Adapting to the Dark

Click here to listen to this feature

It's tough to see in dim light immediately after we move from a bright room.  The explanation for our temporary blindness is found in the way the two kinds of cells in the eyes' retina, cone cells and rod cells, are connected to the brain.

In dim light it is the rod cells that provide most of our vision.  The rods are more sensitive to light because they are arranged so that several of them connect to the same nerve cell for transmission of information to the brain.  So, when light falls on several rods in close proximity, the signals are pooled and go to the single neuron to which they are connected.  This pooling of impulses provides increased sensitivity, so that the neuron can be triggered by a much lower light level.

However, since the brain can't identify which of the several rods received the light, sharpness of vision is diminished.  In order for rods to adapt to the dark certain chemical changes must take place within them.  A visual pigment normally bleached out by bright light must be regenerated in the rods.  This chemical change requires about twenty minutes for all the pigment to be regenerated, which is about the length of time it takes the eye to fully adapt to the dark.  The transition to dark at sunset is close to the time needed for the eye to adapt to darkness.

So next time the light goes out, wait, and you'll be surprised what you can see.