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The image below is included in the BBC article Amazing white rainbow snapped over Scottish moor. Apparently this phenomenon is sometimes called a "fog-bow" and is characterized by the washing out of perceived color relative to a conventional rainbow.

Is this related to the geometry (fog being far closer to the viewer) or properties of the droplets themselves? If so, which properties and why?

Higher resolution in flickr

enter image description here

uhoh
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  • I did not find any more tags related to rainbows, or any other optical or visual effects caused by sunlight interacting with the atmosphere, if there are additional tags that apply please add them, thanks! Also, I am not sure if this question better asked in physics SE, or is on-topic here. – uhoh Nov 23 '16 at 02:50
  • I found the article Rainbows in nature: recent advances in observation and theory and it is not paywalled. It does mention "fogbow" but I am having trouble extracting a simple, clear explanation for the reduction in perceived color. That article was found here. – uhoh Nov 23 '16 at 02:58

1 Answers1

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The reason why fog bows lack colour, compared to rainbows, is due to the size of the drops of water.

Fog is composed of very small drops of water - less than 0.05 mm diameter. Because of this the wavelength of light is critical, with diffraction smearing out colours that the larger drops of water in a rainbow would make.

Fred
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  • Wow! I never thought about that, but it makes perfect sense. The ray tracing diagrams we draw for refraction in a spherical drop assume it is far bigger than a wayvelength, but now that you point this out, sure diffraction must take place. It's only a small "window" (figuratively) on a given droplet where light can enter and then reach our eye. – uhoh Nov 23 '16 at 15:37
  • I have an issue to this answer but up vote. Can you answer this? – Muze Mar 29 '18 at 20:48
  • https://earthscience.stackexchange.com/questions/13424/why-are-all-rainbows-are-not-the-same – Muze Mar 29 '18 at 20:48