Walt Their net effect on the game would be similar - you could focus multiple ion beams into a small space. However, I believe mirrors+lenses would leave more room for creativity, design, and unusual applications than prisms do. Prisms (as far as I understand it), would be a block that you shoot 2 ion beams into. The block combines the ion beams into a single more powerful one and shoots it back out. It only has one real function and only one main way to use the prism. It's also very simple to use - the same basic unit can be duplicated for every ship.
Mirrors and lenses could be used for the same thing - focusing multiple ion beams into a small area. But there's a lot more mirrors and lenses could be used for as well. You could bounce ion beams at unusual angles around the edges of your ship, or use self-destruct to change where beams exit your ship mid-fight (hinge/piston synergy if those get added!). You could use lenses to create a pinpoint stream of small lasers, or use cleverly placed mirrors to focus a dozen ion beams onto a single focal point - better make sure your target isn't too close or too far from that point. You could cover the outside of your ship with mirrors to deflect ions and lasers back at your enemy (for a while, the mirrors would break eventually), or bounce your own ion beam off the enemy's internal ion mirrors to snipe otherwise well-protected reactors. Even if all you do with mirrors and lenses is combine ion beams like the prism on the trello (which would still take quite a bit of ship engineering), there's more design decisions than just aiming 2 ions at one prism.
In terms of the actual mechanics of mirrors and lenses, they would work like their real-world counterparts. Mirrors would reflect and lenses would refract based on the beam's incident angle. Ion beams would never actually physically combine, but they could be manipulated into overlapping so heavily that the effect would be similar. The physics couldn't be exactly like real life (real life lenses, for example, would be very difficult for players to work with), but they could be similar enough that everything is completely intuitive.
At least that's my take on it. It's possible that mirrors and lenses would just be too complicated for anyone to use effectively - and it's a spaceship building game after all, not an optics simulator. But overall I think that mirrors and lenses would be a good thing, and would be intuitive to understand but hard to perfect.
