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So the atmosphere scatters light on its way to earth, making the color of our sky. For example, when the sky is blue on a clear, sunny day, the sunlight appears somewhat yellow because the blue light has been scattered away. However, there is something about this that I do not understand. If certain colors are scattered away, why is there 99.5% of the time some white when you look at the sun (e.g. in pictures)? Even at red sunsets with reddish skies, the sun seems to be somewhat white - how can this happen if almost all the original colors/photons have been scattered away and thus the rest should not be able to constitute a whitish color?

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Qmechanic
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That Guy
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  • Of course it's still "whitish" and it's being scattered at thsame time. Being scattered doesn't mean that the 100% of blue light is scattered. Most of it gets lost, but not all of it. If you wore red glasses, you'd see less than in a sunset. – FGSUZ Dec 29 '18 at 18:04
  • So it is because that there still is some parts of all the colors? – That Guy Dec 29 '18 at 18:10
  • you are aware of the difference between the spectrum of light (frequencies) and color perception ? https://en.wikipedia.org/wiki/Color_vision . Though for the picture I think the explanation of paul young is fine – anna v Dec 29 '18 at 18:11
  • @annav Yes, but there still need to be some of all colors(RGB), right? – That Guy Dec 29 '18 at 18:18
  • look at this plot http://hyperphysics.phy-astr.gsu.edu/hbase/vision/colper.html . not overexposed, yes, white needs a lot of frequencies. – anna v Dec 29 '18 at 18:50
  • Plus response of our eyes or camera. As always dealing with these kind of question. Spectral distribution, contrast, atmosphere, saturation of detector,all counts. For instance The sun in the photograph was certainly brighter and whiter than the surrounding! You didn't consider the red sky to be only scattered light. ....... – Alchimista Dec 30 '18 at 09:43

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This is because the light detecting device - which might be your retina or might be a piece of photographic film - is getting "bleached out". So, too little light on a typical detector registers as black and too much light registers as white. You can think of the responsiveness of photographs and your eyes as being a hyperbolic tan(), or tanh(), function of the input. When there is too much light photographers call this "overexposed".

So, even though very little blue is making it through to the detector, the sun is so bright that even that little amount of blue is able to overflow the "blue detectors". Far, far more red and green photons are arriving at the detectors, but "maxed out" is "maxed out" and you just get pure white. Tanh(100) approx equals Tanh(10,000). So the detector loses the ability to realize there is more red light than blue light. And, then, approx equal detection of all colors means ... white.

So, imagine that the detector maxes out at 100 photons per second and there are just three colors, red green and blue. At lower light levels sunlight appears red, because there are 30 red photons, 5 green photons and 1 blue photon per microsecond. Clearly red dominates. But at 100x the brightness, looking directly at the sun, there are 3000 red photons, 500 green photons and 100 blue photons. But since the detectors all max out at 100 photons, all three detectors just say "maxed out" - and the result is white.

If one looked at the setting sun through a neutral filter that blocked out most of the photons it would again seem to be reddish. But looking directly at it and bringing the full power of the imaging device (the lenses of your eyes, for example) to bear is what gathers too many photons. At an oblique angle, or through a dim reflection, one might also see fewer photons and perceive the redness. Filters, reflections and observations at oblique angles all cut down on the number of photons that make it through.

Paul Young
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  • So brightness is what makes it appear whitish?`I still do not really think i completely get it. What makes our retina "bleach out" - not all the detecters are overflowed. – That Guy Dec 29 '18 at 18:00
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    I think this answer is nice to explain the effects in pictures. In fact, it's typical that cameras cannot register the picture as we see it because of that. The photos appear "burned" due to light excess. However, the human eye is wonderful and it's able to balance much more intelligently, althought it can also get blinded by a flare. – FGSUZ Dec 29 '18 at 18:08
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    The image of the Sun is overexposed. Have a look at the two photographs of the same scene here where the sky is overexposed in one of them. – Farcher Dec 29 '18 at 18:15
  • Ahh, I actually almost get it now, thanks. However, how can the brightness of the sun make the blue detector be "maxed out" even though there is a very limited amount of blue? – That Guy Dec 29 '18 at 18:58
  • Perfect, but as the final part: what makes the sun brighter when you look at it and how does that affect the photons? I know it sounds simple, but there surely is a scientific explanation. – That Guy Dec 29 '18 at 20:48
  • @That Guy sun it is bright, it is not looking at it making it brighter. To see direct light from the sun you must look at it in a straight line or via a mirror. There is no other way. Always. It is valid for every object (lensing in GR out of scope). The rest of the sky is diffused light. By contrast to sky dome sun might appear brighter at sunset. But it is not. During day you can't stare at it. .. – Alchimista Dec 30 '18 at 23:41
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To see the color of sunlight with your naked eye without having to correct for the sun's overwhelming brightness, you need to build a pinhole camera. Building a pinhole camera that allows you to see an entire landscape requires a substantial portion of skill or luck. But building a pinhole camera where you can see an image of the sun is so easy that it happens all the time by accident:

sun light on pillow

That white spot is an image of the sun. It is white because the Sun is white. With less stray light around it you could also see the blue sky, white clouds, and green trees that are near the sun from your perspective. With more distance from the pinhole you can make out sunspots on the image. During a partial eclipse the image is a crescent:

crescent suns

Now you have a simple experiment that you can do as early as dusk tonight to determine the unbiased color of the setting sun: go watch the sunset, and use your hand to make a pinhole sun that projects onto your white shirt. I bet it's still pretty white, because the Sun is white.

rob
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