From: henry@spsystems.net   
      
   In article <031107000104234.07Nov03$rookswood@suburbian.com>,   
   John Savage   wrote:   
   >>Also - those X-rays can't reach the ground - Earth's atmosphere is   
   >>equivalent of roughly 10 meters of water...   
   >   
   >Curious. So if I sunbaked on the bottom of a 1 metre deep swimming pool,   
   >(with a suitable snorkle) I should expect to get sunburnt almost as   
   >severely as when lying on the deck (if we can disregard for the moment   
   >the not insignificant reflection at the air-water interface)?   
      
   No, these are two different issues.   
      
   To stop X-rays, you basically need mass.  Most any kind of mass is as good   
   as any other kind; essentially nothing is transparent to X-rays.  So that   
   extra 1m of water does not add very much more shielding against X-rays.   
      
   But solar X-rays aren't what give you sunburn.  Sunburn comes from   
   short-wavelength ultraviolet.   
      
   Stopping short UV is a different problem.  Things that are opaque to it   
   are very very opaque to it; you need only quite a small thickness of such   
   a material.  Normal air is quite transparent to it, so almost all of that   
   10t/m^2 of air makes no useful contribution here.  Ozone is quite opaque   
   to it, so the traces of ozone found in the lower stratosphere cut out a   
   lot of it, even though they are a negligible fraction of the atmosphere's   
   mass.  Small thicknesses of water, glass, or even light clothing will   
   block the remainder completely.   
   --   
   MOST launched 30 June; first light, 29 July; 5arcsec    | Henry Spencer   
   pointing, 10 Sept; first science, early Oct; all well.  | henry@spsystems.net   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)