A light or Alight?
Your choice!
A light
Exemple : ‘’There is a light on upstairs so she must be in.’’
"A light" written as two words has a different meaning to "alight" written all as one word. "A light" is a noun used to denote something which emits brightness.
Alight
Exemple : ‘’During the protest a number of cars were set alight.’’
"Alight" is an adjective used after a verb in order to express that something is shining, bright or is burning.