Light

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Light

• Light can be described in two complementary ways
  as a stream of particles called photons, or as
  electromagnetic waves.
• An electromagnetic wave has an electrical part
  and a magnetic part.

2
Light
3
Light

• An electromagnetic wave is a transverse wave. The
  vibration of the material is perpendicular to the
  direction of travel.

4
Light

• The wavelength is the length of the wave and
  usually has the symbol, ?.
• The wavelength is measured in nanometers (nm) or
  angstroms (Å).
• nm 10-9m
• Å10-10m

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Light

• The wave frequency is related to the wave period.
• f 1/T
• The wavelength and frequency are related to the
  speed of the wave.
• ?f v
• Speed of light in vacuum c3X108m/s
• c ?f

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Light

• The electromagnetic spectrum consists of visible
  and invisible radiation.

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Light
Astronomy Exercises Include electromagnetic
Radiation here
8
Light

• Its important for us to understand how objects
  in space can produce radiation.

9
Tools of Astronomers

• We can determine many properties of stars and
  other objects in our universe just by looking at
  the light from them in different ways.
• We can determine the temperature,
• the chemical composition,and
• the motion by looking at the light being emitted
  from these objects.

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Blackbody Spectrum

• Blackbody curves show a relationship between the
  intensity of light and the frequency or
  wavelength of the light.
• If we heat up a piece of metal, it will glow. We
  can measure the intensity with respect to the
  frequency and we see a relationship.
• We can now look at the emission of light from
  stars and determine its temperature based on its
  color.

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Blackbody Radiation

• The temperature of the glowing object can be
  found by using Wiens law.
• wavelength of peak emission ? 1/ temperature
• We normally express temperature as T and in the
  units of Celsius (C) or Kelvin (K)
• Heat and temperature are different. Heat is the
  transfer of energy from one object to another.
  Temperature is an indicator of the kinetic energy
  of atoms and molecules in a material.

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Spectral Classes

• Spectral class
• OBAFGKM Oh Be A Fine Guy/Gal Kiss Me

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Luminosity and Magnitude

• Apparent magnitude
• How bright a star appears from earth
• Absolute magnitude
• How bright a star appears if it were placed 10 pc
  away from earth
• Luminosity
• Total energy a star gives off in 1 second

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Doppler Effect

• We can use the Doppler effect to determine
  whether an object is moving toward or away from
  us.

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Spectroscopy

• Spectroscopy is the study of spectra.
• A spectrum is breaking the light into its
  wavelength components.
• We use a spectroscope to study spectrum.
• A spectroscope consists of a light source, a
  slit, prism and a screen. Sometimes lenses are
  used.

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Spectra

• There are different types of spectrum.
• Emission
• Absorption
• Continuous

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Spectra

• A continuous spectrum is the colors of the
  rainbow. It is produced by a hot luminous solid
  or dense liquid or gas.

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Spectra

• An emission spectrum is produced by light emitted
  from excited atoms. It appears to be slices from
  the continuous spectrum.

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Spectra

• An absorption spectrum appears to be the
  continuous spectrum with slices missing from it.

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Kirchhoffs laws

• A luminous solid or liquid, or a sufficiently
  dense gas, emits light of all wavelengths and so
  produces a continuous spectrum of radiation.
• A low-density hot gas emits light whose spectrum
  consists of a series of bright emission lines.
  These lines are characteristic of the chemical
  composition of the gas.
• A low-density, cool gas in front of a continuous
  spectrum source, will produce an absorptions
  spectrum.

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Spectroscopy

• We can determine the chemical composition of
  stars and dust clouds based on their spectra.