94 Thermal Radiation Spectra

The Electromagnetic Spectrum

Different names are used for (light waves) with various ranges of : radio waves, microwaves, radiation, visible light, ultraviolet radiation, X-rays, and gamma rays. Collectively these ranges of frequencies make up the shown in the following diagram. The range frequencies that we can see is known as the visible spectrum, and we perceive the different frequencies within the as different colors. The wavelength of light, or any wave, is the distance between successive crests (peaks) of the wave. The frequency and of light waves are directly related and we can sometimes more easily relate to wavelength by comparing it to the length of familiar objects, so we often use instead of to describe colors and the as a whole.

The electromagnetic spectrum. “EM Spectrum Properties reflected” by Inductiveload, via Wikimedia Commons

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We can summarize the previous diagram in tabular form:

The previous diagram in tabular form
Radiation Type  Wavelength (m) Approximate Wavelength Scale Frequency (Hz) Temperature of object with thermal radiation peak at this wavelength (K) Significant penetration through atmosphere ?
Gamma Ray 10-12 Atomic Nucli 1020 No
X-ray 10-10 Atoms 1018 10,000,000 No
Ultraviolet (UV) 10-8 Molecules 1016 No (more at longer wavelength)
Visible 0.5-6 Protozoans 1015 10,000 Yes
Infrared (IR) 10-5 Needle Point 1012 100 Yes (less at longer wavelength)
Microwave 10-2 Butterflies 108 1 No
Radio 103 Humans to Buildings 104 Yes (less at shorter wavelength)

Black Body Radiation

A theoretically perfect emitter for which the is one (epsilon = 1) is known as , because such an emitter would also be a perfect absorber and would thus appear completely black. The shape amount of light emitted at each wavelength defines the of the black body, which depends only on in a well-defined way:

This graph shows the variation of blackbody Radiation intensity with wavelengths expressed in micrometers. Five curves that correspond to 2000 K, 3000 K, 4000 K, and 5000 K are drawn. The maximum of the radiation intensity shifts to the short-wavelength side with increase in temperature. It is in in the far-infrared for 2000 K, near infrared for 3000 K, red part of the visible spectrum for 4000 K, and green part of the visible spectrum for 5000 K.
The intensity of black body radiation plotted against the wavelength of the emitted radiation. Each curve corresponds to a different black body temperature, starting with a low temperature (the lowest curve) to a high temperature (the highest curve). Image Credit:  OpenStax University Physics Volume 3.

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This simulations allows you to see how the depends on temperature:

Blackbody Spectrum

We are often able to approximate the temperature of objects by assuming they are and matching up their with that of  a black body with a known . This is the basic principle behind cameras and handheld (IR) such as the one in the following image. (Note that IR thermometers are often include a low power laser to improve aim, but contrary to popular belief, the laser is not involved in the temperature measurement).

A person points a hand-held, non-contact thermometer at the forehead of another person.
Contact tracers at a hospital in Conakry, Guinea demonstrate how to use a ThermoFlash infrared thermometer to monitor the temperatures of people who have come in contact with Ebola patients. Contacts are monitored for 21 days so that they can be isolated and treated as soon as possible if they develop symptoms. Image Credit: Infrared thermometer training by CDC Global via Wikimedia Commons.

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For example, we can estimate the surface temperature of the sun to be roughly 6000  K (10,000 °F) because the actual emission spectrum of the Sun best matches the of an object at  6000  K, as seen in the following graph. Notice that the peak of the Sun’s emission spectrum is in the visible range, but that significant radiation is found in the UV and regions. The UV light is capable of penetrating the dead out layer of skin (epidermis) and breaking some molecular bonds in your cells, including those in DNA, which can lead to sunburn and increased risk of skin cancer.

This graph shows the variation of blackbody Radiation intensity with wavelengths expressed in micrometers. The radiation curve of the sun (measured above the atmosphere is shown to agree very well with a curve corresponding to black body radiation from an object with temperature of 5777 K. The visible region of the spectrum is highlighted to show that the radiation curve of the sun is peaked in the visible region.
Emission spectrum of the sun as measured above the Earth’s atmosphere (AM0) compared to the black body spectrum of an object at 5777 K. Image Credit: Solar AM0 spectrum with visible spectrum background (en) by Danmichaelo [Public domain], from Wikimedia Commons

Everyday Example: Incandescent vs LED and Fluorescent Light Bulbs

Incandescent light bulbs use to generate light. In order for their to contain significant visible light their must be several thousand , as seen from the previous graph showing black body emission spectra at several temperatures. Temperatures of 3000 K to 4000 K are achieved by running electric current through the narrow filaments of inside the bulbs to cause resistive heating (conversion of electric to ). The filaments are made of high tolerant metals like tungsten to prevent . Additionally, the majority of air within the bulb has been removed to prevent and from heating the glass and to prevent the filaments from quickly oxidizing (rusting). The for objects at 3000 to 4000 show us that much of their radiated is in the range rather than the visible range, and thus doesn’t provide useful illumination. Consequently,  much of the electrical energy used to power incandescent light bulbs goes to waste. In fact, glass does absorb radiation so much of the wasted energy simply goes into making the bulb glass hot, in some cases dangerously so. Fluorescent and LED bulbs don’t use to generate light. Instead they apply voltages to energize electric charges trapped in atoms or in semi-conductor materials. When the electrons de-energize they emitted light at specific , reducing the wasteful production of non-visible light. However, light from incandescent bulbs is sometimes considered more pleasing because it more closely resembles the of fire.

Dangerously Hot Cars

Some materials are to , but readily absorb light (notice how glasses prevent IR light from reaching the camera in this thermal image). Liquid water, water vapor, carbon dioxide (CO2) gas and most types of glass behave this way.  The emission spectrum of the sun shown above has significant emission in the UV, visible, and parts of the .  The visible light gets through the glass, which is why the glass appears transparent to you. The majority of UV is absorbed or reflected, preventing you from getting sunburn inside the car. The glass absorbs much of the IR light, which is re-radiated in both directions, in and out of the car. The visible light that gets through is partially absorbed by the interior of the car (especially if the interior is dark). That absorbed visible light is then re-radiated as IR light because the interior of the car is not nearly hot to enough to radiate visible light like the sun. That re-radiated IR light is absorbed by the glass and re-radiated again in both directions, in and out of the car. Therefore a significant portion of the incoming visible light energy gets trapped inside the car and the interior can rise quickly, even if the outside air temperature is cool.  Green houses use this same phenomenon to keep plants warm in cool weather, so this phenomenon is commonly known as the green house effect.  It’s never a good idea to leave children or pets in cars. Even if you perform thoughtful calculations to predict the interior temperature for a given set of conditions such as air temperature, wind speed, and cloudiness, those conditions can change quickly.  It’s best not to risk injury to loved ones.

The Greenhouse Gas Effect

The Earth’s atmosphere acts like a car’s windshield. The atmosphere lets most UV and through, but significant light is absorbed, primarily by water vapor and carbon dioxide gas. With respect to the Earth, this green-house effect is known as the because the phenomenon is caused by gasses in the atmosphere instead of glass or plastic.

 

Figure shows UV, IR and visible light from the sun striking the earth through its atmosphere. Of these, only IR is reflected.
Illustration of the green house gas effect. UV, visible, and some IR light pass through the atmosphere. The UV and visible light are largely transformed to IR light. Only some of that IR light is able to escape back into space, the rest is trapped and the energy it contains increases the Earth’s average temperature. Image Credit: OpenStax University Physics.

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The helps to keep the Earth’s about 40 °C warmer than it would be without an atmosphere, which is a generally a good thing for us because most water on Earth would be frozen otherwise. Humans have established our modern infrastructure in accordance with the global climate that was present over the last few hundred years, but emission of carbon dioxide and methane (and other greenhouse gases) into Earth’s atmosphere from human activities strengthens the and increases the average of the Earth.  Higher temperature means more is available to drive more powerful and other thermodynamic processes that define weather and climate. The resulting changes in global climate are likely to cause a variety of dangerous and expensive consequences such as higher storm intensity, rising sea levels, and increased flooding in certain areas with prolonged drought in others.[5][6]
The following simulation allows you to examine how the works.

The Greenhouse Effect

Click to Run

  1. "EM Spectrum Properties reflected" by Inductiveload [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)], via Wikimedia Commons
  2. OpenStax University Physics, University Physics Volume 3. OpenStax CNX. Nov 12, 2018 http://cnx.org/contents/af275420-6050-4707-995c-57b9cc13c358@10.14.
  3. Infrared thermometer training by CDC Global [CC BY 2.0 (https://creativecommons.org/licenses/by/2.0)], via Wikimedia Commons
  4. OpenStax University Physics, University Physics. OpenStax CNX. Oct 6, 2016 http://cnx.org/contents/74fd2873-157d-4392-bf01-2fccab830f2c@1.585
  5. "Fourth National Climate Assessment" by U.S. Global Change Research Program
  6. OpenStax University Physics, University Physics. OpenStax CNX. Oct 6, 2016 http://cnx.org/contents/74fd2873-157d-4392-bf01-2fccab830f2c@1.585

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