4.6.3.1 - Emission and Absorption of Infrared Radiation
Infrared radiation is electromagnetic radiation that can transfer energy between objects without the objects touching. In this physics-only lesson, the key idea is that every object emits and absorbs infrared radiation, even when it is cold. The hotter an object is, the more infrared radiation it gives out in the same time.
All bodies emit and absorb
All bodies, meaning all objects, emit infrared radiation and absorb infrared radiation. This is true no matter what the object's temperature is. A block of ice, a desk and a hot metal plate all emit infrared radiation, but not by the same amount.
Emit
To emit infrared radiation means to give out infrared radiation.
An object also absorbs infrared radiation from its surroundings. To absorb infrared radiation means the radiation enters the object and transfers energy to it.
Absorb
To absorb infrared radiation means to take in the radiation rather than reflect or transmit it.
The important temperature rule for this lesson is:
- hotter bodies emit more infrared radiation in a given time
- cooler bodies still emit infrared radiation, but less in the same time
[DIAGRAM: asset_name: Emission and absorption of infrared radiation - diagram 1; asset_slug: 099_4_6_3_1_emission_and_absorption_of_infrared_radiation_diagram1; file: diagram_assets/099_4_6_3_1_emission_and_absorption_of_infrared_radiation_diagram1.png; recommended_method: image_gen; description: Exact qualitative comparison showing a cool object, a room-temperature object and a hot object. Each object has incoming infrared arrows to show absorption and outgoing infrared arrows to show emission. The hot object has more/larger emitted infrared arrows in the same time, while all three objects emit and absorb. Image generated with built-in Codex Image Gen.]
The word radiates means gives out radiation. So "the hotter the body, the more infrared radiation it radiates in a given time" means that a hot object transfers energy away by infrared faster than a cooler object of the same kind.
Absorbed, reflected and transmitted
When infrared radiation reaches a surface, it may be absorbed, reflected or transmitted. The official black-body definition uses all three ideas, so keep them separate:
- absorbed radiation enters the object
- reflected radiation bounces off the surface
- transmitted radiation passes through the material
These words matter because a perfect black body is defined by what it does to incident radiation. The link needed in this lesson is: a good absorber of radiation is also a good emitter. A perfect absorber would therefore be the best possible emitter.
Perfect black bodies
A perfect black body is an ideal object used in physics. It is not just an object that looks black. It is defined by what happens to the radiation that reaches it.
Perfect Black Body
A perfect black body is an object that absorbs all of the radiation incident on it. It does not reflect or transmit any radiation.
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