Heron's Principle of Reflection/Examples/Journey to the River
Example of Use of Heron's Principle of Reflection
However, first she wants to take a drink of water from a river.
She wants to walk as short a distance as possible.
To what point on the riverbank should she walk?
The point $P$ is that point on the river bank directly in line with $S$ and $T'$.
Or you can reflect $S$ in the line taken by the river bank to get point $S'$.
Then $P$ is the point on the river bank directly in line with $S'$ and $T$.