the invisible player

[starless]

http://crawl.chaosforge.org/Invisible

An invisible player gets the following bonuses and penalties:
-5 to their melee to-hit number if you yourself cannot see invisible. (Official justification: you can't see your weapon.)

I didn't know about this provision. Which got me thinking that the official justification might be more accurate like the following:
By being invisible you'd be unable to see anything, since your retina would be invisible(transparent) as well.

[IronJelly]

YES! Playable Player Ghosts! This needs to happen!

Well, player ghosts aren't invisible, but a Phantom species (abbreviated as Ph) totally could be.

Racial features:
1) Permanent Invisibility (as the spell, so things that end it or reveal you would).
- the original proposal
2) You hunger for the essence of recently dead things, and gain sustinence (slowly) by hanging around very fresh corpses.
- adds some danger, since you may need to eat vs running from something and losing the life essence.
3) though invisible, and even having invisible equipment, a few things might make you obvious. if you pick up an item while in LOS of anything, it sees the item vanish, and is alerted.
- makes sense, and is not something we already have. Tactically interesting decision to pick up an item and run, or come back for it.
4) Always have the shout mutation at one level, re-themed as an eerie wail
- because otherwise permanent invis would be broken. This would randomly break it, most often if you just wait around, and occasionally in an already tense situation.
5) The usual undead suite of resists and god limitations.
6) Very slow learning. This is a ghost, after-all.
7) Because this is CYC, potions of invisibility should make you visible for the duration, letting you kite stuff.

Oh, and perhaps maybe you're visible in mirrors if they're ever added to the game.

[duvessa]

This is a common misconception. How invisibility actually works is that it slows down the frequency of your body. So the visible spectrum doesn't interact with your eyes anymore, but because your brain has a lower frequency, you also become able to see infrared light. The main reason your accuracy goes down is that it's really disorienting for the first few minutes. It's a sadly common prank to get trick someone into drinking a concentrated invisibility potion that gets them all the way down to radio waves, which makes you stumble around like a drunk and usually vomit.

[Azrael]

oh physics, my love
i'm so sorry you had to die this way

[sizz]

Who wants to deceive me again?

[ontoclasm]

All the monsters in the dungeon see purely in the infrared spectrum. Potions of "invisibility" just lower your body temperature, making it more difficult for them to sense you. Conversely, they can become transparent to the visible spectrum and still see you just fine, since that's not what their eyes detect.

SInv is just IR/NV.

[sizz]

All the monsters in the dungeon see purely in the infrared spectrum. Potions of "invisibility" just lower your body temperature, making it more difficult for them to sense you. Conversely, they can become transparent to the visible spectrum and still see you just fine, since that's not what their eyes detect.

SInv is just IR/NV.

Why my troll ally did not attack an invisible lich? ;<

[MDvedh]

Isn't invisibility a hex which fogs others' (and own) mind so their brain just ignores you.

If you close your left eye you will be able to see your nose with the right one and vice-versa, but if you open both of your eyes your brain will just ignore nose.

And SInv just adjust your brain not to ignore anything. Including your nose. So any nosed species should get minor accuracy malus while having SInv because their nose is really annoying.

[MyOtheHedgeFox]

In the story about The Invisible Man by H.G. Wells, there was a mention that almost everything except a few pigment spots disappeared, including the small pigment spots on the back of the retina that make you see.
You are not entirely invisible, and that naga might hit you well just because you refused to cut away that pigment spot on your left thigh.

Your question is not the first one: our famous science popularizer, Yakov Perelman, once also stated (albeit mistakingly) that The Invisible Man could not see. Afterwards, Mr. Perelman himself acknowledged this mistake and published an official apology in the next version of his book.

The book in question is Physics For Entertainment, Book II. I think Mr. Randall Munroe ^ XKCD mentioned finding Book I in English, and also said that Book II is one of his favourite educational books on physics.