The Absorption and Emission of infrared Radiation

No matter the temperature, all objects absorb and emit infrared radiation.

  • When an object absorbs infrared radiation, it will get warmer
  • When an object emits infrared radiation, it will get cooler

Eventually, the object will reach equilibrium. At this point, there is a constant temperature, at which the object is absorbing radiation at the same rate it is emitting radiation.

Black Bodies

A black body is an object that absorbs all incident radiation on it, regardless of the frequency. There are no known perfectly black bodies on Earth or anywhere in the universe.

A perfect black body is a theoretical object, but if it existed, it would have these properties:

  • It absorbs all the light and electromagnetic radiation that falls on it
  • It is unable to reflect or transmit any radiation

A good absorber of radiation is also a good emitter of radiation, so a perfect black body is the best emitter of radiation. An object that perfectly absorbs infrared radiation must be black because a black surface absorbs all wavelengths of visible light.

Thermal radiation emitted by hot objects is called black body radiation. Objects emit black body radiation in the form of electromagnetic waves. These electromagnetic waves typically lie in the infrared region of the electromagnetic spectrum. However, it can also be emitted as visible light or other wavelengths, depending on the temperature of the object.

The Effects of Temperature on Emission

Below is a graph of the emitted radiation, with wavelength on the X-axis and intensity on the Y-axis.

  • Keep in mind, that intensity is a measure of how much energy the radiation transfers to a given area, over a period of time.

The hotter an object is, the more thermal radiation it will emit and the wavelength of that thermal radiation will decrease. This is why the curves get higher as you increase the temperature. However, the intensity of the shorter wavelengths increases more than the longer wavelengths. So each curve is more skewed to the left.

This is the reason that the colour of Bunsen burner flames changes as the temperature increases. A blue flame is hotter than a red flame and a blue flame has a shorter wavelength of light emitted.

When an object absorbs the same amount of radiation that it emits over a period of time, its temperature does not change. This is because it is gaining and losing the same amount of heat.