Wien's Law Of Thermal Radiation
To show that hotter blackbodies radiate with shorter wavelength radiation.
Variac, light bulb and filter assembly with two clear 150 Watt light bulbs, red, and blue filters.
A variac is connected to two identical light bulbs in parallel, which are viewed behind red and blue filters respectively. As the voltage is increased by the variac, the lights glow more brightly, and more light is seen through the blue filter relative to that of the red filter. Very little or no blue is seen at low voltages, whereas red is seen to be emitted even at very low voltages.
H. Richard Crane, The cold night sky, TPT 22, 284 (1984). W. G. Rees and C. Viney, On cooling tea and coffee, AJP 56, 434-437 (1988).