This applet traces paraxial light rays through a user-specified system of up to 12 ideal thin lenses and apertures, allowing many simple optical instruments to be modelled. Three examples are given: coupling of light from one instrument to another, a Newtonian telescope, and a projector.

1. Coupling: The light collection properties of any light source or instrument can be parameterized in terms of two apertures, but coupling the light from a source to an instrument can be difficult, especially when constraints are added such as the availability of lenses. The applet allows rapid changing of elements in order to find the optimum solution.
2. Telescope: By varing the angular size of the object effects such as the position and role of the exit pupil can be understood. Try adding a field lens to improve the field of view, or add a field stop to limit the field of view.
3. Projector: Here the problem is to fully illuminate an object (simulated as a aperture) from a diffuse light source, then project it onto a screen. The light source is imaged onto the projector lens, while the object is imaged onto the screen.

The user can modify the optical system as desired by changes to the three parameters of each element:

1. the focal length (APER denotes an aperture)
2. the diameter
3. the distance from the previous element

As changes are made, new ray traces are performed by the applet. Elements may be deleted or inserted by entering the appropriate element number in the delete or insert boxes at the bottom of the screen. A lens may be changed to an aperture by deleting its focal length field. If the first element is given a negative distance, it is not shown on the ray trace (see the telescope example). Two sets of rays can be displayed: the extreme rays defined by the diameters of the first two elements, or a bundle of rays coming from a single point on the object.

Thad G. Walker, University of Wisconsin-Madison, January 1998