Irregular Webcomic! #1318

No. Your eyes are just adapting to the darkness.

2016-03-07 Rerun commentary: In case the above comment is a bit cryptic, I wrote it because I made this strip a bit lighter than the previous one, which on reflection I thought was a bit too dark.

Our eyes are highly adaptive to different lighting conditions. You may know that the pupil (the black part in the centre of your eye that lets light in) expands in dim light to let more light in, but this is not the end of the story by any means.

Our eye detects light by a chemical process, involving the protein molecule rhodopsin, which exists in the rod and cone cells which line the retina at the back of the eye. When exposed to light, rhodopsin does two things:

It begins a chain of reactions called visual transduction, which leads ultimately to the generation of nerve impulses in the optic nerve. This sends signals to your brain that your eye has detected light, and your brain duly processes those signals to understand the stimulus as an image of the world around you.
As part of this process, the rhodopsin is bleached by the incoming light, in that it changes its physical structure by rearranging the shape of the atoms in the molecule. In this bleached state, it is no longer sensitive to light. It takes a few minutes for the rhodopsin molecule to return to its original light-sensitive form.

The impact of this second part is that in bright light conditions, most of the rhodopsin molecules in your eye have already been exposed to photons of light recently, and are in the bleached state. This means that there is only a small population of active rhodopsin absorbing new photons, because the others are all taking a few minutes to return to the active state. This is good, because it means that in bright light your eyes don't get overloaded with signal, which might swamp the optic nerve and result in blinding glare that is impossible to see through.

On the other hand, in dim light, most of the rhodopsin molecules have time to relax back into their active state before more photons arrive, so that now most of the rhodopsin in your eye is in the active state, awaiting more photons to detect. In other words, your eyes become more sensitive to light when the light is dimmer. Which is exactly what you want.

This process of the amount of rhodopsin molecules in an active light-receiving state in your eye being controlled by the amount of incoming light is known as adaptation. And because the rhodopsin takes a few minutes to return to the active state, this produces the familiar time lag of a few minutes when you go from a brightly lit area to a dark one, in which your eyes are "adjusting" to the dark.

Biochemistry is cool.