In 1862 German physicist Gustav Kirchhoff introduced the term  blackbody  to describe a hypothetical object that absorbs all of the electromagnetic radiation (EMR) that falls on it.  In theory, the energy of all the absorbed EMR causes the blackbody to heat up and become a perfect radiator, emitting energy back to the environment in the form of EMR.  Some of the EMR can be seen in red- or white-hot objects and longer wavelengths can be felt as heat.  The heating element on a stove can be considered a blackbody radiator-you can feel the infrared EMR and see the visible EMR at sufficient temperatures.


Blackbody:  any object that absorbs all EMR that falls on it and is capable of being a perfect emitter, releasing energy in the form of EMR

 

The relationship between temperature and colour can be summarized using a Blackbody Spectrum Simulation.  Open the simulation and watch what happens when you adjust the temperature using the slider.

 

The relationship between temperature and colour can be summarized using a Blackbody Spectrum Simulation.  Open the simulation and watch what happens when you adjust the temperature using the slider. 

As illustrated above, the simulation shows the observed colour of an object at the temperature indicated on the slider.  In this example, a  blackbody radiation curve  shows the distribution of energy by wavelengths released by an  incandescent  light bulb when it is 3045 Kelvin (2772 °C).  Notice the bulb emits most of its energy in wavelengths slightly larger than those of visible light, which is infrared heat.  Interestingly, this is why an incandescent light bulb is very inefficient at producing light-90% of the energy it consumes produces infrared heat rather than visible light!

One interesting thing about blackbody radiation curves is that you can use an object's curve, along with its observed colour, to determine the object's temperature!


© The University of Colorado, under the GNU General Public License (GPL)

Try This
Using the Blackbody Spectrum Simulation, complete the three sentences below regarding the relationship between colour and temperature. 

  1. Extremely hot, glowing objects, such as stars, emit a continuous range of wavelengths, making them appear white. 

  2. At a given temperature, the energy emitted by a hot object comprises a specific range of wavelengths, giving it a unique colour. 

  3. At _______ temperatures, an object emits more blue light; at _______ temperatures, it emits more red light. 


Blackbody Radiation Curve:  a graph of the intensity of EMR versus wavelength for an object at a given temperature.



Incandescent:  glowing with heat

 
Self-Check

Answer the following self-check (SC) question then click the "Check your work" bar to assess your response.

 

SC 1. 

Using the radiation curves in the simulation, explain why a light bulb appears white while an oven element only has a slight red glow. What does this imply about the temperature of the filament in a light bulb compared to that of an element in a stove?

 

   Self-Check Answer

Contact your teacher if your answers vary significantly from the answers provided here.

 

SC 1.

 

Light Bulb

© The University of Colorado, under the GNU General Public License (GPL)

 

The bulb appears white because it emits some energy in wavelengths that span across the visible range of wavelengths. The longer, infrared wavelengths are not seen but are felt as heat.

 

Stove Element

© The University of Colorado, under the GNU General Public License (GPL)

 

Zooming in several times along the intensity axis reveals that oven temperatures release energy with wavelengths that approach the red end of the visible spectrum. The element in an oven will start to appear red when it reaches temperatures near 1300 K. (Note the inside of an oven is much cooler than this, which is why the element cycles on and off and the entire oven does not glow red hot.) You can adjust the temperature on the simulation upward to find the exact temperature at which red light will start to appear.

 

The temperature of the filament in a light bulb is much higher than that of a stove element, even during the heating cycle.

              


Read
Read pages 704 and 705 of the textbook for more information on blackbody radiation and curves.  Note that some of the radiation curves shown in the text are related to frequency rather than to wavelength, as seen in the simulation.