What happens to the eye when you enter a darkened cinema hall from bright sunshine? Give reason for your answer.

Initially, we are unable to see anything well as we go into a darkened cinema hall from broad sunshine. But rapidly, our vision gets better. The iris ensures that the pupil is small because outside the cinema hall, the intensity of light entering the eye is very high. But as soon as we walk into the theatre, the pupil opens up, allowing more light to enter and making everything visible.