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**"sinh" in Snell's Law?**

## Homework Statement

I'm trying to understand some equations in my AS level textbook about Snell's Law

## Homework Equations

The equations are in the image below

## The Attempt at a Solution

I tried reading up about sinh but I don't fully understand what it is. Also I'm not sure what the term beside sinh is. It can't be a "1" and to seems like a small letter L which comes out of nowhere?