Cambridge O Level Physics · Syllabus 5054 · Thermal Physics
Infrared Radiation
What is Infrared Radiation?
Infrared radiation is electromagnetic radiation emitted and absorbed by all objects that transfers thermal energy without needing a medium, so it travels through a vacuum; the rate at which an object emits it increases with the surface temperature and with the surface area, and dull black surfaces are the best emitters and absorbers while shiny light surfaces are poor emitters and absorbers but good reflectors.
This definition is part of the Thermal Physics chapter in Cambridge O Level Physics.
Infrared Radiation in context
Thermal physics runs along a single chain of reasoning. Particles are arranged and move in a way that depends on the forces between them; heating makes them move faster, which raises temperature; faster particles collide harder and more often, which creates gas pressure and drives expansion; energy supplied at a change of state pulls particles apart instead of speeding them up; and thermal energy travels from hot to cold by conduction, convection and infrared radiation.
Thermal energy always travels from a hotter region to a cooler one, by three distinct routes. Conduction passes energy from particle to neighbouring particle through lattice vibrations in all solids, and much faster in metals because free delocalised electrons carry energy through the structure. Convection occurs only in fluids: the heated fluid expands, its density falls, it rises, cooler denser fluid sinks to replace it, and a convection current carries energy by bulk movement of the fluid itself. Infrared radiation is emitted and absorbed by every object, needs no medium at all, and is emitted at a greater rate by hotter objects and by objects with a larger surface area; dull black surfaces are the best emitters and absorbers while shiny light surfaces are the best reflectors.
Common mistakes with Infrared Radiation
- 14. “A vacuum stops infrared radiation.” Why it is wrong: radiation is the one route that needs no medium at all — energy from the Sun crosses space to reach us. Corrected model: a vacuum removes conduction and convection; radiation is dealt with by the silvered surfaces. Exam-safe: “The vacuum reduces conduction and convection because there are almost no particles between the walls; the silvered surfaces reduce radiation.”