By means of this image I want to explain something that I do often.
In this portrait the light source comes from the top left. Consequently the shadow on the face is on the right side, on the chin. Sometimes it works well to have a high contrast in the values between the cast shadow and the background. That means I make this part of the background a lot lighter than the rest of the area.
When teaching I explain it this way: It’s possible to give a portrait some fresh air.