Thermal radiation is another name for
blackbody radiation. Both terms refer to the emission of light and
other non-ionizing radiation from a heated object based solely on the
object's temperature. So sometimes it's also called "temperature
radiation." Also, we're not concerned with the perfect blackbody here, except as a standard for comparision.
So, how else does light get produced other than the heat-it-till-it-glows method? Well, there's fluorescence, the process that goes on inside a fluorescent bulb, where ultraviolet radiation is absorbed by an atom and visible light is emitted. Fluorescent lights use high voltage to stimulate mercury atoms to emit ultraviolet photons which hit the visible-light emitting phosphor coating on the inside of the tube. (A related, delayed type of light emission is called phosphorescence.) LEDs emit light by electroluminescence, where the "bandgap" energy in a semiconductor connected to a DC voltage is turned into light. And then there's the chemiluminescence of fireflies and green-light glow-in-the-dark thingies, but you can click the link to read about that.
So, how else does light get produced other than the heat-it-till-it-glows method? Well, there's fluorescence, the process that goes on inside a fluorescent bulb, where ultraviolet radiation is absorbed by an atom and visible light is emitted. Fluorescent lights use high voltage to stimulate mercury atoms to emit ultraviolet photons which hit the visible-light emitting phosphor coating on the inside of the tube. (A related, delayed type of light emission is called phosphorescence.) LEDs emit light by electroluminescence, where the "bandgap" energy in a semiconductor connected to a DC voltage is turned into light. And then there's the chemiluminescence of fireflies and green-light glow-in-the-dark thingies, but you can click the link to read about that.
For
thermal radiation to be visible, the heated body must be hot enough to
produce light in the visible spectrum (390 to 700 nanometers, violet
wavelength to red wavelength). Incandescent light bulbs are one example
of that, producing light by being heated by an electric current to
temperatures around 4500° F. Stars, including our Sun, are also
examples, with surface temperatures in the range of 10,000° F. But
thermal radiation is emitted by any object that's not at absolute zero
of temperature--so all objects emit thermal radiation, mostly at
very long wavelengths. The Earth, for instance, emits thermal radiation
in the infrared region of the electromagnetic spectrum ,
which goes from 750 nanometers on up to about a million nanometers, or
one millimeter. This emission is due to Earth's own internal heat, and from light it absorbs from the sun and re-emits as heat. Our bodies also emit thermal radiation in the infrared
region. Our average power output is about 100 watts, another
way of saying 2,065 Calories/day. We are visible not because of
radiation we emit but because, like the things around us, we reflect
most of the visible light hitting us. This is called diffuse reflection rather than mirror-type (specular) reflection.
The hotter an object is, the shorter the wavelengths of
light emitted by the object. (Shorter wavelength means higher frequency.) The webpage I just linked to shows the
blackbody curves (spectra) of energy emitted versus wavelength. Optical pyrometers use
this spectral relationship--the relationship Max Planck explained by
assuming quantized emission of thermal radiation--to measure the temperatures of
hot objects such as molten steel.
An aside: Lighting techniques in stage lighting and photography make use of something called "color temperature," which uses the idea of thermal or blackbody radiation as a reference standard, although it associates "warm" colors with what are actually cool objects and "cool" colors with what are actually hot objects.
An aside: Lighting techniques in stage lighting and photography make use of something called "color temperature," which uses the idea of thermal or blackbody radiation as a reference standard, although it associates "warm" colors with what are actually cool objects and "cool" colors with what are actually hot objects.
