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Rainbows and fogbows

The appearance of a rainbow depends on the size of the water drops - as indicated by the following MiePlot simulations of rainbows (as might be seen by a 35 mm camera with a lens of 70 mm focal length):

Fig. 1a    r = 10 µm Fig. 1b    r = 20 µm Fig. 1c    r = 50 µm
Fig. 1d    r = 100 µm Fig. 1e    r = 200 µm Fig. 1f    r = 500 µm

For very small water droplets (e.g. for r = 10 µm), the rainbow (or fogbow) is colourless and not clearly defined.  For r = 50 µm, the primary rainbow is predominantly white with a hint of red at its outer edge.  The colours of the primary rainbow become more obvious for even larger drops, together with the secondary rainbow and Alexander's dark band.  If the drop size is uniform, supernumerary arcs inside the primary rainbow can be clearly seen for r = 50 µm and r = 100 µm.  However, as the spacing between the supernumerary arcs decreases with drop size, it is difficult to see the supernumerary arcs for r = 500 µm.

Further information on how the size of water droplets affects the appearance of the primary rainbow is shown in the Lee diagram generated by the MiePlot computer program.

Page updated on 21 December 2003
 
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