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Rainbows and Halos

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

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Rainbows and Halos

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  1. Rainbows and Halos Tony Signal IFS Massey University

  2. Rainbows • Formation of Rainbow depends on three optical phenomena • Refraction of light at boundary • Total Internal Reflection • Dispersion

  3. Rainbows • Formation of Rainbow depends on three optical phenomena • Refraction • Total Internal Reflection • Dispersion

  4. Rainbows • Formation of Rainbow depends on three optical phenomena • Refraction • Total Internal Reflection • Dispersion

  5. Rainbows • White light shining on spherical water droplet

  6. Rainbows • Rainbow forms from light coming from many droplets • Your rainbow is unique!

  7. Rainbows

  8. Rainbows • Why is the rainbow at ~42° deviation? • Most rays have this deviation, so its brightest. • Also Fresnel theory predicts rainbow is highly polarized!

  9. Rainbows • Double (secondary) rainbow at ~51° deviation from double internal reflection • Between rainbows is darker area (Alexander’s band)

  10. Rainbows • Double (secondary) rainbow at ~51° deviation from double internal reflection • Now red on inside

  11. Rainbows • Also visible in moonlight – moonbow • Best seen when moon is low (<42°), full and sky is dark • Long exposure is good to see colours

  12. Rainbows • Fogbows and lunar fogbows also seen • Smaller droplets mean colours are washed out – diffraction effect • Can also see glory from backscattered light

  13. Supernumerary Rainbows • Extra bows can be seen inside principal rainbow • Diffraction • First explained by Thomas Young 1804

  14. Supernumerary Rainbows • Extra bows can be seen inside principal rainbow • Diffraction • First explained by Thomas Young 1804 • http://www.atoptics.co.uk/rainbows/supdrsz.htm

  15. Rainbows • Other interesting rainbows can be seen: • Reflection rainbow • Water must be smooth • Seawater rainbow • http://www.atoptics.co.uk/rainbows/seabow.htm • Larger ref. index • => smaller radius

  16. Halos • Halos are coloured or white arcs, spots, pillars in sky. • Ice crystals in cirrus cloud • usually responsible • Halo • Sun dog • Parhelic circle

  17. Halos • Ice crystals tend to form as hexagonal prisms • Minimum deviation about 22° • Slight dispersion (inner edge reddish)

  18. Halos • Easily seen in both sun- and moon-light • Ring makes angle of 22° with source

  19. Halos • Sun dogs (parhelia) formed when light passes through near horizontal plate ice crystals • Can be very bright, reddish on inside • Noted by Aristotle and Cicero

  20. Sun dogs at sunset

  21. Halos • Light pillars formed when light reflects from faces of near horizontal plate ice crystals • Seen when sun (or other source) is low in sky

  22. Halos • Sun (+ moon) pillars can rise 22° in sky, • but pillars from artificial lights can go • very high.

  23. More Information • Classic 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.htm • Nice figures, good explanations, pictures

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