In paintings such as "An Experiment on a Bird in the Air Pump" by Joseph Wright, the use of strong contrasts of light and dark may be used to discuss the nature and location of the light source as well as the inverse square law. The rather sharp shadows suggest a point source such as a candle. Notice too that even though the light source cannot be seen, its location can be inferred. And perhaps most importantly, the shadows establish the mood of the painting.
The scene in Edward Hopper's "The Night Hawks" is totally devoid of harsh shadows. Why? When this work was created, fluorescent lights had become commonplace. The uniform lighting produced by a collection of extended sources does not produce sharp shadows. The result: a mood of detachment and loneliness.
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