Daniel65 wrote:
R Kym Horsell wrote on 18/2/21 6:04 pm:
Daniel65 wrote:
R Kym Horsell wrote on 17/2/21 11:37 pm:
In alt.astronomy Andrew W wrote:
...
Bragalia said it was a "stunning admission" from the US government and the
documents reveal that some of the retrieved debris possesses "extraordinary
capabilities" including the potential to make things invisible or even slow
down the speed of light.
It's disheartening to hear this last bit.
Many materials "slow down the speed of light". E.g. glass. The "refractive
index" of various glases on which optical instruments depend is the ratio of
the speed of light in the material to the speed of light in air.
Slowing down the speed of light in a vaccum would be the trick.
.... whereas slowing down the speed of light with a sheet of Lead is
quite easily done!!
What is lead's refractive index?
Umm!! At a guess, I'd say "Infinity" or very, very, close!!

They dope glass with lead to increase its refractive index (usually
written as symbol "n").
Making compound lenses for optical instruments from flint and lead glass
was a bit of a dark art when I was in school.
I presume these days it's all about selecting the right sensor
from an online catalog.
Scientists measured the refractive index of lead around 2.
I.e. the speed of light inside the metal is around 50% of the vaccum speed.

The "opacity" (usu written as symbol "k") of lead which I think you're
thinking of is a different but related issue.

--
Refractive index of METALS - lead
Refractive index n = 1.9076.
-- Bing