Submarines using red lights
 

So a friend of mine asked me this today and I didn't have a good answer.. I'm sure one day I will.

Why do submarines use red lights?

I understand surface ships use it for better night vision, and because it makes it much harder for enemy ships to spot the lights in the ship...

But submarines do not have windows. The light wouldn't really be escaping the sub..

It's in case the lights go out for whatever reason, you're eyes don't have to adjust to the dark.