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This may sound like a daft question, considering a filter would be the easiest way to make this change. However, I was wondering if white light could be changed into blue light by manipulating scattering, or something like that? I am NOT a physicist and I have barely grasped the fundamentals of light so please correct me if my knowledge of the technicalities are wrong.

If the sky appears blue because blue light is scattered more than other colors, then why is the light that comes through the window during the day not distinctly blue? Would there be a way of achieving a blue light stream, without using a filter that blocks out other wavelengths? So for instance, a hollow glass cube that contains certain particles? Would it do anything to have this cube packed with nitrogen, for example?

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  • $\begingroup$ When you say "the light that comes through the window during the day not distinctly blue," what do you mean by "distinctly blue?" You will find that the light is much bluer than daylight, with less long-wavelength light. However, your brain does a LOT of processing on the scene it sees, and corrects for this. The famous example of this is The Dress $\endgroup$
    – Cort Ammon
    Jan 28, 2021 at 0:18
  • $\begingroup$ I see! I’m not familiar with the physics behind this, so I think my wording may have been a bit off. What I mean is that the light coming in through the window in the morning does not appear to be blue light, it simply appears white. Do you mean that it IS actually blue light? Sorry if I’m misunderstanding. $\endgroup$
    – Snfkin
    Jan 28, 2021 at 0:47
  • $\begingroup$ Yes, it actually is quite blue (obviously not blue like what comes off an LED, but far more blue than broad daylight). Our brains do an astonishing amount of processing before we get to the colors that we actually perceive. If you go to a photography forum and grumble about "white balance" they'll talk you ear off about this effect! One popular theory is that we evolved it to better track prey when they bound in and out of shadows (which changes the color of light falling on them) $\endgroup$
    – Cort Ammon
    Jan 28, 2021 at 1:16
  • $\begingroup$ It is especially noticeable in underwater photography. The water filters out red, similar to the situation you speak to, but it does so much stronger. Your eyes compensate rather quickly, and you see red hues, but if you take a picture, they look black. It's really frustrating! On the other hand, if you bring a strobe down, with a very white light, you get dazzling colors in the picture that you couldn't even see with your eyes! $\endgroup$
    – Cort Ammon
    Jan 28, 2021 at 1:33
  • $\begingroup$ That’s quite interesting! So assuming our brains did not do all of this processing, would our surroundings (during the day) look blue, or at least slightly blue in hue because of the light? $\endgroup$
    – Snfkin
    Jan 28, 2021 at 14:28

3 Answers 3

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If the sky appears blue because blue light is scattered more than other colors, then why is the light that comes through the window during the day not distinctly blue?

Because most of the light that comes through the window is not sky-scattered light. On a clear day, much more light makes it to the ground, where it is reflected by objects. This light is not distinctly blue, and the intensity is much greater than that of the sky.

To change white light (a collection of multiple frequencies) into blue light, you need to do something with a large proportion of the non-blue frequencies. Here's what I can come up with:

  • frequency dependent absorption of transmitted light (filtering)
  • frequency dependent reflection (structural coloration)
  • frequency dependent scattering (reflection/diffraction)

The first is simple absorption which it sounds like you don't want to do.

The second is also a type of selective absorption, but the method is different. Structural coloration may be different enough from a filter for your purpose.

The third is harder unless you have a narrow source. With a narrow source you can use a prism or a diffraction grating. The narrow source means the different angle of reflection can be controlled and you only select the frequencies you want. But if your source is diffuse, you can't control the angles. Light from one direction might give you your blue light, but the same prism will take light from another direction and give you red light.

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  • $\begingroup$ Thank you for the insight! I’ll look into the options you listed. I had in mind a large object (say, a curtain) so perhaps diffraction grating won’t work since the stream of light would be quite broad. $\endgroup$
    – Snfkin
    Jan 28, 2021 at 14:31
  • $\begingroup$ Also, say if you had a large glass block completely packed with particles in front of a window, could the light shining through appear blue because of scattering? $\endgroup$
    – Snfkin
    Jan 28, 2021 at 15:11
  • $\begingroup$ Maybe a bit. Smoke can often have a blue cast. But the diffuse nature of the source means that the efficiency is low. $\endgroup$
    – BowlOfRed
    Jan 28, 2021 at 16:32
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why is the light that comes through the window during the day not distinctly blue?

Are you so sure that it's not?

Color perception is a slippery concept. Our eyes and our brain are evolved to see a consistent view of the world even under changing lighting conditions. If a thing is "white" it will look white regardless of whether it is illuminated by the light of the midday Sun or, by the reddish light of the setting sun or, by the scattered blue light from the sky.

This used to be a problem when color photography was new. People would take pictures on so-called "daylight" film, and when the prints or the slides came back, they would look at photos taken by the light of the setting Sun and ask, "Why did these all come out red?" They would look at photos taken in the shadows on a cloudless day and they would ask, "Why did these all come out blue?"

Pretty soon, photographers talked about "white balance" and finding the "white point," which meant, adjusting the colors of photographs to compensate for a colored light source. In the digital age, "auto white balance" happens in the camera, and it usually does a pretty good job...

...until you try to take a photo of a beautiful sunset, and you ask, "Why are the colors all washed out?"

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Certainly. You can buy something called a diffraction grating which is a thin piece of transparent plastic containing a huge number of extremely tiny grooves in its surface. When you shine white light on the grooves, they break up the light into its constituent colors and reflect it back towards you. One of the resulting rainbow color bands will be blue, and there's your blue light.

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  • $\begingroup$ Ah, thank you for your reply! Again, I’m a beginner when it comes to anything physics related, so I wanted to clarify. Is this diffraction grating only able to show the entire spectrum of colour (in separate bands)? I’m trying to think of a way to get ONLY blue light, so if this is the case, it wouldn’t quite work. $\endgroup$
    – Snfkin
    Jan 28, 2021 at 0:45
  • $\begingroup$ the grating yields a complete rainbow of color. You then block off everything but the blue. $\endgroup$
    – niels nielsen
    Jan 28, 2021 at 2:50
  • $\begingroup$ I see! Thank you $\endgroup$
    – Snfkin
    Jan 28, 2021 at 14:32

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