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                                 Dark and Light adaption

                                Dark Adaption

Dark adaptation occurs when going from an illuminated area to a dark area.  Initially blackness is seen because our cones cease functioning in low intensity light. Also, all the rod pigments have been bleached out due to the bright light and the rods are initially nonfunctional. Once in the dark, Rhodopsin regenerates and the sensitivity of the retina increases over time for healthy eyes (Normally 20/30 Mins) to reach their Maximum. During these adaptation processes reflexive changes occur in the pupil size.

Rhodopsin is an extremely sensitive molecule.  As long as the light is of low intensity, rods quickly regenerate rhodopsin and the retina continues to respond to light stimuli.  In high light intensity rhodopsin is bleached as quickly as rhodopsin can be produced making some of the rods nonfunctional.  If this happens the cones take over.  There is an automatic adjustment of retinal sensitivity to the amount of light present.  This automatic adjustment is not only explained by the breakdown of photoreceptor pigments, other retinal Neurons are involved.                                                                                                 

                                Light Adaption

This occurs when we move from the dark into light environment. The light momentarily dazzles us and all we see is white light because the sensitivity of the receptors is set to dim light. Rods and cones are both stimulated and large amounts of the photopigment are broken down instantaneously, producing a flood of signals resulting in the glare we experience.