Nature of Light

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Nature of Light

• Photon- Stream of particles that are extremely
  small that can push matter
• Force photons exert is called radiation pressure
• responsible for pushing material away from a
  comet to produce a tail
• each photon has a specific amount of energy
• related to wavelength. Shorter wavelengths have
  more energetic photons

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Spectroscopy

• Study of the properties of light that depend on
  wavelength.
• light given off by stars has many colors
• The light appears white to us
• Spectroscope Instrument used to break up light
  from a star into its characteristic colors

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1. Continuous Spectrum

• produced by an incandescent solid, liquid or gas
  under high pressure
• spectrum is an uninterrupted band of color from
  red to purple

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2. Absorption Spectrum

• produced when visible light is passed through a
  relatively cool gas under low pressure
• spectrum appears continuous, but with dark lines
  running through it at various wavelengths
• spectra of most stars

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3. Emission Spectrum

• produced by a hot gas under low pressure
• spectrum is a series of bright lines of
  particular wavelengths
• depends on the gas that produced them
• bright lines appear in the exact location as the
  dark lines that are produced by the same gas in
  an absorption spectrum

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Spectroscopy Conclusions

• Each element or compound in its gaseous form
  produces a unique set of spectral lines
• spectral lines act as fingerprints as they
  identify the elements present in the stars
  composition
• ex) The SUN has thousands of dark lines more
  than 60 elements have been identified by matching
  these lines with those elements known on Earth

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Doppler Effect

• the apparent change in wavelength of light that
  occurs when the object is moving toward or away
  from the Earth.
• Using the spectroscope, astronomers can determine
  whether a particular object is moving toward
  Earth or away from Earth

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Ex) Ambulance Siren Approach observer High
pitch Move away from observer low
pitch
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BLUE SHIFT RED SHIFT
Object approaching earth Object moving away from earth
Light waves are compressed (pushed together) Light waves are expanded
Wavelengths appear shorter (blue end of spectrum) Wavelengths appear longer (red end of spectrum)

• Larger Doppler shifts indicate higher speeds and
  smaller Doppler shifts indicate slower speeds.