Black Body Radiation (OCR GCSE Physics A (Gateway))

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

  • This moves to a lower wavelength and a higher intensity

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

  • At room temperature objects emit thermal radiation in the infrared region of the spectrum

  • At around 1000 °C an object will emit a significant amount of red light

  • At 6000 °C an object will mainly emit white or blue light (and some ultraviolet)

  • At even higher temperatures objects will emit ultraviolet or even X-rays