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George,

You wrote, much as you have before:

> You can demonstrate the effect for yourself, rather convincingly. Hold your
> finger and thumb up a few inches in front of an eye (the distance isn't
> critical), such that there's a bright diffuse background behind: a white
> cloud, a lit lampshade, even a bright computer screen. Now bring finger and
> thumb together. Just as they meet, you see a dark shadow jump across the
> narrow gap between them. Part them, ever so slightly, and that shadow
> suddenly vanishes. I haven't met anyone who is immune to this effect. It
> surprises all who notice it, and they find it hard to explain.
>
> What's happening, it seems to me, is this. As long as there is the
> slightest sliver of light, illuminating the narrow isthmus between finger
> and thumb, the effect of irradiation makes it look wider than it actually
> is (by an arc-minute or so, perhaps). Only when the gap closes completely,
> so there's no light shining through that isthmus at all, does that bright
> sliver disappear. That's why it appears to vanish so suddenly. Try it for
> yourself.

The problem with that physical model is that it involves two dark areas,
with some light between, not one boundary between one dark area and one
light one. Perhaps in consequence, I see (or think I see) the shadow
jump between my fingers before they touch (or before I can feel them
touch, which may or may not require more-than-minimal contact in order
to activate the touch receptors in my skin). I also think that I can see
  diffraction patterns in the gap between my fingers before the shadow
jumps across -- which may simply be my defective eyesight but wouldn't
be too surprising if real. Shouldn't there be diffraction as well as
irradiation, since the two-fingers model involves a bright slot, not
simply a boundary between light and dark areas?

The better demonstration of irradiation, for members of this list, might
be to measure the semi-diameter of the Sun, under various conditions and
with various filters and/or magnifications. Comparison with the
tabulated value in the Almanac should show whether irradiation is
significant or not, for the individual observer, with his or her
instrument and with a realistic selection of filters.


Trevor Kenchington


--
Trevor J. Kenchington PhD                         Gadus@iStar.ca
Gadus Associates,                                 Office(902) 889-9250
R.R.#1, Musquodoboit Harbour,                     Fax   (902) 889-9251
Nova Scotia  B0J 2L0, CANADA                      Home  (902) 889-3555

                     Science Serving the Fisheries
                      http://home.istar.ca/~gadus

   

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