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Rainbows and Halos. Tony Signal IFS Massey University. Rainbows. Formation of Rainbow depends on three optical phenomena Refraction of light at boundary Total Internal Reflection Dispersion. Rainbows. Formation of Rainbow depends on three optical phenomena Refraction - PowerPoint PPT Presentation
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Rainbows and Halos
Tony SignalIFSMassey University
RainbowsFormation of Rainbow depends on three optical
phenomena• Refraction of light at boundary• Total Internal Reflection • Dispersion
n1 sin1 n2 sin2
RainbowsFormation of Rainbow depends on three optical
phenomena• Refraction• Total Internal Reflection • Dispersion
sinC n2n1
48.7
RainbowsFormation of Rainbow depends on three optical
phenomena• Refraction• Total Internal Reflection• Dispersion
RainbowsWhite light shining on spherical water droplet
RainbowsRainbow forms from light coming from many droplets
Your rainbow is unique!
Rainbows
RainbowsWhy is the rainbow at ~42° deviation?
Most rays have this deviation, so its brightest.Also Fresnel theory predicts rainbow is highly
polarized!
RainbowsDouble (secondary) rainbow at ~51° deviation from
double internal reflection
Between rainbows is darker area (Alexander’s band)
RainbowsDouble (secondary) rainbow at ~51° deviation from
double internal reflectionNow red on inside
RainbowsAlso visible in moonlight – moonbow
Best seen when moon is low (<42°), full and sky is dark
Long exposure is good to see colours
RainbowsFogbows and lunar fogbows also seenSmaller droplets mean colours are washed out –
diffraction effect
Can also see glory from backscattered light
Supernumerary RainbowsExtra bows can be seen inside principal rainbow• Diffraction• First explained by Thomas Young 1804
Supernumerary RainbowsExtra bows can be seen inside principal rainbow• Diffraction• First explained by Thomas Young 1804
• http://www.atoptics.co.uk/rainbows/supdrsz.htm
RainbowsOther interesting rainbows can be seen:• Reflection rainbow
Water must be smooth
• Seawater rainbowhttp://www.atoptics.co.uk/rainbows/seabow.htmLarger ref. index => smaller radius
HalosHalos are coloured or white arcs, spots, pillars in sky.Ice crystals in cirrus cloud usually responsible
Halo
Sun dog
Parhelic circle
HalosIce crystals tend to form as hexagonal prismsMinimum deviation about 22°Slight dispersion (inner edge reddish)
HalosEasily seen in both sun- and moon-lightRing makes angle of 22° with source
HalosSun dogs (parhelia) formed when light passes
through near horizontal plate ice crystalsCan be very bright, reddish on inside
Noted by Aristotle and Cicero
Sun dogs at sunset
HalosLight pillars formed when light reflects from faces of
near horizontal plate ice crystals
Seen when sun (or other source) is low in sky
HalosSun (+ moon) pillars can rise 22° in sky, but pillars from artificial lights can govery high.
More InformationClassic text: Light and Color in the Outdoors, M. G. J. Minnaert
(Springer, 1993)
Good websites:• Atmospheric Optics www.atoptics.co.uk
Simulations, clear explanations, many pictures
• Polar Image, Pekka Parviainen’s site, 100’s of pictures www.polarimage.fi
• The Weather Doctor www.islandnet.com/~see/weather/general/site_map.htmNice figures, good explanations, pictures