Is it normal to still see what's on the screen when it's fully dimmed? I'm using an external monitor, so most of the time I've got the MacBook Air's internal screen dimmed to 0. But it doesn't switch off completely at 0 brightness as I can still see some graphical elements. They're hardly discernible but still there.

I was wondering if this is normal.

Unfortunately I couldn't find any other way to disable the built-in screen.
If you use only the second screen, you can safely close the lid of your MacBook Air and work only on the second screen, it will automatically switch it as the main and only screen.