RADIATION AND SPECTRA

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RADIATION AND SPECTRA

• Chapter 4

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WAVES

• A stone dropped into a pool of water causes an
  expanding disturbance called a wave.

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WAVES

• A stone dropped into a pool of water causes an
  expanding disturbance called a wave.

• Sound is a wave caused by a pressure disturbance.

• Light and radio are waves (called electromagnetic
  radiation) caused by charged particles (mostly
  electrons) oscillating.

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PROPERTIES OF RADIATION

• Speed 3 x 105 km/s in vacuum.
• Radiation often behaves as a wave.
• Wavelengths (1nm 10-9 m)
• Radio 1m (109 nm)
• Infrared 10 ?m (104 nm)
• Visible 0.5 ? m (500 nm)
• Ultraviolet 10 nm
• X-ray 0.1 nm
• g-ray 10-4 nm
• m metre, ? 10-6, n 10-9

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ELECTROMAGNETIC RADIATION
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ELECTROMAGNETIC WAVES
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HUMAN SENSITIVITY to WAVES

• Sound Waves
• l (wavelength) pitch
• Short l high pitch
• Long l low pitch
• Light Waves
• l (wavelength) colour
• Short l bluer
• Long l redder

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NANOMETER

• Usual unit of l for light is nm
• (nano-meter 10-9 metres)
• Blue light 400 nm
• Red light 700 nm

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NANOMETER

• Usual unit of l for light is nm
• (nano-meter 10-9 metres)
• Blue light 400 nm
• Red light 700 nm

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PROPERTIES OF RADIATION

• Speed 3 x 105 km/s in vacuum.
• Radiation often behaves as a wave.
• Wavelengths (1nm 10-9 m)
• Radio 1m (109 nm)
• Infrared 10 ?m (104 nm)
• Visible 0.5 ? m (500 nm)
• Ultraviolet 10 nm
• X-ray 0.1 nm
• g-ray
• m metre, ? 10-6, n 10-9
• Generally the hotter the source the shorter its
  emitted wavelength

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ELECTROMAGNETIC RADIATION
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PROPERTIES OF RADIATION

• Speed 3 x 105 km/s in vacuum.
• Radiation often behaves as a wave.
• Wavelengths (1nm 10-9 m)
• Radio 1m (109 nm)
• Infrared 10 ?m (104 nm)
• Visible 0.5 ? m (500 nm)
• Ultraviolet 10 nm
• X-ray 0.1 nm
• g-ray
• m metre, ? 10-6, n 10-9
• Propagation of radiation

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PROPAGATION of RADIATION
INVERSE SQUARE LAW
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PROPERTIES OF RADIATION

• Speed 3 x 105 km/s in vacuum.
• Radiation often behaves as a wave.
• Wavelengths (1nm 10-9 m)
• Radio 1m (109 nm)
• Infrared 10 ?m (104 nm)
• Visible 0.5 ? m (500 nm)
• Ultraviolet 10 nm
• X-ray 0.1 nm
• g-ray
• m metre, ? 10-6, n 10-9
• Propagation of radiation
• Spectrum of radiation (blackbody)

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WHITE LIGHT SPECTRUM
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BLACKBODY RADIATION

• Astronomical objects emit energy at different
  wavelengths

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ORION CONSTELLATION
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BLACKBODY RADIATION

• Astronomical objects emit energy at different
  wavelengths

• WHY?
• Temperature

• Blackbody

• a source that absorbs all radiation hitting it.
• Energy is re-emitted at all wavelengths.
• Spectrum emitted depends on the temperature
• At higher temperatures, more energy is emitted.
• F sT4
• The higher the temperature, the shorter is the
  maximum wavelength.
• lmax(nm) 2.9 x 106 /T(ºK)
• ºK ºC 273

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BLACKBODY CURVES
WIENS LAW
T Temp ºK ?max in nanometers
EMITTED ENERGY




0 500 1000 1500
2000 2500 3000
WAVELENGTH (nm)
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ORION CONSTELLATION
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FLASHCARD
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FLASHCARD
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DOPPLER SHIFT

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Doppler Shift Formula

• Change in wavelength (original wavelength)
  times (v/c)
• c 300,000 km/sec

• eg wavelength 400 nm from source moving ½ c away
  from you.
• change in wavelength (wavelength) times (v/c)
  (400) times (½) 200 nm
• wavelength thus observed at 600 nm colour
  change from blue to red

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FLASHCARD