Most simulatuions only show the ray tracing of light paths for converging lens and indicate where the real/virtual image will be formed.
If a screen is located at the real image focus point, image will be seen.

If a screen is not located at the real image focus point:
if it is close to the real image focus point, it will be a suposition of many images out of focus, and the intensity is also smaller.
if it is too far away, no image will be formed.
Lookang was looking for ways to let image out of focus, and he is still working on it.
Please check out [url=http://www.phy.ntnu.edu.tw/ntnujava/index.php?topic=1391.msg5263#msg5263]Re: how to model intensity and blurring of image in Ejs ?[/url]

I would say the best way is to show a real experiment in front of the students, instead of a simulation.
May be what you need is an animation (not a real simulation) to illustrate the effect.

I will try to make one such simulation if I can figure out a way to present it.
Do you have any suggestion?