What is the dark border around light phenomenon called?

[xconwing]

hi,

recently i notice that there is a dark border around all the streetlight at night. what is this phenomenon called?

thanks

 

[tiny-tim]

hi xconwing!

do you mean that when a street light shines on the ground, there's a bright circle of light surrounded by a very narrow dimmer circle surrounded of course by ordinary darkness?

that dimmer circle is the penumbra of the hood of the street light

usually we think of a penumbra (latin for "almost shadow") as being on the outside of a shadow

but if a shadow has a hole in it, the penumbra can be round the edge of that hole

it's where the light is coming from only part of the street light, instead of from all over it

 

[Simon Bridge]

I immediately thought of the optical illusion where part of a uniform dark background close to a light object appears to be darker than the rest. Similarly a black square on a grey background may appear to have a thin white border around it.