10. Black-body Radiation

This demo shows the shifting colors as a function of filament temperature. Also, this demo shows prism dispersion, radiation intensity as a function of wavelength, and infrared radiation. The black body radiation curve can be demonstrated by using a radiation sensor hooked to a digital millivoltmeter. The carbon disulfide prism (see Dispersion) is used to spread out the light of a slide projector lamp onto a screen . As you scan across the spectrum with the radiation sensor, the millivoltmeter shows the peak of the 3000 K tungsten filament in the infrared with the tails of the curve in the visible spectrum and further infrared.

The same demonstration can be done more qualitatively. Turn down the room lights and show the spectrum of "white" light on the wall. As you reduce the voltage to the lamp with the variac, the blue color dies away, and then the green, leaving only dull red of low intensity. (Of course, the 3000 K tungsten filament already peaks in the infrared so the initial "white" light is already quite red. Infrared itself can be demonstrated; see Infrared, Radiometer, and Maxwell's Spectrum)

The applet below shows the blackbody curve and colors corresponding to the given temperature.

 

Physlet by Wolfgang Christian

webPhysics, Davidson College

Instructions on how to use the animation above
  • Slide the bar appearing at the bottom of the animation in order to set the temperature. The value of the temperature, in units of Kelvin, is shown to the right of the sliding bar and in the border on the left side of the animation.
  • You may change the location of the maximum of the energy spectrum by dragging the "color spectrum", appearing below the curve, to the left or right.
  • The energy density (y axis) is given in units of Joules/meter3. The actual value of the energy density is obtained by multiplying the y coordinate by the power of ten appearing near the top of the y axis.
  • The length (x axis) is given in units of meters. The actual value of the length is obtained by multiplying the x coordinate by the power of ten appearing near the bottom (and to the right) of the x axis.
  • The actual color seen by an observer looking at the thermal radiator is displayed in the disc appearing above the word Appearance in the border on the left side of the animation.