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Red Hot?

Graham Phillips's picture

The color of temperature 

 Have you ever heard or used the expression, "red hot"?  You've probably heard (perhaps in art class) that colors on the left end of the visible light spectrum (red, orange, yellow) referred to as warm colors, while those on the right side of the visible light spectrum (green, blue, violet) referred to as cool colors.  But, do our observations in the world of science match our observations in the world of everyday living? 

Begin by writing down at least five things that you use every day that are or would be hot to the touch.  What color are they?


 Object  Color

Now, we will examine an object's black body radiation, and the color of its heat signature.  As you now know, heat is a form of energy, and travels in waves.  Objects that give off heat can radiate those waves at different speeds and intensities.  Go to the following website:

 The switch on the right side of the graph is a heat switch.  Lifting the switch raises the temperature of the system, and therefore, increases the heat given off.  Lowering the switch does the opposite--it decreases the temperature, and therefore, the amount of heat given off.  What are the three objects that are listed on the graph?


Now, line up the switch to a temperature of 4800 K (Kelvin is the standard SI unit of temperature, which is simply the temperature in degrees Celsius +273).  What color is the star meter in the top right hand corner?  


Now, look at the graph.  Notice where the peak in the red heat curve is?  It is in the middle of the visible light spectrum.  In which color(s) do you find the peak of the heat curve?


This is because even amounts of red, green, and blue light are in combination around the peak of the heat curve.  As you now know, mixing equal amounts of the primary colors of light gives what color?


Now, lower the switch so that it is about 2000 K.  What color is the star meter now? 


What colors are underneath the heat curve in the graph? 


Finally, raise the temperature up to 9000 K.  What color is the star meter, and what colors are underneath the heat curve the most?


Can you draw any conclusion about color and temperature based on the amount of heat an object gives off?