When a person walking in bright Sunlight enters a dark room, he is not able to see clearly for a little while because

When moving from bright sunlight to a dark room, the photopigments in the retina have been partially bleached by the intense light and need time to regenerate. This photochemical adaptation (dark adaptation) is the primary reason for temporary vision loss. While pupil dilation also occurs, it happens relatively quickly and contributes less to the adaptation process.