5.37 Black Body Radiation

• Black body radiation is the name given to the thermal radiation emitted by all bodies (objects)
• All objects, no matter what temperature, emit black body radiation in the form of electromagnetic waves
• These electromagnetic waves usually lie in the infrared region of the spectrum but could be emitted in the form of visible light or other wavelengths, depending on the temperature
• The hotter object, the more infrared radiation it radiates in a given time

The infrared emitted from a hot object can be detected using a special camera

• A perfect black body is defined as:

An object that absorbs all of the radiation incident on it and does not reflect or transmit any radiation

• Since a good absorber is also a good emitter, a perfect black body would be the best possible emitter too
• As a result, an object which perfectly absorbs all radiation will be black
  • This is because the colour black is what is seen when all colours from the visible light spectrum are absorbed

Absorption and Emission For Different Colours Table

• All bodies (objects) emit a spectrum of thermal radiation in the form of electromagnetic waves
• The intensity and wavelength distribution of any emitted waves depends on the temperature of the body
• This is represented on a black body radiation curve
  • As the temperature increases, the peak of the curve moves

Black body spectrum for objects of different temperatures

• From the electromagnetic spectrum, waves with a smaller wavelength have higher energy (e.g. UV rays, X-rays)
• When an object gets hotter, the amount of thermal radiation it emits increases
  • This increases the thermal energy emitted and therefore the wavelength of the emitted radiation decreases

• An ideal black body radiator is one that absorbs and emits all wavelengths 
  • This idealised black body is a theoretical object
  • However, stars are the best approximation there is
• The radiation emitted from a black body has a characteristic spectrum that is determined by the temperature alone

The intensity-wavelength graph shows how thermodynamic temperature links to the peak wavelength for four different bodies