Is it possible to create something I can hold in my hand, feel it’s weight, but not be able to detect visually?
To put some parameters around the question, I mean anything with mass, either solid or liquid. And I mean invisible to the unaided human eye.
One line of thought I had was about very clear glass or plastic. It’s not invisible, but it’s utterly transparent when viewed from a particular angle, therefore, from that angle it is for practical purposes invisible. But if you look at it on edge, it fails the transparency test.
I understand how vision works and that photons have a tendency to bounce off matter, but there are arrangements of atoms that allow almost all light to pass right through (from certain angles), e.g., the clear glass mentioned above. Seems like if you could get the atoms arranged just right, you could construct a solid that was transparent from all angles.
And if perfect transparency can’t work, perhaps if you had a substance that absorbed every scrap of light energy that fell on it. Seems like that be substance be invisible…
Or if you took an invisible gas and began condensing it…the instant it turns liquid, does it become visible, or is there even an infinitesimal time span where it is liquid yet invisible? I’ve never read it anywhere but it almost seems that visibility is one of the defining properites of liquid (and solid) matter.
So, I ask again, is it possible to construct a solid (or liquid) that is invisible? And, if not, why?
Off the top of my head, I’m going to say that it’s impossible for a solid to be invisible as you seem to be defining the term. If I understand your question, you seem to be looking for a solid which has identical visual properties to air. And I don’t think that’s possible because the higher density of a solid would cause a noticeable refraction of the light passing through it.
Actually, it would be the exact opposite. A substance which absorbed all light would appear absolute black.
I agree that holds true for the solids we find in nature, but couldn’t one be “engineered” that allows light to pass through it? (or around it?) or is there something inherent to the atomic structure of solids that always makes them refract some amount of light?
That’s right, you need something that not only passes visible light through it completely, but does not refract the light in any way. The idea of something absorbing all light wouldn’t work, because you have to see what’s behind the object; a perfect absorber would be a black hole.
My thinking:
If the different interactions with light between liquids and gasses were just a question of how close together the molecules were, then the visibility of air would change with pressure. Since this effect, if it occurs at all, seems to be negligible, I don’t think that’s the problem.
If you had a region of compressed air somehow magically confined in your hand, would it be visible? Yes, it would. Picture it - a little ball of air in your hand that is much denser than the air around it. It therefore has a definite boundary. As light passes through the boundary it will be bent, and you will see a distortion.
So it’s not the distance between molecules that affects the light per se, but rather the existence of boundaries between areas of different density. This is why you can see heat ripples rising off a road. You have two “objects” that are invisible in a background of uniformity (cold air in cold air and warm air in warm air). But when put together, before complete mixing has occurred, there are boundaries that form and bend the light. You see the same thing in the schlieren that forms when you mix a salt solution with water.
In order to have an invisible object in your hand, it would have to have optical properties of the exact same kind as the air (i.e. be made of the same kind of molecules) and be of the exact same density as the air. Only in this way can you avoid boundaries that will distort the light. Hence, you could not have a truly invisible solid or liquid object in air.
A diamond in water would still be visible, although not to the same degree as in air. Water has an index of refraction of 1.5, if I recall correctly, and diamond is over 2. One that you can do, however, (and for considerably cheaper than diamond, too), is glass in paint thinner.
By the way, water, glass, diamond, and paint thinner can all be made essentially perfectly transparent; the problem is not transparency, but refraction.
Well, water is invisible if you’re submerged in it; I imagine that air is invisible for the same reason. Water, air, and glass all bend light passing through them at different angles. I recall hearing that corn syrup or glycerine bends light at an angle so similar to glass that if you fill a tank with said liquid and then put glass in it, the glass will be indistinguishable from the liquid.
A little larger than hand-held, a Klingon Bird-of-Prey with its cloaking device operational would be so invisible that you’d bump into it in a sunny park while looking for the two kooks you had pizza with the other night. :D:D
I’d say that if I were immersed in molten lead I would be utterly unable to sense it visually in any way. And remember, in space no one can hear you scream.
As some people said above, I believe the key issue is the “index of refraction.” I recall from my high school physics book an interesting technique that was used to help capture “hit-and-run” drivers.
A special chemical is put in a cylinder which has an index of refraction that is known to vary with tempurature. Some shards of glass from the accident scene are dropped into the cylinder. Then the chemical is slowly heated up.
Eventually, the index of refraction of the chemical matches that of the glass. At this point, the glass seems to disappear. The temperature of the chemical is read, and the exact index of refraction of the glass can be looked up in a table. Another table cross-references indices of refraction with makes and models of cars. Thus (assuming that the car has original glass, that the right glass is tested, etc.) you can make a good guess as to the make and model of the car involved in the “hit and run.”
Anyway, I’m not sure what the index of refraction of air is, (and I wouldn’t be surprised if it varied with temperature, pressure, etc.), but I bet that if you had a very uniform solid object of matching index of refraction, it would be very difficult to see in the air.
I don’t know if the right substances exist - it could be that nothing that is solid at room tempurature has anywhere near the index of refraction of air. But, IIRC, index of refraction depends on two quantities - permittivity and conductivity. So it doesn’t seem outrageous that two very different substances could have the same index of refraction.
Another issue to consider is that for many (most? all?) media, the index of refraction changes depending on the wavelength of the light passing through. I believe this is why you can see rainbow-like effects in a prism. So you would want an object with the same (or close) dispersive properties as air. I imagine this would be a tall order . . .