Is there a paradox over the sky being blue?

[Jez]

If I picture going out in an all sky blue boat dressed in a sky blue body suit with a sky blue mask and I take a piece of white paper, I picture a clear sky with an occasional cloud.

If I imagine waiting for the Sun to be obscured behind a cloud, I look down at the piece of paper in the bottom of the boat and, oh, it's white.

Is there any paradox that it looks white?

 

 

Edited June 8, 2023 by Jez

[StringJunky]

Try taking a photo of the experiment. You will likely find the paper tinged blue... this is because our brain is tuned to represent things as it thinks the way they should be, and so it filters out the blue*. Make sure the camera doesn't automatically white balance as that will nullify the blue. This is a quirk of brains rather than a physics issue. iirc the wavelengths add up to a more purple sky than we see.

* Our visual system has evolved to maximize contrast for the better detection of threats. In this case it's colour contrast rather than light-dark contrast.

Edited June 8, 2023 by StringJunky

[exchemist]

53 minutes ago, Jez said:

If I picture going out in an all sky blue boat dressed in a sky blue body suit with a sky blue mask and I take a piece of white paper, I picture a clear sky with an occasional cloud.

If I imagine waiting for the Sun to be obscured behind a cloud, I look down at the piece of paper in the bottom of the boat and, oh, it's white.

Is there any paradox that it looks white?

 

 

The brightness of the cloud obscuring the sun will be generally greater than that of the sky or blue-coloured objects, so most of the light falling on the paper will still be white, even if there is a cloud preventing direct sunlight from reaching it. Consider: you can gaze at a blue sky far from the sun without discomfort, whereas looking at a cloud in front of the sun often involves screwing up your eyes. 

I suppose that, if the cloud were very dark (which it would never actually be in an otherwise blue sky, but never mind) then most of the light reaching the paper would be blue and it would consequently appear blueish. 

[Jez]

30 minutes ago, exchemist said:

The brightness of the cloud obscuring the sun will be generally greater than that of the sky or blue-coloured objects, so most of the light falling on the paper will still be white, even if there is a cloud preventing direct sunlight from reaching it. Consider: you can gaze at a blue sky far from the sun without discomfort, whereas looking at a cloud in front of the sun often involves screwing up your eyes. 

I suppose that, if the cloud were very dark (which it would never actually be in an otherwise blue sky, but never mind) then most of the light reaching the paper would be blue and it would consequently appear blueish. 

The way I have tested this is to look at a white piece of paper down the bottom of a deep blue bucket with the opening pointed away from the Sun. It's still white. How can Sunlight get from the Sun and scatter at just the right angle to go into the bucket. What is it scattering off above?

[mistermack]

It's what Stringjunky said. Your brain is what 'sees'. Your eyes just process the light. Your brain automatically interprets the image, before it displays it in your mind. If everything has a slight blue tinge, the brain compensates.

In your bucket example, the sides of the bucket will have the same blue tinge as the white paper, but your brain removes it, and displays in your mind what colours it has calculated are actually there. I wouldn't think that it could do it so well for stronger tinges, but the blue tinge is usually very slight, as there is nearly always a bit of haze making the sky look closer to white. 

I would expect the same thing to happen when there is a red sky above, the brain would correct for it, as everything would have the red tinge, including the paper.

[Jez]

1 hour ago, StringJunky said:

Try taking a photo of the experiment. You will likely find the paper tinged blue... this is because our brain is tuned to represent things as it thinks the way they should be, and so it filters out the blue*. Make sure the camera doesn't automatically white balance as that will nullify the blue. This is a quirk of brains rather than a physics issue. iirc the wavelengths add up to a more purple sky than we see.

* Our visual system has evolved to maximize contrast for the better detection of threats. In this case it's colour contrast rather than light-dark contrast.

As you have alluded to there, I think this is a perception issue. An optical illusion of which there are many brilliant examples.

My wife and I would disagree all the time on what was green or blue. I think perception of green is the issue.

I snapped the plots off wiki page about this, https://en.wikipedia.org/wiki/Diffuse_sky_radiation, and realigned them;-

 

Seems to me that 'most of the photons' from the sky are actually 'green' (if one can say an individual photon even has a colour), but the ways our eyes work (we can't see the peak colour of black body green radiators) and the relatively small but positive shift due to Rayleigh scattering give us the sense that it is blue. I think the sky would be more accurately described "bluish-white light" but our visual attention focuses on the blue because that's an unusual colour in nature.

With all those green and red photons filling my bucket, more than blue, there is plenty enough of all colours to expose the true colour of the page.

 

Another case of this, I think that 'recent' dress example is amazing.

I see white and gold, and simply can't imagine how anyone can see anything else.

 

1 hour ago, StringJunky said:

Try taking a photo of the experiment. You will likely find the paper tinged blue... this is because our brain is tuned to represent things as it thinks the way they should be, and so it filters out the blue*. Make sure the camera doesn't automatically white balance as that will nullify the blue. This is a quirk of brains rather than a physics issue. iirc the wavelengths add up to a more purple sky than we see.

* Our visual system has evolved to maximize contrast for the better detection of threats. In this case it's colour contrast rather than light-dark contrast.

I have done this and there is a problem. Cameras can't automatically pick a white balance. We can make cameras follow an algorithm that repeats our expectations of what we see, but (I think we already realise) it's the brain that paints a picture, not the eyes seeing a reality.

I can take a picture and make it look white or bluer, I just have to toy with the white balance. I can't figure what is reality. Like the dress. It is unbelievable to me that anyone can see black and blues.

Edited June 8, 2023 by Jez

[swansont]

4 hours ago, Jez said:

The way I have tested this is to look at a white piece of paper down the bottom of a deep blue bucket with the opening pointed away from the Sun. It's still white. How can Sunlight get from the Sun and scatter at just the right angle to go into the bucket. What is it scattering off above?

It’s scattering off the molecules in the atmosphere, which is everywhere. There’s always going to be some light scattered in your direction

As far as the viewing perception goes, I have another anecdote: in my lab I would wear laser safety glasses, which blocked the red (and NIR) part of the spectrum. When I took them off, everything looked pink. My brain had adjusted the perception to include the red that was absent- i.e. it “assumed” white light was present - so I would see the mentally-added red for a minute or so, until my brain readjusted.

You can also see more spectacular afterimage effects

https://en.m.wikipedia.org/wiki/Afterimage