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Name:  Luke N.
Status: student
Age: 20s
Location: N/A
Country: N/A
Date: 2000-2001


Question:
I understand that all heated objects emit radiation. The intensity depends on the emissivity value of the material. The frequency range of the emitted electromagnetic waves is a function of the temperature.

So why, when we talk about heat radiation, are infrared frequencies focused on as opposed to any other range of electromagnetic waves? I have read several articles which seem to imply that we only obtain heat energy from the Sun's infrared spectrum and not so from the other frequencies (like those of visible light).


Replies:
Infrared frequencies are not different from other frequencies in their ability to carry heat energy. However, near room temperature, the black body spectrum peaks in the infrared, and most real objects we encounter behave pretty much like these idealized radiators, so we've come to associate infrared with heat.

Tim Mooney


Well, Luke, if that is what they say, they are wrong. The energy intensity of the solar spectrum is at a maximum in the visible, at the wavelength of yellow-green light. (This is why the Sun is considered a yellow star.) In fact, the sensitivity of the human eye to different frequencies of light roughly parallels the intensity of those frequencies in the solar spectrum. (No coincidence: we evolved seeing sunlight.)

We get heat from other wavelengths in the solar spectrum because many substances on earth (leaves, rocks, dirt, animals, etc.) absorb light, converting the energy to heat. It is not necessary for the infrared component even to be present. It is not a very large portion of the energy anyway.

Richard E. Barrans Jr., Ph.D.
Assistant Director
PG Research Foundation, Darien, Illinois


You are correct, objects at any temperature emit electromagnetic radiation of all frequencies (wavelengths) that depends upon the temperature (black body radiation) and the relative amounts are given by Planck's distribution law.

The reason that the infrared portion of the electromagnetic spectrum is stressed when we talk about heat energy is that is where the maximum in the Planck distribution occurs for what we nominally call "hot objects".

Of course some hot objects emit radiation in the visible range -- they glow.

Radiation from the sun or other sources is not necessarily a continuum. In addition, light of various wavelengths can be converted to other wavelengths by a variety of processes. Some examples are:

microwave (oven) radiation converted to infrared radiation (hot food).

visible radiation converted to infrared radiation (heat) when it is absorbed by the ground, or even more so by a black asphalt driveway.

ultraviolet radiation converted to visible radiation by fluoresence ("black light" U.V. lamps)

incandescent lamps that produce both visible light and heat.

fluorescent lights that produce a higher amount of visible light compared to infrared radiation because of the phosphors.

It is just not correct that the sun provides heat only from its infrared portion of the solar spectrum. All of these processes above and others produce "heat radiation" from solar radiation.

Vince Calder


As you know, heat is just energy.

The sensation of heat from a radiative source requires that the energy be present in the radiation AND that the radiation is absorbed. Almost all infrared wavelengths are absorbed in the skin and thus creates the sensation of heat very efficiently.

Visible light, on the other hand, is mostly reflected. Thus, while there is much energy available in the visible light portion of the solar spectrum it is not efficiently converted to heat -- unless a good absorber is present (such as black paint).

Solar heaters are black to improve the efficiency of absorption of radiation, in both the visible and infrared wavelengths.

Greg Bradburn


Dear Luke,

The answer to your question is already in your question; the intensity of the emitted radiation is a function of the wavelength and that function depends on the temperature of the emitter. It is a smooth curve which extends from very short wavelengths to infinitely long wavelengths, but most of the energy transmitted is in the region of the maximum.

To become a little quantitative, the maximum is given by the Wien Displacement Law:

LT = 3E6 nm K

Here L is the wavelength in nanometers (1 nanometer = 1 nm = 1E-9 m) and T is the temperature of the emitter in degrees Kelvin. If you're not happy with exponential notation, 3E6 = 3,000,000 and 1E-9 = 0.000000001.

To get some feeling for these numbers, 300 K is approximately room temperature (around 80 F) and the temperature of the sun is around 6,000 K. The sun is the brightest (emits the highest intensity) around a wavelength of 500 nm (plug in the above equation to get this), right in the middle of the visible range of wavelengths. (Obviously our eyes developed to take advantage of the properties of our sun -- luckily the molecules we are made of could take advantage of these properties).

Here are some wavelengths to keep in mind: (These numbers denote regions and are not exact).
   < 400         470       570      670        700
Ultraviolet     Blue    Yellow     Red      Infrared

Now, if you take the temperature of the earth to be 300K, the above equation will tell you that the maximum intensity of the radiation emitted is be at a wavelength of 10,000 nm, in the far infrared.

And that is why the sun's energy mostly gets through to the earth, since the earth's atmosphere is quite transparent in the visible region, but the earth's radiation is increasingly trapped because CO2 and other "greenhouse" gases are transparent to visible light, but tend to reflect light of infrared wavelengths.

Best, Dick Plano...


Hi, Luke !!!

As you know, all the radiant energy is in the form of electromagnetic waves, and they have a broad range of frequencies, coming from radio waves, microwaves, infrared, visible, ultraviolet, X-rays and gamma-rays. When you say : "I understand that all heated objects emit radiation." I would point out that not only heated objects emit radiation, but lets say, every object ( with the temperature greater than 0o Kelvin) emits radiation. All objects emit a mixture of radiant wavelenghts of energy. If the temperature of the body becomes hotter, some of the radiant energy is in the visible range of wavelenghts like - for instance - the filament of a light bulb. The Sun emit energy across the whole range of wavelenghts. You mention that you have read several articles which seem to imply that we only obtain heat energy from the Suns INFRARED SPECTRUM and not from other frequencies. Well, for sure that thermal radiation is defined as electromagnetic radiation in the wavelenght range of 0.1 to 100 micra, which contains the visible light ( ca.from 0,7 till 0,4 micron ). Our atmosphere filters out many harmful types of electromagnetic radiation.

I belive that your question focus attention on how much energy is delivered by the Sun when you take into account the INFRARED SPECTRUM. Certainly depending upon the temperature of a body, you have a different wavelenght allocation. Just to recall, the energy of a vibrating molecule is proportional to its frequency of vibration. And they came - as Planck said - in discrete lumps. The Plancks radiation law describes the wavelenght distribution of electromag- netic emission of a blackbody

Well, it is necessary to count all the frequencies in to correctly evaluate the delivered energy of the Sun.

Best regards

Alcir Grohmann


Luke,

At common temperatures: 0 to 100 degrees Celsius, most radiation emitted is in the infrared range. At much higher temperatures, objects begin to have a red glow: visible light is emitted. As temperature increases, the color passes through the visible spectrum: orange, yellow, white(full spectrum), eventually blue.

As for the sun, the earth receives the full spectrum. Most of the radiation above visible light is reflected back by the ozone layer of our atmosphere. The ultraviolet light that does get through causes sunburn. Radio waves don't have enough energy to be noticed: some pass right through the earth. The two parts of the spectrum that have greatest effect are infrared and visible. Since visible light is such a narrow range of frequencies, infrared does provide a great deal of the heat we receive.

Dr. Ken Mellendorf
Illinois Central College


I believe that near the normal body temperature (say from 60 deg F to 120 deg F), objects tend to give up waves in the infrared frequency range.

-Wil Lam



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