We feel blinded for a short while entering a dark room when coming from bright light.

When we are standing in bright light the muscles of the iris contract and the pupils constrict, so that less light can pass into the eye. When we move into a dark room from bright light the eyes need time to adjust and the iris muscles need time to dilate the pupils so that more light can enter the eye. So, in that duration when the eye muscles are trying to adjust the eyes to the amount of light we feel blinded.