Electromagnetic Spectrum

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Electromagnetic Spectrum
Energy low ? medium ?
high
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Electromagnetic Radiation Quick Facts

• There are different types of EM radiation,
  visible light is just one of them
• EM waves can travel in vacuum, no medium needed
• The speed of EM radiation c is the same for all
  types and very high (? light travels to the moon
  in 1 sec.)
• The higher the frequency, the smaller the
  wavelength (?? f c)
• The higher the frequency, the higher the energy
  of EM radiation (E h f, where h is a constant)

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Visible Light

• Color of light determined by its wavelength
• White light is a mixture of all colors
• Can separate individual colors with a prism

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Three Things Light Tells Us

• Temperature
• from black body spectrum
• Chemical composition
• from spectral lines
• Radial velocity
• from Doppler shift

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Temperature Scales
Fahrenheit Centigrade Kelvin
Absolute zero ?459 ºF ?273 ºC 0 K
Ice melts 32 ºF 0 ºC 273 K
Human body temperature 98.6 ºF 37 ºC 310 K
Water boils 212 ºF 100 ºC 373 K
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Black Body Spectrum

• Objects emit radiation of all frequencies, but
  with different intensities

Ipeak
Higher Temp.
Ipeak
Ipeak
Lower Temp.
fpeakltfpeak ltfpeak
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Cool, invisible galactic gas (60 K, fpeak in
low radio frequencies)
Dim, young star (600K, fpeak in infrared)
The Suns surface (6000K, fpeak in visible)
Hot stars in Omega Centauri (60,000K, fpeak in
ultraviolet)
The higher the temperature of an object, the
higher its Ipeak and fpeak
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Wiens Law

• The peak of the intensity curve will move with
  temperature, this is Wiens law
• Temperature wavelength constant
• 0.0029 Km
• So the higher the temperature T, the smaller
  the wavelength, i.e. the higher the energy of the
  electromagnetic wave

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Example

• Peak wavelength of the Sun is 500nm, so
• T (0.0029 Km)/(5 x 10-7 m) 5800 K
• Instructor temperature roughly 100 F 37C 310
  K, so
• wavelength (0.0029Km)/310 K
• 9.35 10-6 m
• 9350 nm ? infrared radiation

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Measuring Temperatures

• Find maximal intensity
• ? Temperature (Wiens law)

Identify spectral lines of ionized elements ?
Temperature
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Color of a radiating blackbody as a function of
temperature

• Think of heating an iron bar in the fire red
  glowing to white to bluish glowing

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Kirchhoffs Laws Dark Lines

• Cool gas absorbs light at specific frequencies
• ? the negative fingerprints of the elements

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Kirchhoffs Laws Bright lines

• Heated Gas emits light at specific frequencies
• ? the positive fingerprints of the elements

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Kirchhoffs Laws

1. A luminous solid or liquid (or a sufficiently
   dense gas) emits light of all wavelengths the
   black body spectrum
2. Light of a low density hot gas consists of a
   series of discrete bright emission lines the
   positive fingerprints of its chemical elements!
   
3. A cool, thin gas absorbs certain wavelengths from
   a continuous spectrum
   ? dark absorption (
   Fraunhofer) lines in continuous spectrum
   negative fingerprints of its chemical elements,
   precisely at the same wavelengths as emission
   lines.

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Spectral Lines Fingerprints of the Elements

• Can use this to identify elements on distant
  objects!
• Different elements yield different emission
  spectra

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Spectral Lines

• Origin of discrete spectral lines atomic
  structure of matter
• Atoms are made up of electrons and nuclei
• Nuclei themselves are made up of protons and
  neutrons
• Electrons orbit the nuclei, as planets orbit the
  sun
• Only certain orbits allowed ?Quantum jumps!

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• The energy of the electron depends on orbit
• When an electron jumps from one orbital to
  another, it emits (emission line) or absorbs
  (absorption line) a photon of a certain energy
• The frequency of emitted or absorbed photon is
  related to its energy
• E h f
• (h is called Plancks constant, f is
  frequency)