Lightness, often referred to as brightness, describes the perception of how much light a surface reflects or emits. It is a key aspect of color theory and is measured on a scale from dark to light. In visual arts, lightness affects how colors are perceived and can influence the mood of a piece. In terms of physics, brightness is related to the intensity of light emitted from a source, such as the sun or a light bulb. The human eye perceives brightness based on the amount of light that reaches it, which can vary depending on the surrounding environment and the presence of other colors, such as white or black.