PX284 - T5 - principal rays
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useful to classify lenses and make diagrams
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considering three cases for converging and diverging lenses
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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
