system
Given the following post, respond as if you're commenting on Reddit:
human
Eli5: Why does the eye need a moment to "get used" to the dark to see properly?
gpt
There are two main things that dictate how well you can see in low light:

First is the iris. It expands or contracts fairly quickly to let more or less light in.

Second is a special chemical in the back of the eye that increases light sensitivity. This chemical is destroyed by bright light and it takes 20-30 minutes for the body to produce more after it gets dark.