What I found out about index of refraction

Hi everyone, This is for newbies like me. I was checking out a
gemstone website and was trying to understand what is meant by the
“Index of Refraction” and found this:

(sine i)/(sine r) n

Where i is the angle of incidence and r is the angle of refraction
and n is the index of refraction. I was stumped as to what the
“index of incidence” and “index of refraction” meant. so I found
this website that helped me understand these terms and more. It’s
actually a highschool physics tutorial:

http://tinyurl.com/igzg

Most gemstone sites use the words like refraction and incidence but
don’t explain what the terms mean. I’ve found that a little “brush
up” on your physics helps when trying to understand gemstone
properties.

Dan T