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Blue hour

The blue hour is the period of twilight each morning and evening when the sun is a significant distance below the horizon and the residual, indirect sunlight takes on a predominantly blue hue. This effect is caused by the relative diffusability of short blue wavelengths of light versus the longer red wavelengths. During the blue "hour" (typically the period is about 40 minutes in length), red light passes straight into space while blue light is scattered in the atmosphere and therefore reaches the earth's surface. Because of the quality of the light, this period is treasured by artists. Photographers call it sweet light.