Seawater and other bows

Günther Konnen has drawn attention to this famous image showing disconnected rainbows. J Dijkema imaged it in the Pacific Ocean.

The upper primary bow (with a fainter supernumerary) was formed by falling rain.

The lower bow was made by drops of seawater thrown up by waves against the ship’s side. The seawater bow has a slightly smaller radius (by about 0.8°). The difference would not normally be apparent but here it is obvious by comparison with the rain water bow.

Seawater, because it contains dissolved sodium, calcium and magnesium salts, is slightly denser than pure water and also has a greater refractive index. The lower diagram shows two minimum deviation rays going through spheres of different refractive index to form primary bows. Seawater and rainwater are too alike to show clearly distinguishable rays and so instead, ray paths are shown for a water sphere (n = 1.33) and a glass sphere (n=1.51).

As the refractive index increases, the incoming ray that forms the primary bow (minimum deviation) moves inwards to so that if it were undeviated it would pass closer to the drop centre. At a sufficiently large refractive index the ray actually passes through the centre, the deviation angle approaches 180°, and there is no longer a rainbow. Highly refractive substances cannot form rainbows.

Advertisements

Posted on May 13, 2011, in rainbow and fogbow, theory and tagged , . Bookmark the permalink. 3 Comments.

  1. Claudia Hinz said…

    Wow, it””s physics live! Thanks also to Les for the extensive explanations.

    Enthusing greetings
    Claudia

  2. Günther Können said…

    In a BBC TV documentary about Antarctica called ””Live in the Freezer”” there is a shot of a rainbow at the coast (in Hawaii, I believe)in which the salt water bow is also visible — growing and shrinking with the size of the sea-spraywater. The documentary is produced together with the British Antarctic Survey.

  3. GV said…

    interesting, I can””t explain the fact that particles has been cut, however the sames still keeping in the same patron, this is intrigant, because if you cut the frequence the signal and patron too have to be cutted and for law the rainbow must dissapeared.

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s

%d bloggers like this: