image credit: NASA/IPAC; image source; no larger image available

Cooling of the Human Body

These faces were photographed in infrared light.  
-- Notice how different parts of the face are brighter or dimmer, and compare the two noses.  
-- Infrared images are sensitive to the temperature of different parts of the face because the heat radiation emitted per second varies as the fourth power of the temperature, so a small difference in temperature makes a big difference in the brightness of the image.  
-- For more on this temperature dependence, see Hyperphysics' Cooling of the Human Body.  For related images, see Caltech's Our IR World Gallery

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Exploratorium Science Snacks: Hot Spot

In this Exploratorium activity, you focus a beam of invisible heat radiation on your skin with a large parabolic mirror and then use your skin as a radiation detector.  To learn more about this activity, visit Exploratorium Science Snacks: Hot Spot, and while you're there, notice the image of the radiant heater made by the mirror in visible light.

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image credit: Mark J. Harrison, Kansas State University; image source; larger image

Quartz Liner Tube Inside Tube Furnace

This image shows a quartz liner in an oven at 1150° C, and the orange glow is heat radiation.
-- This radiation is much brighter than that given off by a face (see Physics in Your World) and has its maximum brightness at a much higher frequency--in the visible spectrum rather than infrared.
-- For another example of a substance at 1100°-1200° C giving off heat radiation, see the San Diego State University page on basaltic lava.

Blackbody Radiation

"Blackbody radiation" is radiation emitted by a body that is in equilibrium with its own radiation.
-- 19th-century physics was only partially successful in explaining blackbody radiation, and quantum ideas were required to achieve full agreement between theory and experiment.  
-- For an introduction, see Blackbody Radiation, and for more on the physics, try the Hyperphysics page Blackbody Radiation.