From: luweitest@gmail.com   
      
   On 2018-5-27 2:05, Eric wrote:   
   > Dear all,   
   >   
   > I have quite a naive question but I can't manage to really figure the   
   > explanation and even if my assumption is true.   
   >   
   > let's suppose you have an object, a lens and the object image ; with   
   > the object at the exact same location if you turn the lens backwards   
   > so so frontal face is now the back, will the image be the same as   
   > before ?   
   >   
      
   Not in the general case.   
      
   > I presume that for a thin simple lens this is true, but I also guess   
   > that it's not true for more complex optical systems which do not   
   > behave the same if you return them backwards.   
   >   
   You are right. For complex "thick" systems there is a only special point   
   (maybe called origin point) by which you can turn the system backwards   
   and the image remains at the same place.   
      
   > But I also think that the reason why it is so is due to reversibility   
   > of light, so that I'm confused because thsi principle should work in   
   > both situations wether with a simple thin lens or with a complex   
   > systems with multiple elements (like a microscope or a telescope).   
   >   
   No, the reason is the symmetry of imaging formula:   
   1/od + 1/id = 1/f   
   in which object distance and image distance is exchangeable -- if turned   
   at the origin point.   
      
   --   
   Regards,   
   Lu Wei   
   PGP key ID: 0xA12FEF7592CCE1EA   
      
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