Lines per inch, dots per inch (or pixels per inch) and physical image size are interrelated. The three concepts are so closely connected that they seem to conspire against students attempting to master them.
After a long, and potentially boring, lecture about the three attributes, the sophomore students seem to be grasping them.
The visual aids used to teach the concept hark back to days of light sensitive paper and stinky chemicals. An old PMT, or photomechanical transfer (the car image) and a mylar PMT screen were used.