This is the phenomena that takes place when we move from dark room to bright light or else from bright area to dark room. Our eyes take some time to adjust and adapt to the total change of light that it experiences and thus images appearing to us seem unclear with a little bit of darkness or blindness appearing to our eyes.
This adaptation takes place automatically without our knowledge due to ability of our visual system to adjust to the changing light. It is a complex phenomena which is based on many factors like cone and rods of eyes, pupil size etc..... Adaptation first begins with iris of the eyes whose main function is to control image quality.
When we move from dark room to bright room, this phenomena takes place. Sensitivity of receptors of our eyes is set to dim light and when we suddenly move from dark room to bright room then all that we can see is white light. Resultant glare is due to instant breaking of photo pigments. It is an active defense mechanism. It prevents damage to vision by bright light.
In this case, sensitivity of retina of our eyes decreases. Rods and cones both gets stimulated. Neurons present in retina here adapts quickly. They favor cones and inhibit rods. Within a few minutes our eyes adapt to the bright light and we no longer feel discomfort in viewing and that is because cones are excited by brightness to take over. In this phenomena, retinal sensitivity goes away. For next few minutes, one can experience more clarity in vision and color.
When we move from bright room to dark room then adaptation that our eyes have to do to adjust to the darkness is known as dark adaptation. Here, function of cone is inhibited. In the beginning of the phase, rods too seem to lose their functioning as its pigments are bleached by bright light. In this phenomena, sensitivity of retina is not lost unlike light adaptation. In fact it increases. Reflexive changes are seen in pupil.