When we enter from bright sunlight to a dark room, we cannot see initially, why?

There is a slight correction in my answer. Please refer to this one: 

When we are in bright sunlight, the iris of our eyes adjust the pupil in such a way that less light enter our eyes (i.e. the pupil shrinks). But, when we enter a dark room, the iris needs to adjust the pupil in such a way that more light enter our eyes (i.e. the pupil needs to be enlarged). This is because we cannot see anything in a dark room if less light enter our eyes. The adjustment of pupil takes time. So, when we enter from bright sunlight to a dark room, we cannot see initially.