Black Body Radiation (Edexcel International A Level Physics)

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 (or emits) 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

Black Body Radiation Curves

• 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