Diffraction of Thermal Waves in Liquid Helium II

Diffraction effects in thermal radiation have been produced in liquid helium II by means of a thermal grating radiating a primary (zero-order) beam accompanied by higher order beams at appropriate Bragg angles. These results at once demonstrate unambiguously the true wave nature of this thermal wave propagation and its conformity to Huygens' principle in the most demanding test to which second sound has been subjected. The general nature of the angular pattern, including the fine structure, conforms to the requirements of diffraction behavior for more customary wave motions. A determination of wave velocity ($v_2$) for second sound is obtainable from the observed Bragg angles.