4B40.50 - Thermal Radiation - Absorption/Transmission

See paragraph one in the procedure section
See paragraph two in the procedure section
See paragraph three in the procedure section
See paragraph four in the procedure section
Code Number:
Demo Title:
Thermal Radiation - Absorption/Transmission
Radiation of Heat from Different Surfaces, Absorption & Transmission
Area of Study:
Heat & Fluids
Multimeter, Light bulb (Standard), Sensitive thermocouple, Variac, Heater (coil in ceramic base), Glass Plate, Silicon Wafer, Grill Lighter, Heat lamp, filters on stands, Radiometer.

Place either the light bulb or the heating coil directly in front of the thermocouple. As you turn up the Variac there should be an immediate voltage increase on the meter. An ordinary 100 watt light bulb works just as well as the type of bulb shown in the picture. 

Use the coil heater hooked to the Variac as the heat source. Hook the thermocouple horn to the voltmeter, (reading microvolts) as the detector. Turn the Variac to about 40 which will give heat but no visible radiation. By putting the glass between the detector and the heat source almost all the radiation can be absorbed. (Reading on the detector will go to background levels.) Remove the glass plate and insert the silicon wafer to show that even though you can't see through the wafer it is fairly transparent to I.R. Note: Wafer is fragile. BE CAREFUL. A light bulb will not work as the I.R. wavelength is to short.  

An interesting variation of this is to use the silicon wafer filter in front of the thermocouple.  Place a heater coil in front of this and plug it into the watt meter.  Set the output energy at 300 watts and allow the system to come to equilibrium.  (The reading of the voltmeter should be around 16.40.)  Now take off the heater coil and put the 1000 watt light bulb in its place.  Again turn up to 300 watts and allow the system to come up to equilibrium.  (The reading on the voltmeter should be 14.40.)  This basically shows the component of energy that is turned into light and not infrared radiation.

Substituting the voltmeter for a radiometer may be preferred in some situations.  In this case use a heat lamp as your heat source.  Point it at the radiometer from a distance of about 5 feet and observe the rotations rate of the vanes.  Insert the heat absorbing filter and notice that the rotation rate slows to a stop.  Insert the heat transmitting filter and notice that the vanes continue to turn although at a slightly slower rate than without the filter.

  • Carl Mungan, "Activating a Radiometer with a Hair Dryer", TPT, Vol. 53, #4, April 2015, p. 196.
  • V. I. Hlafkouski, A. I. Pinchuk, "A New Way to Demonstrate the Radiometer as a Heat Engine", TPT, Vol. 53, # 2, Feb. 2015, p. 109.
  • "Figuring Physics", TPT, Vol. 40, # 4, Apr. 2002, p. 252.
  • "Figuring Physics", TPT, Vol. 34, # 9, Dec. 1996, p. 569.
  • Harry Manos, "Demonstrating Heat Absorption", TPT, Vol. 24, # 8, Nov. 1986, p. 487.
  • H- 3b:  Wallace A. Hilton, "Radiation", Physics Demonstration Experiments.
  • Janice VanCleave, "5, Coming Through", Engineering For Every Kid: Easy Activities That Make Learning Science Fun", p. 35.
  • Yaakov Kraftmakher, "7.11, Detection of Thermal Radiation", Experiments and Demonstrations in Physics, ISBN 981-256-602-3, p. 349.
  • "Infrared Demonstration Set: A Striking Demonstration of Selective Transmission", Sargent-Welch Catalog, 2003.
  • Curt Suplee, "How heat gets around", Everyday Science Explained, National Geographic, p. 68.
  • Curt Suplee, "Of sunlight and Earth", Everyday Science Explained, National Geographic, p. 106.
  • Julius Sumner Miller, Q109 & A109, Millergrams I – Some Enchanting Questions for Enquiring Minds, p. 70 & 116.
  • Julius Sumner Miller, Q124 & A124, Millergrams II – Some More Enchanting Questions for Enquiring Minds, p. 17 & 80.

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