No matter the temperature, all objects absorb and emit infrared radiation.
Eventually, the object reaches equilibrium. At this point, it maintains a constant temperature, absorbing radiation at the same rate it emits radiation.
A black body is an object that absorbs all incident radiation, regardless of frequency. There are no known perfectly black bodies on Earth or anywhere in the universe.
A perfect black body is a theoretical object, but if it existed, it would have these properties:
A good absorber of radiation is also a good emitter of radiation, so a perfect black body is the best emitter of radiation. An object that perfectly absorbs infrared radiation must be black because a black surface absorbs all wavelengths of visible light.
Thermal radiation emitted by hot objects is called black body radiation. Objects emit black body radiation in the form of electromagnetic waves. These electromagnetic waves typically lie in the infrared region of the electromagnetic spectrum. However, depending on the object’s temperature, it can also be emitted as visible light or other wavelengths.
Below is a graph of the emitted radiation, with wavelength on the X-axis and intensity on the Y-axis.
The hotter an object, the more thermal radiation it emits, and the wavelength of that thermal radiation decreases. This explains why the curves rise with increasing temperature. However, the intensity of the shorter wavelengths increases more than the longer wavelengths. Therefore, each curve skews more to the left.
This explains why Bunsen burner flame colours change with increasing temperature. A blue flame is hotter than a red one and emits light with a shorter wavelength.
When an object absorbs as much radiation as it emits over time, its temperature remains unchanged. This is because it is gaining and losing the same amount of heat.