PX284 - T5 - principal rays

useful to classify lenses and make diagrams
considering three cases for converging and diverging lenses
parallel rays (from infinity)
- converging: converge at focus
- diverging: diverge, seems to come from a point source at focus

intersecting at the centre of the lens
- converging & diverging: no change

exit as parallel rays
- converging: coming from a point source at focus
- diverging: coming from a source that would converge at the focus