GOOD Point by my wife! Why can’t we have a blower on a belt to accomplish most of the sorting? Plastics blow one way and glass stays on the belt. Throw in a magnet and make eddy currents to trap metals.
When I was a child I saw one system that expected all aluminum that just used an electromagnet to lift any steel out of the stream. That was quite a ways up in the air so I suspect the owner would have gotten pretty pissed if we threw a bunch of nails or soup cans in the bag for fun. But most of the other ones I've seen have two conveyors with a gap, and they use an AC magnet to fling the aluminum onto the second conveyor (via eddy currents). Most of the ferrous stuff just drops into the first bin, or funnels off to be sorted again.
For your plastic or glass suggestion, that may take breaking it up first, since large items might not lift or go in weird directions. And then some plastics are pretty dense. I have the vaguest notion that I might have seen a system that sorts plastic from glass while washing it, since the glass sinks and some plastics don't, but that could have been a prototype or a misfiring neuron for all I can remember.