Atmospheric effects

This page is devoted to atmospheric effects and particularly to rainbows. All photographs presented in this section have been taken by the authors with a conventional camera and standard positive films. No color correction has been performed during the digitalization. For some sunset images, relatively long focal lengths have been required, like telescopes.

WARNING : Never view the sun through optical instruments like telescopes or binoculars without reducing light intensity with appropriate filters. Irreversible eye damage may result !

· Sundogs

Parhelia or sundogs are the brightest and most common halo that originates in an oriented crystal. They are found on either side of the sun at the same altitude and tend to show bright colors. They are red on their sunward side and fainter and more bluish away from the sun. At low altitudes parhelia are found 22° from the sun but at higher altitudes they separate from it.
Image taken from Affoltern am Albis (Switzerland), September 2001. © M. Willemin

· Rainbows images

The second photograph has been taken just before the sunset, which explains the red dominent.

· Rainbows "theory"

This very interesting, natural and nice effect is due to the refraction and dispersion of the light through the water drops. Sun and rain together is the condition for rainbows. The primary rainbow is easy to observe. It results from a single internal reflexion and two refraction of the light of the sun in water drops. The index of refraction of water depends on the wavelength (dispersion), which produces a decomposition of the light during the two refraction processes, like a prism. Under good conditions, a secondary rainbow can also be observed (see second photograph). In this case, two internal reflexions of light within the water drops occur. The intensity of the secondary rainbow is much lower than for the primary or main rainbow.

· Primary rainbow · Secondary rainbow

For calculating your own rainbow online, click HERE !

Wavelength (nm) Index n Primary angle (°)  Secondary angle (°)
300 1.3532 38.72 56.80
350 1.3501 39.66 55.26
400 1.3440 40.51 53.73
450 1.3411 40.91 52.99
500 1.3364 41.27 52.33
550 1.3344 41.64 51.68
600 1.3335 42.01 51.02
650 1.3318 42.25 50.58
700 1.3309 42.38 50.34
800 1.3294 42.60 49.92
900 1.3285 42.73 49.70
1000 1.3277 42.86 49.49

 · Collection of sunsets

© M. Willemin

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