The degree of polarization of rainbows of order with is bounded in the interval [75%, 78%], where 75% is the limit for . A polarization filter can improve the signal-to-background ratio of the third and fourth rainbows by a factor of 2, which may lift their visibilities in natural circumstances above the threshold of human visual perception. Under optimal circumstances, the latter may be true for the recently photographed green fingerprint of the fifth rainbow, even without the aid of a polarization filter. The prospects for observing the sixth rainbow are unclear. There exists a possibility that the signal of the natural seventh rainbow (appearing at 64° from the Sun) may be separated from its background if photographed under perfect conditions through a polarization filter.
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Geometrical optics; only single scattering is considered.
For the fifth-rainbow Descartes angle in the region of the second rainbow ( scattering; values are for ; ).
For the fifth-rainbow Descartes angle in Alexander’s band (values are for ; ).
Table 2.
Rainbow Signal/Background Ratio and Improvement Factor for (), and Drop Radius of 0.5 mm, as Calculated Using Debye Seriesa
refers to the component of the background solely caused by single scattering by the rainbow-generating drops. Observations without a polarization filter are denoted by unpol; observations through a polarization filter maximally transmitting the rainbow light are denoted by pol1.
Of the main maximum.
Normalized with the value at the third rainbow.
Obscured by the coinciding main maximum of the second-order rainbow (values are for ; ).
In Alexander’s band (values are for ; ).
Table 3.
Rainbow Visibility for , where to an Unpolarized Background Radiance of a Multiple of is Added, in which is the Background Proposed by Lee and Laven [5]a
Observations without a polarization filter are denoted by unpol; observations through a polarization filter maximally transmitting rainbow light are denoted by pol1. Values are for drop radius of 0.5 mm (calculated using Debye series). ; . ; .
Table 4.
Photographic Records of Higher Rainbows through 31 December 2013
Edens’ and Antipov’s pictures are all taken through a polarization filter; their visual observations without.
Discovery picture.
Edens’ pre-2012 fifth-rainbow pictures surfaced during a search in his pre-2012 digital images [4].
Tables (4)
Table 1.
Higher-Order Rainbow Polarization and Improvement Factor for , Corresponding to Water/Air at and a
Geometrical optics; only single scattering is considered.
For the fifth-rainbow Descartes angle in the region of the second rainbow ( scattering; values are for ; ).
For the fifth-rainbow Descartes angle in Alexander’s band (values are for ; ).
Table 2.
Rainbow Signal/Background Ratio and Improvement Factor for (), and Drop Radius of 0.5 mm, as Calculated Using Debye Seriesa
refers to the component of the background solely caused by single scattering by the rainbow-generating drops. Observations without a polarization filter are denoted by unpol; observations through a polarization filter maximally transmitting the rainbow light are denoted by pol1.
Of the main maximum.
Normalized with the value at the third rainbow.
Obscured by the coinciding main maximum of the second-order rainbow (values are for ; ).
In Alexander’s band (values are for ; ).
Table 3.
Rainbow Visibility for , where to an Unpolarized Background Radiance of a Multiple of is Added, in which is the Background Proposed by Lee and Laven [5]a
Observations without a polarization filter are denoted by unpol; observations through a polarization filter maximally transmitting rainbow light are denoted by pol1. Values are for drop radius of 0.5 mm (calculated using Debye series). ; . ; .
Table 4.
Photographic Records of Higher Rainbows through 31 December 2013
Edens’ and Antipov’s pictures are all taken through a polarization filter; their visual observations without.
Discovery picture.
Edens’ pre-2012 fifth-rainbow pictures surfaced during a search in his pre-2012 digital images [4].