I think to be one hundred percent accurate I should say that the prism
doesn't require a silver mirror coating to work. It works on the principle
of total internal reflection; ie when rays of light meet a glass to air
interface at an angle greater than the critical angle for that particular
glass (related to the refractive index) they reflect totally rather than
refract through the interface. Prisms are the most efficient mirrors as
they reflect 1000f the incident light unlike coated surfaces. They also
don't accumulate dust etc like a mirror would. This is also how Porro and
Dach (roof) prisms in binoculars work and why they are used rather than
mirrors.
The problem described that occurs with the foam and prism is that the foam
rots and becomes bound to the prism surface. When stuck on the glass it
alters the glass to air interface that is required for the total internal
reflection to occur. What you see instead is the foam through the glass.
Thats why cleaning the prism of the gunk and restoring the original glass
air interface will restore things.
Just a technical thing....
Oben
At 10:47 PM Thursday 19/07/2001, you wrote:
That's the prism coating being eaten away by foam that is pressed against it
under the top cover. For best results, replace pri$m. Otherwise "most" of
the black stain can be cleaned off.
John Hermanson
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