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Crepuscular rays are shafts of sunlight which look as though they are coming from a specific point in the sky, most classically the Sun, although they can appear on the horizon opposite the Sun as well. A number of alternate names are used to describe crepuscular rays, often integrating religious references, reflecting the fact that this striking visual phenomenon can look like the hand of God in the sky. You may know crepuscular ways as Jacob's fingers, cloud breaks, God's rays, Buddha's finger, or sun drawing water.
In fact, these shafts of light are really parallel to each other, they just look they are are coming from a unified point because of perspective. Think of crepuscular rays as a set of railroad tracks. When you view railroad tracks straight-on, you may note that they appear to converge at a point in the distance, even though this is not the case. Crepuscular rays work in the same way.
This visual effect is created when the sunlight is alternately scattered with dust, rain, snow, or mist, and obscured by something which casts a shadow. Classically, crepuscular rays appear as shafts of light coming from a cloud, with the sunlight piercing the cloud in thin places while the cloud shadows other regions, creating an illusion of pillars of light. Crepuscular rays can also be created by the interplay of light and shadow in a forest, near a mountain, or in a heavily-built region such as a downtown financial district.
Typically, crepuscular rays occur around dawn and dusk, explaining their name: “crepuscular” means “pertaining to twilight.” Conditions are prime for seeing crepuscular rays at twilight because of the increased contrast, which makes these rays of light much more visible. However, they can be seen at any time of the day, depending on conditions.
Most of the time, crepuscular rays appear to be radiating out from the Sun. However, sometimes anticrepuscular rays, which appear opposite the Sun, can be seen. These crepuscular rays are created through a reflection of the light from the Sun against a cloud or another surface opposite the Sun. On some occasions, anticrepuscular rays can even frame the moon as it starts to rise, creating a very memorable visual image.
These shafts of light are commonly used in works of art, especially religious art, to illuminate a scene and create more contrast and visual interest. Photographers are very fond of working with crepuscular rays, since the interaction of light and shadow can make for a very interesting photographic composition.