Have you ever experienced a sudden darkness in front of your eyes when you come back inside your home on a sunny day? Human eyes experience such phenomenon when we enter from a bright lighting to dim one. Even in opposite situations when we travel from inside of a tunnel to outside, there is a split second of darkness. We try to find out the reason behind the sensitivity of our eyes.
I recently did some search on the topic and found that the reason behind the phenomenon is pretty intriguing.
Little background: Our eyes are made up of two kinds of photoreceptor, called rods and cones. Rods are responsible for processing light when photons are scarce. Cones require more photons to fire and help us see in bright environment.
Main Reason: The rods are very sensitive to photons. Researchers say that single photon is enough to fire rods. The rods require a visual pigment for photo-activation, which takes long time to replenish. So, when we are out under a bright light, the visual pigment of our rods get used up making the rods inactive. However, we are able to see in bright light due to cones which require much more photons to fire and gets quickly replenished with visual pigment. When we enter inside our house having dim light, there is scarcity of photons required to trigger the cones. Meanwhile, rods got inactive due to brightness outside. This creates a temporary blindness for the period our rods reactivate. If you have noticed closely, blindness caused on entering dark from bright is way longer than entering bright from dark. When we travel outside a tunnel, the rods quickly gets bleached and gives a sense of blindness for a very short period, before cones take over.
So, that was the reason why we face blindness when brightness changes. The mechanism of our eye is fascinating and its complexity amazes me every time. Take care and have a good day!