Physics Of Dramatic Blue Sky

Recently, I’ve been studying/viewing some Ansel Adams work to learn more about his approach to black and white photography and one theme popping up over and over was his frequent use of red filters for landscapes. It’s been years since my biophysics classes, so I’ve decided to do some minor reading, study some more, dust off old forgotten knowledge of mine and write it all down. If not for anyone else’s benefit, then for my own and re-visit these notes in future.

Back to physics

As you may already know the wavelengths of light spectrum visible to human eye are “roughly” between 380nm in violet spectrum and 720nm where visible red spectrum ends (or 400-700, depending on whom you ask). Because the sun as a source of light emits waves at all the lengths within the above band + much much more, the sunlight is more or less perceived as pure white light. First point: humans see colors, which are reflected from objects, not emitted by the objects, so if we see sky being blue it is a result of blue light being reflected and scattered in all directions as the sun rays pass through the atmosphere.

Simplified light spectrum infographic (not proportionately accurate). Star marks 590nm cutoff wavelength of my IR camera

That’s where Tyndall Effect  and Rayleigh Scattering come in, where the latter postulates that the intensity of scattering is inversely proportional to the 4th power of the wavelength. Meaning the smaller the wavelengths (violet/blue side of spectrum) the more they tend to scatter and the opposite applies to longer wavelengths (red side of the visible spectrum). As blue part of the light passes through air it scatters more than red light spectrum does, hence if you look up into the sky perpendicular to the sun (you look at the sun rays from their side of travel), you see deeper blue, but when you look at the sky facing the sun (obviously don’t look into the sun directly!) the blue sky color will become much more pale and grow more or less white the closer to facing the sun you get (typical light halo around the sun).
It is also worth noting that Rayleigh Scattering is dealing with scattering of light by very small particles (molecules) as we know from a blue sky, while Tyndall Effect, although very similar in its theory, is more interested in scattering of light in colloid substances with larger particle size as we know from sunrise/sunset skies where the air is polluted by other solid or liquid particles (dust, droplets of water, etc..).

Notice the halo around the sun. The farther you go away the deeper the blue becomes. This is not due to vignetting or editing. Out of camera JPEG

Why blue sky?

What’s causing that blue then? Unlike previously believed, the color of the sky is not caused by presence of water. Because, if that were correct, then the sky’s hue would change based on how much water (precipitation, mist) was present in it at any given moment and clouds would be blue. Let’s pretend for a bit that we are talking about clear cloudless sky with minimal humidity where the sky is always blue no matter what, and when the sky will be full of hot humid air on a clear day it too will be just a blue sky. So, the only explanation is that the diffraction (scattering) of the blue spectrum more than the green or red bands is caused by the air molecules themselves (oxygen and nitrogen) and not by the water or other larger particles (Tyndall’s effect of light scattering in colloid substances).

Infrared 590nm out of camera JPEG. Sun was just to the left of the camera. Notice the dramatic drop off on right perpendicular to the sun, which is bright deep blue scattered light that IR cannot see


As we read above, the blue light spectrum is scattered in all directions away from the original heading of the sun ray, interacting with other molecules and being reflected again and again in all directions and phases of the energy waves. This causes the blue to become more of a blue “hue” (as we know and love) rather than a strong primary blue color. This diffraction of blue spectrum as the light travels through the atmosphere also has one interesting property – helping us in photography – if we limit that blue light from flying in all directions and try to limit it to travel in only one heading, the blue light will become way more prominent and deeper and that’s what polarizer filters do. If you looked at a polarizer under a microscope you’d discover a fine “mesh” allowing the energy waves of blue frequency band to travel in a single phase, exaggerating the final result of what we see. Hence those deeper blue skies…
This is why it is so imperative that when you use a circular polarizer filter on your lens you always want to have the sun to your side and never be facing into the sun, because while heading in the direction of the sun, there is not much blue spectrum left and the filter’s effect is greatly diminished. See sunset section below on why that is.
As an aside, any time you point your camera in a different direction while using a circular polarizer filter you should always adjust it, because your change in heading also changes the phase under which the blue light is being reflected towards you, so if you play with the filter and constantly rotate it to find the deepest blue sky as you move around, instead of setting it up once and forgetting about it, your pictures will be better for it.

Red sunsets

When the sun is low above the horizon (sunset/sunrise) its rays travel through a much longer length of atmosphere, unlike when it is right above our heads and only has to penetrate the thickness of the air mass before reaching us, therefore, light diffuses in greater amount while the sun is low. As I’ve describe above with Rayleigh Scattering color blue diffuses at way higher rate than red spectrum, so by the time the sun ray travels and meets our eye head on most of the blue spectrum has been depleted/scattered while green and red spectrum light still remains, therefore we see yellow, orange, red sky hues during sunrise or sunset and the lower the sun the closer to red hues we get. This would explain that weird phenomenon when you look towards the sun during sunset the sky would tend to be in yellow-ish or red-ish hues and if you looked perpendicular to the sun (sun on your side) the sky would still look blue. That is the difference between observing the sunrays head on (red/orange) vs. seeing them from their side (blue). Tyndall Effect, which talks about scattering of light on larger particles, also plays a role here: the lower the sun above the horizon the longer the light travels through lower strata of the atmosphere where more pollution is (be it natural weather elements or man-made air pollution, which further increases the diffraction and loss of blue light a the ray travels. That’s part of the physics behind the famous warm light of the Golden Hour…

And the red filters?

Again, everything human retina is capable of seeing is reflected light, not emitted light. The way color filters work is that they let their predominant color pass through while blocking the other colors to varying degree. So, one could think of these as band pass reject filters: they pass their own color and reject all other colors. When you look through a red filter, everything looks red as that filter passes mostly red light. If you look at the spectrum chart at the beginning of this post, you will see that blue and red are on the opposite sides, so the farther from each other the colors are on the spectrum chart the more pronounced the attenuation (suppression) of the opposite color is.
This is why a yellow and green filters will have quite subtle effect on B&W photo film, their frequency bands sit in the middle of the spectrum with not to much distance to the left or right extremes of visible light colors, but red and blue filter will gave rather dramatic effect as these colors sit far apart. Since the sky is seen as blue, using a red filter will block significant amount of blue frequency (not letting it into the camera), thus turning the blue sky darker. Conversely, a blue filter will block lots of red band, making red objects way darker. In IR photography specifically, blue filter is redundant as IR cameras cannot see it to begin with, so blue filter will have no significant effect.
If you want to read more in-depth on what each filter could be used for, this article has a very nice write up with examples.
In the age of digital photography, this may not seem to matter, because the camera sensors capture all of the visible spectrum and beyond. Where this knowledge comes in handy is when you use post production to convert your images to monochrome. I will be writing a quick post on that as well, to remind myself of the theory.

Out of camera JPEG in IR with simulated red filter darkened the sky. If processed in RAW, I would be able to turn upper part of the sky into pitch black.

Alright, so I just wrote well over a thousand words talking about the physics behind color while studying how to improve black & white photography and still barely scratched the surface. Weird and exciting at the same time.

Any comments or suggestions? Hit me up. I’m always looking to learn and improve.

Categories: TutorialsTags: , , , ,

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