Rainbows and Halos

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 (Alexanders
  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 (lt42), 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
• gt 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 Parviainens site, 100s of
  pictures www.polarimage.fi
• The Weather Doctor www.islandnet.com/see/weather/
  general/site_map.htm
• Nice figures, good explanations, pictures