'treatment of light and shade in drawings and paintings, an effect of contrasted light and shadows created by light falling unevenly or from a particular direction on something'
Before this effect was 'discovered?' paintings looked very flat because lighting wasn't considered as much, this effect added depth to paintings.
Still don't fully understand how it works, i keep finding images and i don't know if its using the effect or not.
This image (Above) claims it uses the effect, it says the main light is on the girl whilst the background stays pretty dark. don't know if its a good example or not.