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I am looking for an end-result where one would not be able to differentiate between land and horizon. And I want it in a desert. So either some new, exotic material has to make-up the sand. Or it has to be mirage everywhere in the distance instead of some spots in the distance.

This is in a desert on a tidally locked planet where I want this 'transparency/mirror' effect. However, from what I know, sand has impurities and hence is not transparent.

  1. Are there materials that can be used to create near transparent sand / dust?
  2. Or can I increase the temperature so much that the entire desert is in a state of a constant mirage from any point of view so that you always see water on the surface which gives the illusion of transparency/mirror effect?
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    $\begingroup$ From some sentences I feel like you want a mirror and not transparency. Could you clarify this beyond doubt? $\endgroup$
    – Trioxidane
    Nov 6, 2021 at 19:44
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    $\begingroup$ Sand cannot be transparent, at least not under air. Even if made of the finest optical glass, sand will appear white and not transparent. (Because sand consists of many many many tiny little grains, and the reflections on their surfaces will dominate over the transmission of light. It is the same reason why ice is transparent but snow is white although they are made of the exact same material.) (But you can have invisible glass or quartz sand if immersed in water, because water and glass or quartz have almost the same index of refraction.) $\endgroup$
    – AlexP
    Nov 6, 2021 at 19:52
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    $\begingroup$ Mirage is way easier. $\endgroup$
    – Willk
    Nov 6, 2021 at 19:53
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    $\begingroup$ @trioxidane , yes, mirror is the right word. I am sorry for the confusion I caused. I have edited the question. $\endgroup$
    – mukul215
    Nov 7, 2021 at 7:34
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    $\begingroup$ (1) Did you mean to write 'between land and ground'? (2) A desert is a waterless, desolate area of land - do you need yours to be made of some kind of sand, or could it be a solid crystalline plane? $\endgroup$ Nov 7, 2021 at 8:45

6 Answers 6

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It's not just a matter of impurities, it's the physical properties of the material making up the sand. The most obvious one is the refractive index. At first it doesn't seem like that big a deal because water (1.33) doesn't seem that different from clear silica (1.52), and silica is used for optical glass, but the important thing to remember is that the refractive index kicks in when light transitions from one material to another. When you look through a body of water, you're only looking at one location where the light is "bent", namely the interface between the body of water and the surface. But if you weren't looking through a single interface, then things become a lot more complicated.

For instance, look at this image:

enter image description here

You can't see through relatively thin layers of water making up the waterfalls and rapids because you aren't looking at one transition where light gets refracted, but many as water and air are mixed and any given light ray transitions between water and air and back again many times, effectively scattering it. The total combined refraction means the material becomes opaque.

Here's another example from underwater:

enter image description here

Notice how the mass of bubbles completely hide what's behind them. It's not a matter of the bubbles individually being hard to see through; after all, they're composed simply of air which, in the total amount present, would be totally transparent. It's all the refraction between individual bubbles and the water which causes the scattering of light which makes the whole thing opaque.

The same thing would happen if you had a bowl of optical-grade glass grains. Even if each grain was optically transparent by itself, the total refraction of all the transitions between glass and air would combine to make the material in the bowl opaque.

In fact, here's a picture of that very thing:

enter image description here

And the smaller you make the glass particles, thus creating more and more surfaces light is refracting through, the more and more opaque the material becomes:

enter image description here

Eventually, finely ground glass looks like a plain white powder like salt, sugar, or talc.

The only way your desert could appear transparent is if the material making it up has a refractive index exactly identical to the air, fluid, and any other material that may be between the grains, which all have to be the exactly the same as each other as well.

ADDENDUM

Even given the above, just because you have materials with the same optical properties doesn't guarantee transparency because you need near atomic-level matching perfection at the interfaces. Any kind of imperfection will result in diffraction and scattering. For an example, consider automobile safety glass.

enter image description here

The exact same material as perfectly clear glass, only now it has fractures in it, and those fractures cause interfaces within the material itself where diffraction happens even though the material on both sides of the fracture is exactly the same. The result? Harder to see through. Enough fractures in three dimensions and on a small enough scale and you can't see through it at all, even though the fragments are right against each other. Internal fracturing is why, for instance, otherwise perfectly pure silica, calcite, or salt crystals can look cloudy.

Long story short, not only would all those materials making up the desert have to have the exact same optical properties, each would have to have homogeneous internal structure as well. So yeah, good luck with that.

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    $\begingroup$ Absolutely this, I came here to say this, Keith has done a better job though. $\endgroup$
    – Ash
    Nov 7, 2021 at 5:09
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    $\begingroup$ wet sand could be almost transparent. Water(1.333) and silica(1.458) are a bit far apart still, but how about clean sand wet with vegetable oil(1.47)? Or a 60% saturation sugar solution in water, which is an exact match? ((explaining why the desert feels and tastes more like a dessert is an exercise left to the student)) $\endgroup$
    – PcMan
    Nov 7, 2021 at 6:51
  • $\begingroup$ Wow, what an excellent explanation of why refractive index matters so much. $\endgroup$ Nov 7, 2021 at 16:16
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    $\begingroup$ @PcMan, no, it couldn't. Even though the diffraction index might only have a small difference, you're multiplying that small effect hundreds, thousands, perhaps millions of times, and you're back to where you started. To put it another way, you might be able to pull off the old detective novel trick of hiding one diamond or some other gemstone in a fishtank or some other fluid, because it's "invisible", but the more you have the less it works. $\endgroup$ Nov 7, 2021 at 18:14
  • $\begingroup$ C.F. physics.stackexchange.com/a/206591/6698 $\endgroup$ Nov 8, 2021 at 19:36
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Temporary mirror

If you want a temporary mirror you can look at Salar de Uyuni. This is the largest salt flat on the Earth. It looks like a white flat plains with hexagonal lines on top. It is exceptionally flat for nature's terms.

The magic happens after rain. A thin layer can sit on top and be practically still. Then the whole salt desert is transformed into a 129km (80mi) in diameter mirror. Pictures there are amazing, as the sky and the ground seem to merge into one.

It might be possible to increase the time it looks like a mirror. To achieve this you need a steady supply of water, which is calm when it is added to the salt flats. It then must evaporate, never taking any salt with it. My best guess would be many springs in the earth slowly pushing up water. The springs are fed by a large underground river, or cave systems pressurised by sea water and heat. Or the like. The problem with anything except rain is the movement. Moving water can transport and erode the salt layers. Still it might be possible to have long periods if time on a large scale a visually practically flat mirror. But if you only need it to be a mirror sometimes the Salar de Uyuni is your best bet.

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  • $\begingroup$ Perhaps this salt flat plus an extremely heavy dewfall. Maybe it evaporates only on some days, only for a few hours, very late, so it's almost always a mirror. $\endgroup$ Nov 7, 2021 at 23:40
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"I am looking for an end-result where one would not be able to differentiate between land and horizon"

I believe that a warm and very flat desert ought to be enough (the same can be obtained with a thin layer of water), but if you have clouds in the sky, then you will be able to differentiate very easily even if, technically, the horizon border itself isn't (easily) distinguishable:

enter image description here

Alternatively, and probably more easily obtained, you can use dust to obscure the farther details:

enter image description here

Finally, what I call the "Mesklin trick" (described in Hal Clement's Mission of Gravity): if the air density gradient is exactly right due to its composition and temperature at all heights - actually the first 400-500 meters is enough - then you can have the illusion of the horizon being at an infinite distance. Light attenuation and extinction will then obscure that same horizon (it will disappear in a bluish-white haze due to scattering), making it impossible to tell exactly where land and sky meet; that might also appear to happen at a higher altitude than the viewer, wherever they are, as if they were at the bottom of a bowl (which is why Mesklinites believe the world is a bowl with them at the bottom).

enter image description here

The Mesklin effect appears identical to a reversed Fata Morgana, but it obscures everything below those 400-500 mt of height (Fata Morgana on Earth is limited to a few meters at most: "above" that limit, things return visible).

enter image description here

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"Do you know of materials that can be used to create near transparent sand / dust?"

Fill the gaps between the grains with a liquid that has the same refractive index. The liquid doesn't have to be water. It could be some biological goo, or a clear oil. If the match isn't perfect, you would likely get something translucent, like slush (ice crystals in liquid water). If you get a perfect match, the solid can seem to disappear entirely.

You would still be able to see the horizon, though. It would look like a lake.

Or sometimes, the simplest approach is best. Instead of sand or dust, simply fill a valley with a lake of once-molten glass. (Long-term volcanic activity melted the sand, or something.)

"EDIT: to clarify further, I am looking for an end-result where one would not be able to differentiate between land and horizon. And I want it in a desert."

That's much easier! If you can't make the ground look like the sky, then make the sky look like the ground. A constant dust storm raising a 'fog' can make it impossible to see long distances, and hide the horizon. It's a common problem in snow blizzards.

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  • $\begingroup$ This suggests a mechanism, too--the goo is emitted by some kind of photosynthesizing organism. So instead of being covered with glass particles and prevented from getting sunlight, it is now protected by glass particles that still let in plenty of light. You would then have a group of life forms develop around that--predators finding ways to bypass the glass defense, prey animals taking advantage of the glass defense as protection for themselves, etc. $\endgroup$ Nov 8, 2021 at 16:10
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Table salt

Some regions of the world are covered in meters of a salt crust due to ancient lakes evaporating and geology being a cruel mistress. The evaporation process also makes these places uncannily flat.

The most famous salt flat in the world is Salar de Uyuni. Some parts of it look like what you ask in your requirement no. 2:

Or can I increase the temperature so much that the entire desert is in a state of a constant mirage from the pov of viewer so that the viewer always sees water on the surface which gives the illusion of transparency?

A picture of the Salar showing extreme reflection

Another picture of the Salar showing extreme reflection

Notice that at a 3,656m elevation above sea level (~12,000 ft), the salar is rather cold than hot.

In these places you can tell where the horizon is by tracking the clouds. But if you have a planet that is all salt plains, like a water world that evaporated, you will have no clouds and hence it will be very HARD to tell where the horizon is. Ground far from you will just look like sky.

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  • $\begingroup$ You just blew my mind because I had no idea all the water could evaporate. Is it possible? Can we have a planet with no clouds? $\endgroup$
    – mukul215
    Nov 8, 2021 at 14:51
  • $\begingroup$ @mukul215 lots of deserts used to be salt water lakes or part of the ocean. Because of the salt it's difficult for life to trive, which makes the area not able to hold on to moisture to grow life. A negatively reinforcing circle. The problem in the answer is that the salt alone doesn't produce a mirror. On Earth you need a (tiny) layer of water to make this happen. If there's water on the planet, there's near certainly clouds. If you use something else that doesn't evaporate you'll likely not get a flat enough surface. $\endgroup$
    – Trioxidane
    Nov 8, 2021 at 17:54
  • $\begingroup$ @mukul water molecules are lighter than breathable oxygen, CO2 and nitrogen molecules so it is possible for the planet to reach a temperature in its history for which most water is lost to space but other gases are not. Please notice that oxygen is highly reactive and might not persist in its purest form (O2) for geological eras. Most likely you will have a planet with an atmosphere like that of Mars but with a thicker atmosphere. Or maybe Titan, which is mostly methane. $\endgroup$ Nov 8, 2021 at 18:39
  • $\begingroup$ I can do with the salt desert or dust option. One last thing before I close this, can the water on my planet be ammonia solution or does it have to be H2O for the salt desert explanation to work? If the water is ammonia solution, does salt have to be NaCl or can the salt then be something else? If it is workable with ammonia solution, I will simply state it as salt and leave it at that (salt will be like a black box in my story, internal composition not important to users). I ask this because ammonia worlds are cold, so I don't know if I can have a hot, dry desert there. $\endgroup$
    – mukul215
    Nov 9, 2021 at 1:31
  • $\begingroup$ @mukul215 I don't know enough chemistry to give a proper answer but I think at low enough concentrations the ammonia should have no effect on the water's color, refraction and reflection. $\endgroup$ Nov 9, 2021 at 3:02
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As Keith Morrison explain in his answer, even if you have a transparent material, it can't be transparent if you have a lot of interfaces with air. Therefore, sand won't be transparent even if individual grains of sand are.

For an actually transparent desert, you need to fuse the material in a single piece, which will look more like a pond of glass than a sand desert. Definitively, that would be a desert, because no plant is going to grow in a solid glass surface, and a cool place.

To explain such a glass desert you need:

  • The right composition: That's the easier part (in Wordbuilding), because optical glass is just made of mineral components, you can have them in the same place. In fact, some accounts of how glass was discovered involve some natural sand having the necessary components together.
  • Fusing the sand into glass: You need a lot of heat rather incompatible with conditions in an habitable planet (you want your planet to be inhabited, don't you?). A past volcanic event could have melted the sand, or the star on your planet could have gone supernova in the past and melted the planet surface long ago - now it's a white dwarf. How to get back an atmosphere could be a separate but solvable question.
  • Finishing the surface: Not all processes leading to glass leave an smooth surface. If you want your desert to be truly transparent, erosion might have played a role finishing the surface.

Btw, a transparent desert might be even cooler if there were something interesting to see through it. Embedded gems or fossil sandworms would look great.

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  • $\begingroup$ Could I have a surface layer of glassy or transparent material and worms and snakes crawling underneath? Would be very spooky. But that would pose a problem of habitable conditions underneath the surface, no? $\endgroup$
    – mukul215
    Nov 7, 2021 at 12:38
  • $\begingroup$ @mukul215 the worms would have a hard time getting air... plus the glass pane above them would probably get in the way of digging. $\endgroup$ Nov 7, 2021 at 12:41
  • $\begingroup$ @mukul215 as soon as you have a grainy layer below the surface layer, it becomes intransparent again. $\endgroup$ Nov 7, 2021 at 12:43
  • $\begingroup$ @PaŭloEbermann the grainy layer would be opaque, but the glass layer above it would remain transparent $\endgroup$ Nov 7, 2021 at 12:46
  • $\begingroup$ I wrote "fossil" sandworms, and I meant sandworms that became fossil when the glass melted. Of course, that poses its own set of problems about how it can happen. $\endgroup$
    – Pere
    Nov 7, 2021 at 12:48

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