Foundation 1 - Discovering Astronomy

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Chapter 4 Light
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Ever since we crawled out of that
primordial slime, thats been our unifying cry,
More light. Sunlight. Torchlight. Candlelight.
Neon, incandescent lights that banish the
darkness from our caves to illuminate our roads,
the insides of our refrigerators...Little tiny
flashlights for those books we read under the
covers when were suppose to be asleep. Light is
more than watts ... Light is metaphor. Light is
knowledge. Light is life...
- Diane Frolov and Andrew Schneider Northern
Exposure,1993
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What can we learn by analyzing starlight?

• A stars chemical composition
• A stars temperature
• A stars speed and direction of motion

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How fast does light move?

• 186,000 miles per second!
• or
• 3 x 105 kilometers per second!

Nothing travels faster than light!
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Particle or Wave ?

• 1905, Einstein verified that light sometimes
  behaves as a wave and sometimes as particles.
• This is called the wave-particle duality of
  light!

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Particle or Wave ?
Wave Electromagnetic wave
Particles Photons (packets of energy)
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Light as a Wave
Different wavelengths correspond to different
colors!
Units 1 nanometer 10-9 meters!
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Light as a Wave

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Some Important Relationships
if frequency ?, wavelength ?
if frequency ?, then energy ?
if wavelength ?, then energy ?
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Concept Question
Which color light has more energy blue or red?
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Blue coats beat the red coats!
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If you pass white light through a prism, it
separates into its component colors.
Spectrum
ROY G. BIV !!
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A hot object or a hot, dense gas produces a
continuous spectrum -- a complete rainbow of
colors without any specific spectral lines.
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A hot, less dense gas, when heated, produces
an emission line spectrum - a series of bright
spectral lines against a dark background.
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A cooler gas in front of a hot dense gas
produces an absorption line spectrum - a series
of dark spectral lines among the colors of the
rainbow.
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Each chemical element produces its own unique set
of spectral lines when it burns
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Spectral Lines
By observing the location of the emission and or
the absorption lines, one can identify the
element!!!!
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Twinkle, twinkle little star I dont wonder what
you are. For by spectroscopic ken I know that you
are hydrogen! - anonymous
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Electromagnetic Spectrum
Figure 3.5
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Not all radiation can penetrate Earths
atmosphere.
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What is Color?

• Color of objects due to how much light is
  reflected, absorbed, or emitted.

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Kelvin Temperature Scale
Kelvin Temperature Celsius Temperature
273
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Peak color (wavelength) shifts to shorter
wavelengths as the temperature increases
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Which is HOTTER???
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Peak color (wavelength) shifts to shorter
wavelengths as the temperature increases
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Humans emit infrared light!
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The Doppler Shift

• Christian Doppler 1842
• the observed frequency of an object
• is affected by its apparent motion
• Doppler shift occurs when the source
• of the waves is moving with respect to
• the observer
• occurs for all waves
• (e.g. water, sound, light)

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The Doppler Shift
Sound Waves
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The Doppler Shift
Light Waves
Stationary Star
constant wavelength detected by both A and B
A detects longer wavelength ? redshift B detects
shorter wavelength? blueshift
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The Doppler ShiftIn Astronomy
Spectrum of Approaching Source
Spectrum of Stationary Source
Spectrum of Receding Source
400nm 550nm 700nm
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Detection of Extrasolar Planets Stellar Wobble

• gravity of the planet causes
• the star to wobble back
• and forth
• 1990s, used Doppler effect
• to detect stellar wobbles

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Concept Question
If a star is moving away from us, which
statement best describes what is happening to
the stars light?
A) the light is blueshifted we perceive that
the wavelength increases B) the light is
redshifted we perceive that the wavelength
increases C) the light is blueshifted we
perceive that the wavelength decreases D) the
light is redshifted we perceive that the
wavelength decreases
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What can we learn by analyzing starlight?

• A stars chemical composition
• by spectrum
• A stars temperature
• by color (peak wavelength)
• A stars speed and direction of motion
• by spectrum and Doppler Shift