Note on page 938 in HRW a description of three principal rays for lenses.
The file lens_rays.pdf in this directory contains two diagrams:
a converging lens (f=+5) with object and a diverging lens (f=-5) with object.
Print out this file and then accurately (using a ruler) draw in the
three principal rays for each case.  Using the principal
rays determine the image size and location.  Draw in this image.
Calculate the image location and size.  Does the calculated image
location and size match the results of the principal rays?
Turn in your hardcopy showing this work.

Work the following problems from HRW.  For each single lens problem sketch
the lens/object/image and include at least two principal rays.
These sketchs, while not as exacting as those for lens_rays.pdf,
should display the the general topology of the rays.

70,72 (draw ray diagrams for both); 80,82,84