Dear student,
Wien's Law, sometimes called Wien's Displacement Law, is a law that determines at what wavelength the intensity of radiation emitted from a blackbody reaches its maximum point. After this point, the intensity decreases as temperature increases. This creates the characteristic shape of blackbody radiation curves.
The
displacement in Wien's law refers to the way that the position of the peak can be displaced with a change in temperature. Qualitatively this property can be seen quite simply. As an object is heated, it goes from red-hot to white-hot. This shows that as the temperature is increased, the wavelength that is emitted with the greatest intensity becomes smaller. The wavelength will decrease linearly as the temperature is increased Thus, Wien's Law helps to illustrate the connection between colour and temperature. Note that Wien's Law only helps to determine at what wavelength the radiation of a blackbody peaks at.
Wien's black body and Ferry's black body are considered to be almost black. A fine hole in a double walled spherical cavity, evacuated and painted black, represents Ferry's black body.
The Ferry black body is designed so no ray coming in from the outside can get out again - so if you have a reflection of eg sunlight you don't get any sunlight out.
The inside surface should be black, although if you have enough reflections, so that all of any incident light is absorbed the spectrum will be blackbody whatever the surface. Soot works well because the microscopic structure has lots of small cavities that absorb, re-emit and scatter light.
The dual wall is so that the inside is at a uniform temperature even if different parts of the outside are seeing different sources of heat
Regards
