[ATM] Card in the foucault test

["David Williams" <dwilliams1128@earthlink.net>]

Hello

I have taken the lists advice about using the card to cut the cone of light returning from the mirror in the foucault test.  The only thing is, that instead of using a card or playing card, I'm using a piece of paper like a piece of dot matrix paper.  This has produced an interesting effect.  I can see the cone of light to a pin hole like area if I hold it right.  And while I have the paper at this area and the knife inside the ROC and move the knife to the left, the image on the paper moves to the right.  Does this mean that the knife is out side the ROC or the paper is outside the ROC?  Also when I move the paper back to get the image to where the it's not a point anymore, the image looks like a donut like this:

                     _______
                  /              \
                /   /              \ 
             /    /                    \ 
            |    |                        | 
            |    |                         | 
             \   \                         /
                \  \                      / 
                    \  \ _____  _  /
                         



So it looks like a cresant moon on the left hand side.  Does any one know why it's doing this?

Thanks
David







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