The Eye As A Lens (Part 1)

Sources of light very far away (at infinity) emit rays that are (and stay) parallel to each other.
Shown are two separate sources at infinity, that emit rays forming an angle between them.
This angle is the angular separation between the two sources.
The observing eye is "relaxed", i.e. focused at infinity.

  • Drag the sources up and down along the vertical constraining line.
  • Note that each set of rays is focused on the retina at the same spot regardless of elevation.
  • Conversely, if the eye changes elevation, the focused image on the retina will not change.

    How "big" a distant object appears to the eye depends on the angular separation only.
    (A mountain appears just as big from the fifth floor of a building as it does from the first floor.)

    Credits
    Authored by Kevork Spartalian, University of Vermont, based on Davidson College physlets.