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Re: Cat's eyes.



What is the evolutionary advantage of eyes with reflecting
retinas?

I asked our biology teacher and he said it increases visual
sensitivity of nocturnal creatures (cats, rabbits, dear, crocodiles).
Apparently there is reflective layer made from crystals called
"tapetum lucidum" behind the retina. Light which is not
absorbed in photosensitive cells of retina is reflected back
and has "a second chance" to activate cells.

A transparent sphere whose back has a mirror does behave
as a multidirectional retroreflector, provided the incoming
light is focused on the back layer. This is a good model for
what we were discussing, I think. The cats eyes must be
focused on where the light is being scattered from, focusing
on the infinity, for example, may take the retroreflection
away. That is my speculation.

By the way the so-called "reflective tape" I bought in the
auto supply story was not a retroreflector, it scattered light
in all directions. But a "reflecting strip" on my vest does
behave (approximately) as a true retrorelector. Thus I see
the "strong shining" only when light is coming to it from
the direction which is nearly the same (plus or minus
about ten degrees) as the direction of my looking. Light
scattered near my eyes, as opposed to that scattered
from my legs or chest, is mostly responsible for the
strong shining. Both the reflecting strip and the cat's
eyes behave the same way. That does not mean
mechanisms of retrorelection are identical.
Ludwik Kowalski