Why are we not able to see immediate after we enter a dark hall from a lighter place.

Technically this is a Biology question;

The 'amount' we can see depends on how much light can get through our pupil to hit our retina.
When there is a lot of light the pupil is small; it doesn't need to be big to let a lot of light in.
When we move to a dark space there is much less light, so the pupil 'dilates' to let enough light so we can see properly.
The period in which one cant see is simply when the pupil hasn't had time to change shape yet so doesn't let in enough light.

You can find results and justify them? you can test your hypothesis

Because the sun is the biggest and if the sun orbits the earth we will be dead probably