Yes, the light area is air sac. It gets bigger over time, whether the egg develops or not.
The dark area is everything else-- white, yolk, and (maybe) chick.
Try candling the egg somewhere very dark (like inside a windowless room, with the lights off and the door closed.)
https://www.backyardchickens.com/ar...ion-through-incubation-of-chicken-eggs.47879/
^Here's an article with a lot of photos of eggs being candled.
They are labeled by how many days of development.
At first, there is nothing to be seen in the egg (except the yolk, which looks like a big dark shadowy thing).
But then as they develop, you start to see veins, and one stage looks rather like a big spider sitting in there, and then the embryo gets bigger (etc, etc)
If you look at those pictures, and try candling your eggs in a very dark place, you might be able to tell better what you are seeing. For me, it's usually clearer when I look at the egg in person, and less clear if I try to take photos-- so you might be able to see more details in person than what we can see in photos you take.