting area
• Telescopes can see more detail than our eyes ⇒ angular resolution (magnification)
• Telescopes/instruments can record light more sensitively than our eyes, and detect electromagnetic radiation 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 resolutioncan see fainter things
can see smaller things
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Bigger is better
can see fainter:
with higher resolution:
For a telescope with mirror of diameter D,
b−1
∝ D2
θ ∝
λ
D
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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 reflectors
Reflecting telescope Gemini North 8-m
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4 mKitt Peak National Observatory (AZ)
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2.1 m WIYN (3.5 m)Burrell Schmidt
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Different designs for different wavelengths of light
Radio telescope (Arecibo, Puerto Rico)Longer wavelengths need larger “mirrors”
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FAST 500 m (China)