Typo and clarify that the negative lens figure is not a reflection.

opticsExercises.tex

@@ -331,14 +331,14 @@ \subsubsection{3. Virtual images}
 \label{fig:mirror}
 \end{figure}
 
-A virtual image is also formed by a negative (concanve) lens, as shown in Fig.~\ref{fig:neglens}. 
+A virtual image is also formed by a negative (concave) lens, as shown in Fig.~\ref{fig:neglens}. 
 Examine the ray diagram of the negative lens. 
 What do you predict you will see if you look through a negative lens?
 Verify this with the $f=-50~mm$ lens.
 \begin{figure}[h]
 \center
 \includegraphics[width=3.5in]{negative_lens.eps}
-\caption{Ray diagram of a negative lens.}
+\caption{Ray diagram of a negative lens. Note that the image is not a reflection, but a virtual image. }
 \label{fig:neglens}
 \end{figure}