$$
\newcommand{\partd}[2]{\frac{\partial #1}{\partial #2}}
\newcommand{\partdd}[2]{\frac{\partial^{2} #1}{\partial {#2}^{2}}}
\newcommand{\ddt}{\frac{d}{dt}}
\newcommand{\Int}{\int\limits}
\newcommand{\D}{\displaystyle}
\newcommand{\dA}{\; \mbox{dA}}
\newcommand{\dz}{\; \mbox{dz}}
$$
Wave travel and reflection