Crepuscular rays in Taipei, Taiwan in 2018.
Sunbeams as seen from space, illustrating their parallel nature

A sunbeam, in meteorological optics, is a ray of sunlight that appears to radiate from the point in the sky where the Sun is located. Shining through openings in clouds or between other objects such as mountains, these columns of sunlit scattering particles are separated by darker shadowed volumes. Despite converging toward the light source, the rays are essentially parallel shafts of sunlit and shadowed particles. Their apparent convergence in the sky is a visual illusion from linear perspective. This illusion is the same as railway lines' or long hallways' appearing to converge at a distant vanishing point.[1] The scattering particles that make sunlight visible can be air molecules or particulates.[2]

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Crepuscular raysEdit

 
Sunbeams in Nevada during a sunset

Crepuscular rays or "God rays" are sunbeams that originate when the sun is below the horizon, during twilight hours.[3] Crepuscular rays are noticeable when the contrast between light and dark are most obvious. Crepuscular comes from the Latin word "crepusculum", meaning twilight[4]. Crepuscular rays usually appear orange because the path through the atmosphere at sunrise and sunset passes through up to 40 times as much air as rays from a high midday sun. Particles in the air scatter short wavelength light (blue and green) through Rayleigh scattering much more strongly than longer wavelength yellow and red light.

Antisolar raysEdit

The rays in some cases may extend across the sky and appear to converge at the antisolar point, the point on the sky sphere directly opposite the sun. In this case they are called antisolar rays.[5] This apparent dual convergence (to both the solar and antisolar points) is a perspective effect analogous to railway tracks appearing to converge to opposite points in opposite directions.[6]

Alternative namesEdit

  • Backstays of the sun, a nautical term, from the fact that backstays that brace the mast of a sailing ship converge in a similar way
  • Buddha rays[2]
  • God rays, used by the computer graphics industry[7]
  • Jacob's Ladder[2]
  • Light shafts, sometimes used in the computer graphics industry, such as the game engine Unreal Engine[8]
  • Ropes of Maui, originally taura a Maui—from the Maori tale of Maui Potiki restraining the sun with ropes to make the days longer[2]
  • Sun drawing water, from the ancient Greek belief that sunbeams drew water into the sky (an early description of evaporation)[2]
  • Sunburst
  • Tyndall rays

See alsoEdit

ReferencesEdit

  1. ^ Schaefer, Vincent J.; Day, John A.; Pasachoff, Jay (1998). A Field Guide to the Atmosphere. Houghton Mifflin Harcourt. p. 169.
  2. ^ a b c d e Lynch, DK; Livingston, W (1995). Color and light in nature. Cambridge: Cambridge University Press.
  3. ^ Naylor, John (2002). Out of the Blue: A 24-Hour Skywatcher's Guide. Cambridge University Press. pp. 77–79.
  4. ^ Edens, Harald. "Crepuscular rays". Weather Photography lightning, clouds, atmospheric optics & astronomy. Retrieved November 1, 2011.
  5. ^ Cowley, Les. "Anti-solar (anti-crepuscular) rays". Atmospheric Optics. Retrieved March 19, 2015.
  6. ^ Day, John A. (2005). The Book of Clouds. Sterling. pp. 124–127. ISBN 978-1-4027-2813-6. Retrieved 2010-10-09.
  7. ^ E.g. this term is mentioned in: Krüger, Jens; Bürger, Kai; Westermann, Rüdiger (2006). "Interactive screen-space accurate photon tracing on GPUs" (PDF). Proceedings of the 17th Eurographics conference on Rendering Techniques (EGSR'06).
  8. ^ "Light Shafts". Unreal Engine 4 Documentation. Archived from the original on 2018-11-17. Retrieved 2018-11-17.

External linksEdit