>...
>
> Hi all,
> ...the ability to resolve an object which subtends one minute of arc at
> the eye. Is that not right?
Yes but it depends on kind of fearture you are looking at. separating
two adjacent dots or two adjacent lines yeils slightly different
resolution limits.
>
> If so, then how is it that I can easily see, with my unaided eyes, a
> single black human hair lying on the off-white floor of my bathroom
> from a distance of 2 metres or more?
Non contradiction since you are addressing two different physical problems.
The first problem is : when do we merge two separate wires or spots a
single spot ? The answer is : as long as the centre-to-centre distance
is greater than about 1 minute of arc.
+-------------------------------------------------------------------------+
| |
| |< 1 minute>| |
| * * |
| ** ** ** ** |
| * | * * | * |
| * | ***** | * |
| * | | * |
| * | | * |
| * | | * |
| * | | * |
| * | | * |
| * | | * |
| * | | * |
| * | | * |
| * | | * |
| * | | * |
| * | | * |
| * | | * |
| ******* * | | * ******* |
***********-----*****----------|-----------|----------*****-----***********
The second problem is : given a single object with a very high
contrast, when do I stop to perceive that there is actually one object
to see ? The answer is : you can perceive a single wire on a bright
sky or a single hair on a white floor even if the objet as a diameter
smaller than one minute of arc. Whta you see is a diffraction spot of
about 1 minute width, you'll perceive is if there is a sufficient
signal to noise ration between the object and the background.
+-------------------------------------------------------------------------+
| |
| |
| |
| |
| |
| |
| |
| |
| |
| |
| |
| |
| Any object smaller than one minute of arc is imaged as a small |
| diffraction spot of about one minute of arc |
| this spot is visible is the contrast vs. background is good. |
| |
| ******* |
| *** | *** |
| *** | *** |
*****************************--------|--------*****************************
--
Emmanuel BIGLER
<bigler@xxxxxxxx>
