The idea here is clear: any concept is better explained with the feature of interest to look at and to touch.
You can basically use anything you can get your hands on: merchandising articles, broken equipment, special presentation equipment from the companies, ...
In our case we use for example:
Show them the intact side first and let them guess of how many lenses such an objective consist. This usually puts quite some emphasize on the sentence "Handle your objective with care"
How fast does light really travel? May give a better understanding of the distance that (2photon laser-) light pulses really travel.