Bat eyes have night vision just about as good as you do – that is to say, not that great compared to something like a cat. However, a lot of bats make up for it by being able to echolocate. This means they send out really high pitched sounds from their mouths and noses that bounce off of objects around them, and they listen to these bounces to figure out where things are and what they are. It works a lot like sonar and radar for submarines or airplanes.
Jeff has done a great job explaining the *how* bats see (and “see”) in the dark, so I’ll add a little on the *why*. Bats are nocturnal, which means that they are most active at night (the reverse of us). They live in caves and other dark shelters so that they’re safe from diurnal (daylight active) animals like us and other predators.
Inside their cave homes, bats often “tune out” their echolocation bc it can get very confusing when tons of bats are doing this at once, and also when the chambers are very large. This is one reason they often smack into cavers that turn up unannounced–it’s like running into a piece of furniture that’s been moved in your house. It’s times like this that bats also are able to use other sensory cues like very highly developed sense of touch on their wings to tell where they’re going in the dark!
Bats aren’t blind like moles or like other cave creatures because they do venture outside their caves (and other roosts) usually starting at dusk, so they do use their vision. It depends on the kind of food the bat eats to determine how well he sees or perceives things in dim light or the dark. Some bats eat fruit or feed on the blood of cattle, neither of which is that hard to find once you’re in the right area. However, those bats that feed on insects or small animals really have their work cut out for them since those things are small AND moving pretty fast! Echolocation is very good for locating these guys!
So, the *why* is that bats are nocturnal and need to be able to both find dinner and evade predators so that they don’t become someone else’s supper!