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I was walking outside one cold afternoon with my mask on and my glasses began fogging up. The mist was initially gray.

I kept walking without cleaning my glasses and eventually enough mist collected that that it transformed into clear water droplets.

This got me thinking: why is mist gray but water clear? Or perhaps more specifically, why are smaller water droplets gray and larger droplets clear? I couldn't find any explanation online. What is the physics behind such shenanigans?

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Mist is a suspension of tiny water droplets in air.

Light traveling through the mist gets randomly scattered, mainly by bouncing of the droplets. That makes mist far less transparent than bulk water.

I don't think mist is literally gray in colour but the fact that mist is far less transparent than pure air (or bulk water) causes it to look the way it does.

Other suspensions like smoke (a suspension of tiny solid particles in air) look quite similar, due also to light scattering. Another example is very much diluted milk (an emulsion of fat droplets in water, mainly).

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    $\begingroup$ Yes, clouds are white, mist just feels grey. Snow is also white, it is the multiple scattering at all those surfaces. Just like salt, sugar, paper, blond hair, etc. $\endgroup$ – user137289 Feb 21 at 23:20
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    $\begingroup$ Mist will stop feeling gray once you look at the Sun through it (even if you hide the solar disk itself behind an obstacle). In this case it'll be blindingly white—unless there's a really thick layer of it. $\endgroup$ – Ruslan Feb 22 at 9:22
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    $\begingroup$ @Pieter: The color depends on a lot of factors. Clouds can be anywhere from white through various shades of grey to nearly black. Mist or fog often appears white, if it's lit sufficiently, say by the sun above a low-lying fog. And the valley fog that appears grey when you're in it is blinding white when you look at it from above. $\endgroup$ – jamesqf Feb 22 at 17:45
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    $\begingroup$ Gray is like brown, in that neither is a "real" color. There is only "less intense" white or orange. The difference between "white" and "gray" is all about perceived intensity (same with orange vs. brown)... and that's not even getting into "white balance". $\endgroup$ – Matthew Feb 23 at 14:45
  • $\begingroup$ That is called the Tyndall Effect $\endgroup$ – michi7x7 Feb 23 at 20:46
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Regarding water droplets collecting on the surfaces of your glasses:

Those water droplets backscatter the incoming light in random directions, including ones away from your eyes. This means that any glass lens surface populated with water droplets will appear less bright than it would without the droplets, and the random scattering will obliterate anything you might otherwise be able to discern through the lens. In addition, small droplets crowded closely together cannot be resolved by your eye as individual droplets. Result: uniform smooth gray appearance.

As the water droplets begin to merge, you start being able to see objects through the lens and through the water droplets and the result is less dim (gray).

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    $\begingroup$ I prefer this answer to the accepted one. I think the key factor (correct me if I'm wrong) that you touch on is that light is scattered due to the curvature of the droplets. With a big droplet you can see through it because there is a large enough surface where the angle of incidence permits light to pass more or less straight through. A person can see the edge of those droplets which indicates the opacity increases as the exterior of the droplet curves away. Larger droplets tend to be flatter too which helps. With smaller droplets there is a lot more opaque edge regions. $\endgroup$ – Eric Nolan Feb 23 at 11:59
  • $\begingroup$ @EricNolan, I agree that's the essence of it, thanks for your comment. $\endgroup$ – niels nielsen Feb 23 at 16:20
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There are two factors at work here. The first is whether water is "clear", meaning, allows most light to pass through it. And it is, whether it's solid or in a bunch of tiny droplets.

The second is whether you can see through it. In order for a substance to be "transparent", it has to have two properties. First, it has to allow a non-trivial percentage of light to pass through. Materials may absorb a significant percentage of light and still be transparent (e.g. apple juice). In order to see through a material, however, there must be limited scattering; that is, the photons passing through must retain the same relative orientation, otherwise the image you see will be distorted, like a picture taken with an out-of-focus lens, only worse.

This phenomena is actually all around you. To start with, consider a lake. You probably know that if the lake is very still, you can often see through the water. As waves form, what you can see becomes more and more distorted. Now, if you imagine those waves as being really tiny, you start to understand what's happening when you look at mist. It's not that light isn't passing through the water droplets, it's that it's being scattered (refracted) in all directions such that you can't see a clear image. Again, like an out-of-focus photograph, only more so.

You don't just see this in water, either. The difference between "regular" and "frosted" glass is exactly the same; a smooth or (microscopically) rough surface either allows a clear image or a very blurry one. You also see something similar in metals; a rough surface is "shiny" but doesn't produce a clear reflection, but with enough polishing — that is, increasing the surface smoothness — you can get a "mirror finish".

As to the second part of your question, water, due to surface tension, naturally has a very smooth surface. As water collects and transitions from many small droplets with lots of scattering, you wind up with a smoother surface and less scattering.

As to why it's gray... that's the simplest of all. "Gray" is a uniform mixture of all colors of light. By definition (given the way our eyes naturally "white balance" for ambient light), the average of all light you are seeing at any time is gray. If you take all the light in a given environment and mix all those photons together, you will always get gray. Similarly, if you take just about any photograph and blur the ever loving snot out of it, you'll get gray. (This works because water, at least in the sorts of quantities we're talking about here, does not significantly absorb any particular wavelength more than others. If your mist instead absorbs most blue light, you'd get yellow, and so forth.)

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    $\begingroup$ An interesting property of frosted glass: put a piece of clear tape onto it, and you'll suddenly be able to see right through! $\endgroup$ – Dan Henderson Feb 22 at 23:53
  • $\begingroup$ @DanHenderson, indeed. More generally, anything with a similar refractive index can effectively smooth out a rough surface (optically speaking, anyway). In the case of tape, I believe it's the adhesive doing the filling. $\endgroup$ – Matthew Feb 23 at 0:43

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