Lighting - the use of light and darkness to evoke specific moods and draw attention to important features.
Children are afraid of the dark, and because we never really grow up the association between darkness and danger remains within us.
By contrast, light continues to be associated with safety, and is also a helpful way to spotlight important information in an image.
When referring to the lighting within a piece, there are two terms it is worth being familiar with: low-key lighting and high-key lighting.
Low-key lighting - the use of little or no light within a scene to enable high contrast edges and a dark, serious mood.
High-key lighting - the use of bright lighting with minimal shadows to enable low contrast edges and a bright, jovial mood.
Let's consider two examples of images which employ darkness or light to achieve particular effects
Caravaggio was an Italian Renaissance painter famed for his use of contrasting shadow and light. Typically, Caravaggio's paintings feature a dark (often pitch black) background contrasted with brightly lit subjects. This has the dual effect of creating a dark, somber mood, whilst also highlighting the expressions and actions of subjects.
In The Sacrifice of Isaac, the foreground surrounding the subjects is almost entirely black, while the background features only a dimly-lit sky in the distance. This evokes a shadowy, sinister mood which heightens the drama of the piece.
In contrast to the dark surroundings, the three figures of Abraham, his son Isaac, and the angel, are brightly lit, allowing the viewer to clearly witness the gruesome nature of the sacrifice and the horror on Isaac's face.
Arthur Streeton, an Australian artist of the Heidelberg School, was famed for his impressionist paintings which captured the Australian countryside.
Notice how in Streeton's At Templestowe there is almost no shadow present, suggesting the scene takes place around noon with the whole landscape awash in midday sun.
Streeton's use of high-key lighting bathes both the foreground and background of the image in a golden glow, evoking a warm—albeit—stark, mood, effectively capturing the distinct light of the Australian summer.