Infrared Emissions & Black-Body Radiation (Oxford AQA IGCSE Physics)

Explaining Infrared Radiation Emissions

• The particles in all substances vibrate with random thermal motion

• This motion causes objects to emit radiation

• The higher the temperature of the object, the faster the vibrations are, and the more infrared radiation is emitted

• The higher the temperature of the object, the higher the frequency of the emitted infrared radiation

• If the temperature of an object is greater than the temperature of its surroundings, the object will emit more infrared radiation than it absorbs

  • The temperature of the object will decrease as there is a net energy decrease

• If the temperature of an object is lower than the temperature of its surroundings, the object will absorb more infrared radiation than it emits

  • The temperature of the object will increase as there is a net energy increase

• If the temperature of the object is equal to the temperature of its surroundings, then the object will emit and absorb equal amounts of infrared radiation

  • The temperature of the object will remain constant as there is no net energy transfer into or away from the system

  • The object is in thermal equilibrium

Object in thermal equilibrium

Black-Body Radiation

• An object can reach thermal equilibrium if it is capable of emitting and absorbing radiation of every wavelength

• Then the amount of radiation emitted and absorbed at each wavelength per second could potentially reach a point where it is equal

• In this state of thermal equilibrium, the spectrum of wavelengths emitted depends only on temperature

• We call this object a black-body

  • A black body is a theoretical object

  • However, stars are the best approximation

Black-body radiation curve

• On a black-body radiation curve, as the temperature increases, the peak of the curve moves

• Waves with a smaller wavelength have higher energy (e.g. UV rays, X-rays)

• When an object gets hotter, the amount of radiation it emits increases

  • This increases the energy emitted and therefore the wavelength of the emitted radiation decreases