The same (see critical angle picture) is when You are underwater and trying to see the light. Even in a sunny day You can see a circle of light (deeper You are, smaller it is). And this circle is surrounded by dark space - strange isn't it?

But all You have to do is to remember that if the light comes to You with the angle higher than critical, those light is mirrored, and from this direction You have no light - "no signal" for Your eyes, which is dark space.