Combatants often try to escape their foes’ notice by hiding, casting the invisibility spell, or lurking in darkness. When you attack a target that you can’t see, you have disadvantage on the attack roll. This is true whether you’re guessing the target’s location or you’re targeting a creature you can hear but not see. If the target isn’t in the location you targeted, you automatically miss, but the DM typically just says that the attack missed, not whether you guessed the target’s location correctly. When a creature can’t see you, you have advantage on attack rolls against it. If you are hidden—both unseen and unheard—when you make an attack, you give away your location when the attack hits or misses.
Unless something is hidden, it gives away its position, so at least you know what square its in, that why they have invisible minifigures. You still attack at dis-advantage, but at least you can track it until it becomes hidden again, usually by the Hide action. This is what makes rogues so dangerous, they can hide as a bonus action if the conditions are right and then attack with advantage much of the time.
When something is hidden, you have to take it off the battle mat and then PC have to guess what square it is in. When that creature attacks it goes back on the board as it gives it position away until it does something to hide again. An object usually doesn't give itself away and cant take the hide action, in a dark room you just cant see it, but if it make noise it gives always it location like a clock ticking on a wall.
The earlier example of allowing a ranged attack in the dark at a target 300' away that you cant see or hear would be determined by whether you can hear something that far away to target it, otherwise you just have to guess where they are. It also would not give away its location if you cant see or hear that far away either, you just have to guess the square. PC need to do this also against some foes, the elf archer at distance wouldn't give his position away
This is what makes Faerie fire good at fighting invisible creatures for a low level spell, its everything in a 20' cube gets outlined, so once the invisible people attack and give their position away you can cast it to keep them lit up. You can also cast it blind to see what's there. This is also great against the players, its a verbal component only so it can be cast against the players so the enemies can see them and can sometimes be done without giving away your position or where the PCs cant do anything about the caster. Its a great way "pump up" creatures that normally wouldn't be much of a challenge into a much more difficult encounter since the PC will be giving advantage if they fail.
A good way to teach new players about these things is using the Faerie fire spell on them so they will then learn to use it or something like it on others. Also I always use Phase Spiders so new players can get used to using readied actions for monsters appearing out of nowhere. Phase spiders are also intelligent enough to target light sources first and to use their dark vision, but not to tough to take out once you know what to do.
I have always been of a mind that invisible creatures will cast a shadow, like The Shadow, so a bullseye lantern will show them. That's somewhat supported by invisible creatures under water, but I don't think it is anymore. You used to be able to cast invisibility on your self and then put on a suit of armor for example and try to scare people like the headless horseman, I don't think you can do that either. Cant cast Silence on a coin and then throw it into the middle of spellcasters either, it appears that got taken away also. Good times and inventive plays though.
Really you just need to care about light and vision enough to get the mood across, let the players get their stuff in that depends on it, and every now and then throw in an encounter where its a big deal.
Not doing good today, good thing it's a Friday. It is Friday, right?
