Make your own free website on

Ray diagrams

Rules for constructing ray diagrams:

  1. a ray passing through the centre of the lens passes straight through without being refracted – do you know why?
  2. a ray parallel to the axis refracts through the focal point
  3. a ray passing through the focal point refracts parallel to the axis (this is easy to learn – it’s the opposite of 2)

Apply all three rules and where the rays join is the position of the top of the “Object” :





NB.  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.

Extension to these rules to allow understanding of refracting telescope:

1.       rays 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:


2.       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.