When we enter from sunlight or any where there is a presence of light to a dark room then we can't see things for a while. This is because when we there is a presence of light then our iris makes our pupil smaller so that less light can enter our eyes and when we enter into a dark room then less light enters our eye and we can't see the things. It takes some time by our iris to make pupil larger so that more light can enter our eyes and we can see the things.