all three rules and where the rays join is the position of the top of the
It’s usual to draw the lens as double headed arrow because before you
start drawing the construction rays you are not always sure how big the lens
needs to be. It looks weird if the
construction lines “miss” the lens.
to these rules to allow understanding of refracting telescope:
parallel to a ray through the centre of a lens will be focussed at the focal
plane of the lens. This is best
explained by looking the left hand bit of this diagram:
the opposite is true – rays coming from a focal plane will be refracted
back as parallel to each other again -
look at the right hand bit of the above diagram to see what I mean by that.