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