What makes a human dazzled in the eye after being in a dark room and suddenly exposed to light?

The pupil of a human eye opens (gets large) and constricts (gets small) in response to the available light. When it is very bright, the pupil constricts to reduce the amount of light entering the eye. When it is dark, the pupil expands to capture as much light as possible. When you are waiting for your eyes "to get used to the light" of a darkened room, you are waiting for your pupils to open up. Back to your question! So there you are in the dark, your pupils open as wide as possible...and the lights come on! Way too much light floods your vision and your eyes, essentially, short-circuit (a flash of just white). You are momentarily dazzled while your brain's vision circuits (and your pupil sizes) reset.