[quote]I examine the physlet applet, it seems to have a good example of a thickness of lens changing with focus length f.
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Could you provide the URL of the examples ?

I think if you use thin lens approximation in the simulation, the thickness of the lens is only visual effect.

If you want to simulate effect due to thick lens, you need to use Snell's Law to calculate all the refractions occurs at the boundary of lens. You are welcomed to check out [url=http://www.phy.ntnu.edu.tw/ntnujava/index.php?topic=46.0]Thick Lens[/url]

If you want to show how the thickness of a lens affect the focus, that is another story!

Many drawing in Physlet optical bench is only an approximation.
Please check out the attached image. It assumed thin mirror and thin lens case.
The space in the simulation is not the same as real world.
The mirror and lens will be almost like a thin/short line if you want to draw it according to real dimension.