I can't believe my eyes - alternative solution

As you were able to find out from the hint "Properties Of Lenses", we only get an image behind the lens if it is a lens with a thicker centre and thinner edges - a converging lens. The rays are converged behind the lens which results in the image. So if you or Michael shouldn't be able to see directly where a pair of glasses are thinner and where they are thicker, you or Michael could also find this out with the help of a little experiment. The following arrangement would be one possible way:

Naturally the distance between candle, lens and screen have to be varied. If you or Michael should be able to see a clear image of the flame on the screen, then it's clear that it's a converging lens. Now let's assume Michael really manages exactly this with the glasses he had found: