Thermal radiation

1
Thermal radiation

• Any object that is hot gives off light known as
  Thermal Radiation .
• The hotter an object is, the more light it
  emits.
• As the temperature of the object increase, it
  emits most of its light at higher and higher
  energies.

2
The Electromagnetic Spectrum
UV
Wavelength
visible
infrared
(nm)
3
Black body

• A black body is an object that absorbs all light
  that falls on it. Because no light is reflected
  or transmitted, the object appears black when it
  is cold.
• A black body emits radiation at a given
  temperature and wavelength exactly as well as it
  absorbs the same radiation.

4
Black body radiation spectrum
Intensity ( J/m4)
Wave length (nm)
5
Solar radiation Visible
6000 K
6
Radiation from the Earths surface Infrared
300 K
6,000 K
Note To convert to nm multiply by 1000
7
IR images
An image of a human hand taken in the infrared
displayed in false color where white and yellow
correspond to hot regions, blue and green to cool
regions.
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IR Images Some materials are transparent
to infrared light, while opaque to visible light
(note the plastic bag). Other materials are
transparent to visible light, while opaque or
reflective to the infrared (note the man's
glasses)
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IR images
Boy holding cold ball
The temperature difference between a ball that
is cold and one that has been heated. But also
note the various warm areas on the boy's face.
Boy holding hot ball
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IR images
Shown above is an Infrared Map of the Earth. Red
areas represent regions of high heat retention in
the atmosphere.
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IR images
The image shows a cat in the infrared. The orange
areas are the warmest and the white-blue areas
are the coldest.