5.3.3 radiation

• Risks of infra-red radiation

  • can cause serious skin burns if emitted from high-intensity sources
• Infra-red radiation has a high enough frequency to excite particles in food. This increases their temperature.
• Infra-red cameras can detect a range of frequencies. These frequencies can be shown in different colours to depict images
• Uses of infra-red radiation

  • TV controls
  • security purposes, e.g. alarms by detecting body heat
• Infra-red radiation cannot be seen but we can feel it as heat. Infra-red can transfer thermal energy without a medium (substance to travel through)
• The Sun emits infra-red radiation as it transfers thermal energy to the Earth. Infra-red radiation does not require a medium (substance to travel through), so it can still travel through space
• Shiny surfaces...

  • of a colour are poorer absorbers of radiation than dull surfaces of the same colour
  • are poorer emitters and better reflectors than dull surfaces of the same colour
• "A perfect black body" is an object that absorbs all of the radiation incident on it.

  • a black body does not transmit or reflect any radiation
  • since a good absorber is also a good emitter, a perfect black body would be the best possible emitter
• White surfaces

  • good reflectors of infra-red radiation
  • poor emitters and absorbers of infra-red radiation
• Black surfaces

  • good emitters
  • good absorbers
  • poor reflectors
• Houses in hot countries are typically painted white. White is a good reflector and poor absorber of infra-red radiation. So during the day the house is kept cool because of the white walls. At night, the white walls mean that not very much heat is lost because the house is a poor emitter of heat.
• Black surfaces are good emitters, good absorbers and poor reflectors. Shiny surfaces of a given colour are poorer absorbers, poorer emitters and better reflectors than dull surfaces of the same colour.