system
Given the following post, respond as if you're commenting on Reddit:
human
When I look at a source of light in the dark, I see a large halo around it. Why don't I see this effect in a well-lit environment?
gpt
You see the halo because of the contrast between the (presumably very dark) environment and the (presumably rather bright) light source.  Your eyes actually scatter a significant amount of incoming light over a set pattern with broad wings -- so each bright point of light yields a halo-like image on the retina.  The wings of the halo are faint enough that they get washed out in normal life, but at night there is high contrast between (say) a candle flame and a dark room around the candle.  Your retinal response is logarithmic, which downplays how phenomenal the contrast ratio really is (can be a factor of 10,000 to 100,000 between a candle flame and the walls in a typical small candlelit room!).  The stray light from the candle flame washes out the dark room and appears as a halo.

In a well-lit environment the contrast ratio between anything you're likely to view and its surroundings is not nearly so great, since 10,000 times brighter than "well lit" is "painful" or even "damaging".  For example the Sun has a similar optical halo that can be seen on a clear day with dark skies, but you're not likely to have ever noticed it.  (The optical halo there is also masked by a forward-scattering halo in the sky -- you have to use your thumb or something to obscure and reveal the Sun, and carefully observe how the sky around the Sun changes, to notice the optical halo there).