Every object emits and absorbs infrared radiation
Infrared radiation
Infrared radiation is electromagnetic radiation that transfers energy by radiation and is associated with heating.
- All bodies, meaning all objects, both emit and absorb infrared radiation, whatever their temperature.
- To emit infrared is to give out energy as infrared radiation; to absorb it is to take in energy from infrared radiation falling on the object.
- An object need not glow or feel hot to emit infrared: a desk, a person and the walls of a room all emit it.
A hotter body radiates more infrared each second
- The hotter the body, the more infrared radiation it radiates in a given time.
- So a hotter object transfers more energy each second by infrared than a cooler one; the exam idea is the rate of emission, not just whether radiation is emitted.
- A colder object still emits infrared, but less of it in the same time than a hotter object.
Question: Two identical metal blocks are at different temperatures, A hotter than B. Which emits more infrared in one minute?
Block A emits more infrared in one minute because it is hotter: all objects emit infrared, and the hotter a body is, the more it radiates in a given time.
A perfect black body is the ideal absorber and emitter
Perfect black body
A perfect black body is an object that absorbs all of the radiation incident on it.
- A perfect black body absorbs all incident radiation and reflects and transmits none, which is why it is a perfect absorber.
- Because a good absorber is also a good emitter, a perfect black body would be the best possible emitter as well.
Question: Explain why a perfect black body is the best possible emitter of radiation.
A perfect black body absorbs all incident radiation and does not reflect or transmit any. Since a good absorber is a good emitter, it is the best possible emitter.
- Do not say only hot objects emit infrared: all objects emit it, at any temperature.
- Do not say a black body only absorbs: as the best absorber it is also the best emitter.
Use the exact statements:
- all bodies emit and absorb infrared radiation
- the hotter the body, the more it radiates in a given time
- a perfect black body absorbs all incident radiation and does not reflect or transmit it
- a good absorber is a good emitter
- Which objects emit and absorb infrared radiation?
- How does a body’s temperature affect how much infrared it radiates each second?
- What does a perfect black body do to all radiation incident on it?
- Does a perfect black body reflect or transmit radiation?
- Why is a perfect black body the best possible emitter?
