I wrote:
<<If you are going to allow your shadow to pass through two panes of glass,
will refraction start to become important?>>
Fer replied:
<<It is significant>>
I suspected as much, but my maths is not good enough to do the calculations.
I had an idea some time ago to use double glazing as a sundial.
I made a model with two panes of glass mounted in a frame with the marker (two
opposed triangles) and lines both on the INSIDES of both panes of glass.
That one worked OK, but when I put the marker on the outside of the outer glass
and the lines on the inside of the inner glass It didn’t work at all well.
I put the problem down to:
a) the two panes of glass were too close together and
b) refraction shifted the shadow.
BTW, I marked the glass with a spray can of Plasti-Kote 180 Glass Frosting
Spray Paint. You could use this on the outside as it is weatherproof.
I masked off the area and lines with Scotch magic tape cut very thinly with a
craft knife and used Letraset as masks for the numbers. It worked very well.
Photo available on request.
Mike Shaw
53º 22'N 03º02'W
www.wiz.to/sundials
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