Re: [Phys-L] physics and potatoes

[Bernard Cleyet <bernard@cleyet.org>]

On 2015, Jan 12, , at 20:06, Folkerts, Timothy J <FolkertsT@bartonccc.edu> wrote:

> Sure, I think sol  A sheet of metal (low emissivity) formed into a cavity acts like a cavity radiator (high emissivity).  To the extend that the rough side has crevasses/pocks/indents and the shiny side does not, the rough side will be more like a cavity radiator and hence a higher emissivity.  Lots of sources say surface texture matters for emissivity.

Certainly, but the cavities must be large WRT the wavelength, no?

IR peak for 320 C ~ 450 F  is ~ 6 micron.  All but ~ 25% (energy) is longer wavelength.  (GE rad. calculator)  
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The reflectivity of bright aluminium foil is 88% while dull embossed foil is about 80%.[6]  

http://en.wikipedia.org/wiki/Aluminium_foil  
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I suspect the “roughness” is too fine for the 6 + micron to "notice it”.  Notice visible doesn’t notice it much.  ( I assume the 88-80% is for the visible  ~ 1/2 micron.)  


More comment forthcoming.  

bc   never worried about roughness w/ X-band demonstrations, EXCEPT FTIR.