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Re: Random Question violet

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Jeremy Adler-4

Re: Random Question violet

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Another oddity is that sometimes the sensitivity spectra for human photoreceptors appear with a minor peak for red pigment at very short wavelengths - at the end of blue range.
The implication is that at the shorter wavelength end of the spectra some wavelengths can stimulate concurrently blue and red pigments, producing a perception of the combination-violet.

Jeremy Adler

===============================================
B i o V i s P l a t f o r m of Uppsala University
Light & EM microscopy / FlowCytometry & Cell Sorting / Image Analysis ===============================================
Jeremy Adler PhD - Senior research engineer Light, Confocal Microscopy, Image Analysis
E-mail: [hidden email]
070-1679349

Dag Hammarskjölds v 20
751 85 UPPSALA, SWEDEN
http://biovis.uu.se/
===============================================

-----Original Message-----
From: Confocal Microscopy List <[hidden email]> On Behalf Of Henrik Persson
Sent: den 23 augusti 2019 01:55
To: [hidden email]
Subject: Re: Random Question

*****
To join, leave or search the confocal microscopy listserv, go to:
http://lists.umn.edu/cgi-bin/wa?A0=confocalmicroscopy
Post images on http://www.imgur.com and include the link in your posting.
*****

This is indeed a very interesting off-topic discussion, thank you for starting it, Dr. Brown.

I have been thinking about it for a bit in the past few days and also want to weigh in.

Perception of colour is a very interesting topic, combining physics, biology and cognition. We have to keep in mind the spectrum of the sunlight (in space) and the wavelength sensitivity of our cones (maxima around 419,
531 and 539 nm [https://www.ncbi.nlm.nih.gov/books/NBK11059/], assuming we are not tetrachromats). On top of that, our brains will try to adjust what we’re seeing to interpret the colours correctly. A banana will appear more or less yellow regardless of the actual wavelength of the light it reflects, be it in the supermarket or in a spring forest. Recall the obviously black and blue dress from 4 years ago [ https://en.wikipedia.org/wiki/The_dress]

(Or in microscopy terms – we have to be aware of the dye’s emission spectrum, our emission filter’s band pass and camera sensitivity as well as any camera auto white balance we accidentally left on.)

The purple colour can potentially be caused by either purple wavelengths or by blue+red wavelengths hitting the retina. If it is the former, then for whatever reason, there has been very little scattering (purple has shorter wavelengths than blue light and would be even more prone to scattering – there might be conditions where purple light has a chance to scatter but not blue light, in say a very thin atmosphere or one with different composition). Reduced scattering can be caused by light travelling a shorter distance through the atmosphere and/or cleaner air. (Here we can start discussing the colour of the atmosphere on other planets with completely different atmospheric composition and density.)

The explanation of red+blue light for the purple sunsets might be more accurate. This could simply be caused by red light from the sunset being scattered down from the blue areas above. E.g. scattering of the underside of clouds or non-visible particles/vapour/ice crystals in the atmosphere. Indeed, when scrolling through pictures of purple sunsets, they all seem to contain clouds. (The non-cloud areas are also purple but may contain traces of
vapour.)

Searching a bit more on the topic seems to support this. There are several articles talking about the purple skies seen over Florida in the wake of hurricane Michael last October (lots of pictures and live recordings).
Articles attribute the purple to scattering from an increased amount of water in the atmosphere but do a poor job of explaining the phenomenon further (i.e. why purple and not blue).

This turned into a very long email, I hope you made it this far and found something interesting.

Cheers

/Henrik

*Quick recap on blue sky research (literally)*

As I’m sure you know, the light from the sun is white and contains all wavelengths albeit at different intensities. As the light enters the atmosphere, it scatters in a wavelength dependent manner, causing blue (short wavelengths) to bounce around multiple times, spreading out across the sky and reaching our eyes from all over. If you look at the sun (Don’t!), it appears yellow, basically white minus blue.

When the sun is low on the horizon, its rays has to travel through more atmosphere and slightly longer wavelengths are also scattered – only orange to red maintains a roughly straight path from the sun. The sun appears orange-red.

If you add particles to the air, scattering of longer wavelengths (than
blue) will increase and the sky will be more orange, especially at sunset with the long travel distance. One of the many gifts industrialization has given us – bright orange sunsets. Prior to the amount of soot, ash and pollution we have today, we had much more timid sunsets. See for instance this paper [https://www.atmos-chem-phys.net/14/2987/2014/] where Zerefos et al. studied old paintings to figure out what the sunset looked like prior to the industrial revolution. They also compare sunsets before and after volcanic eruptions. For instance, in 1883, the volcano Krakatoa erupted in modern day Indonesia and caused sunsets across the world to turn a deep red
- people even called the fire brigade, thinking nearby towns were on fire [ https://en.wikipedia.org/wiki/1883_eruption_of_Krakatoa#Global_optical_effects].

On Thu, 22 Aug 2019 at 14:20, Claire Brown <[hidden email]> wrote:

> *****
> To join, leave or search the confocal microscopy listserv, go to:
> http://lists.umn.edu/cgi-bin/wa?A0=confocalmicroscopy
> Post images on http://www.imgur.com and include the link in your posting.
> *****
>
> I have really enjoyed reading the replies to this question!
> Fascinating topic that I guess many of us are interested in.
>
> I have only been to Australia once. Last September and I saw a very
> violet sunset. It stood out to me because I had never seen a sunset of
> that colour before. Then a friend of mine posted a sunset photo from
> Australia just a few weeks ago and it had the same violet colour.
>
> Really interesting if this is happening all over the world. I have
> mostly lived in the Northern part of the Northern hemisphere and as I
> mentioned I have never seen a violet sunset before.
>
> I love the idea of testing this out with filters but I'd have to ask
> someone to do that when they witness a violet sunset since I don't
> live in Australia.
> I was trying to understand how uv/violet light would be visible at
> sunset so I favour the idea of it being a mix of colours and like the
> idea that it involves a certain particulate or mix of particulate in the sky.
>
> Thanks for the engaging responses!
>

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Jeremy Adler-4

Re: Random Question violet

*****
To join, leave or search the confocal microscopy listserv, go to:
http://lists.umn.edu/cgi-bin/wa?A0=confocalmicroscopy
Post images on http://www.imgur.com and include the link in your posting.
*****

So, next time anyone sees an unusual sunset and has a spectrophotometer handy-check the wavelengths.

Jeremy

________________________________
From: Confocal Microscopy List <[hidden email]> on behalf of Jeremy Adler <[hidden email]>
Sent: 23 August 2019 10:03:57
To: [hidden email]
Subject: Re: Random Question violet

*****
To join, leave or search the confocal microscopy listserv, go to:
http://lists.umn.edu/cgi-bin/wa?A0=confocalmicroscopy
Post images on http://www.imgur.com and include the link in your posting.
*****

Another oddity is that sometimes the sensitivity spectra for human photoreceptors appear with a minor peak for red pigment at very short wavelengths - at the end of blue range.
The implication is that at the shorter wavelength end of the spectra some wavelengths can stimulate concurrently blue and red pigments, producing a perception of the combination-violet.

Jeremy Adler

===============================================
B i o V i s P l a t f o r m of Uppsala University
Light & EM microscopy / FlowCytometry & Cell Sorting / Image Analysis ===============================================
Jeremy Adler PhD - Senior research engineer Light, Confocal Microscopy, Image Analysis
E-mail: [hidden email]
070-1679349

Dag Hammarskjölds v 20
751 85 UPPSALA, SWEDEN
http://biovis.uu.se/
Start - BioVis - Biological Visualization - Uppsala University, Sweden<http://biovis.uu.se/>
biovis.uu.se
BioVis - Biological Visualization

===============================================

-----Original Message-----
From: Confocal Microscopy List <[hidden email]> On Behalf Of Henrik Persson
Sent: den 23 augusti 2019 01:55
To: [hidden email]
Subject: Re: Random Question

*****
To join, leave or search the confocal microscopy listserv, go to:
http://lists.umn.edu/cgi-bin/wa?A0=confocalmicroscopy
Post images on http://www.imgur.com and include the link in your posting.
*****

This is indeed a very interesting off-topic discussion, thank you for starting it, Dr. Brown.

I have been thinking about it for a bit in the past few days and also want to weigh in.

Perception of colour is a very interesting topic, combining physics, biology and cognition. We have to keep in mind the spectrum of the sunlight (in space) and the wavelength sensitivity of our cones (maxima around 419,
531 and 539 nm [https://www.ncbi.nlm.nih.gov/books/NBK11059/], assuming we are not tetrachromats). On top of that, our brains will try to adjust what we’re seeing to interpret the colours correctly. A banana will appear more or less yellow regardless of the actual wavelength of the light it reflects, be it in the supermarket or in a spring forest. Recall the obviously black and blue dress from 4 years ago [ https://en.wikipedia.org/wiki/The_dress]

(Or in microscopy terms – we have to be aware of the dye’s emission spectrum, our emission filter’s band pass and camera sensitivity as well as any camera auto white balance we accidentally left on.)

The purple colour can potentially be caused by either purple wavelengths or by blue+red wavelengths hitting the retina. If it is the former, then for whatever reason, there has been very little scattering (purple has shorter wavelengths than blue light and would be even more prone to scattering – there might be conditions where purple light has a chance to scatter but not blue light, in say a very thin atmosphere or one with different composition). Reduced scattering can be caused by light travelling a shorter distance through the atmosphere and/or cleaner air. (Here we can start discussing the colour of the atmosphere on other planets with completely different atmospheric composition and density.)

The explanation of red+blue light for the purple sunsets might be more accurate. This could simply be caused by red light from the sunset being scattered down from the blue areas above. E.g. scattering of the underside of clouds or non-visible particles/vapour/ice crystals in the atmosphere. Indeed, when scrolling through pictures of purple sunsets, they all seem to contain clouds. (The non-cloud areas are also purple but may contain traces of
vapour.)

Searching a bit more on the topic seems to support this. There are several articles talking about the purple skies seen over Florida in the wake of hurricane Michael last October (lots of pictures and live recordings).
Articles attribute the purple to scattering from an increased amount of water in the atmosphere but do a poor job of explaining the phenomenon further (i.e. why purple and not blue).

This turned into a very long email, I hope you made it this far and found something interesting.

Cheers

/Henrik

*Quick recap on blue sky research (literally)*

As I’m sure you know, the light from the sun is white and contains all wavelengths albeit at different intensities. As the light enters the atmosphere, it scatters in a wavelength dependent manner, causing blue (short wavelengths) to bounce around multiple times, spreading out across the sky and reaching our eyes from all over. If you look at the sun (Don’t!), it appears yellow, basically white minus blue.

When the sun is low on the horizon, its rays has to travel through more atmosphere and slightly longer wavelengths are also scattered – only orange to red maintains a roughly straight path from the sun. The sun appears orange-red.

If you add particles to the air, scattering of longer wavelengths (than
blue) will increase and the sky will be more orange, especially at sunset with the long travel distance. One of the many gifts industrialization has given us – bright orange sunsets. Prior to the amount of soot, ash and pollution we have today, we had much more timid sunsets. See for instance this paper [https://www.atmos-chem-phys.net/14/2987/2014/] where Zerefos et al. studied old paintings to figure out what the sunset looked like prior to the industrial revolution. They also compare sunsets before and after volcanic eruptions. For instance, in 1883, the volcano Krakatoa erupted in modern day Indonesia and caused sunsets across the world to turn a deep red
- people even called the fire brigade, thinking nearby towns were on fire [ https://en.wikipedia.org/wiki/1883_eruption_of_Krakatoa#Global_optical_effects].

On Thu, 22 Aug 2019 at 14:20, Claire Brown <[hidden email]> wrote: