What are the properties of thermal radiation?

1. Planck's law of black-body radiation for an idealized emitter gives the frequency distribution of the wide range of frequencies that make up the thermal radiation emitted by a body at any temperature.

2. The dominant frequency (or color) range of the emitted radiation shifts to higher frequencies as the temperature of the emitter increases. For instance, a red hot object radiates primarily in the long wavelengths (red and orange) of the visible band. If it is heated further, it also starts to emit noticeable amounts of green and blue light, and the spread of frequencies in the entire visible range cause it to appear white to the human eye; this is known as being white hot. However, even at a temperature of 2000 K, 99% of the energy of the radiation is still in the infrared, as determined by Wien's displacement law. In the diagram, the peak value for each curve moves to the left as the temperature rises.

The total radiation of all frequencies rises steeply with temperature; it grows as T4, where T is the body's absolute temperature. An object at the temperature of a kitchen oven, or roughly twice the room temperature on the absolute temperature scale (600 K vs. 300 K), radiates 16 times as much power per unit area; an object at the temperature of an incandescent light bulb, or roughly 3000 K, or 10 times room temperature, radiates 10,000 times as much energy per unit area. The Stefan–Boltzmann law states that the total radiative intensity of a black body rises as the fourth power of the absolute temperature. In the plot, the area under each curve grows rapidly as the temperature rises.

A surface that absorbs more red light thermally radiates more red light. This principle applies to all properties of the wave, including wavelength (color), direction, polarization, and even coherence. Therefore, it is quite possible to have thermal radiation that is polarized, coherent, and directional, though polarized and coherent forms are fairly rare in nature. 4) The rate of electromagnetic radiation emitted at a given frequency is proportional to the amount of absorption that it would experience by the source.

The properties of thermal radiation include:

1. Wavelength: Thermal radiation spans a wide range of wavelengths, from infrared to visible to ultraviolet.
2. Intensity: The intensity of thermal radiation depends on the temperature and emissivity of the object.
3. Directionality: Thermal radiation is emitted and absorbed in all directions.
4. Speed: Thermal radiation travels at the speed of light in a vacuum.
5. Absorption and Emission: Objects both absorb and emit thermal radiation, with the rate depending on their temperature and surface properties.
6. Inverse Square Law: The intensity of thermal radiation decreases with the square of the distance from the source.