What exactly is happening here? What is the scientific principle involved?
For those who don’t want to watch the video:
They have taken a bit of frosted glass - due to the frosted-ness you cannot see through it.
Then they put tape on it, creating an area under the tape that is no longer opaque (is less opaque).
Here’s my take on it. Frosting glass means you rough up the surface, so there is not a clean smooth plane of glass letting the light through unmolested. The light is scattered by the rough surface, most of the light makes it through, it’s just going in random directions.
Stick a piece of tape on it, and the clear adhesive fills in the rough surface, so the light is no longer exiting the glass through a rough surface, it’s essentially exiting through the smooth surface of the tape.
This is the same principle behind wet T-shirt contests. Cellulose (which is basically what cotton is) is not itself opaque; it just has a different index of refraction than air, so a complicated mess of cellulose fibers going every which way essentially scatters light randomly, making it appear white. Water, while it doesn’t have the same index of refraction as cellulose, is nonetheless relatively close (at least, compared to air), so with less of a difference of index, there’s less scattering, and more light can go through more-or-less unscathed.
So… cloth in a vacuum is transparent?
Edit: I’ve read that intelligence agencies use liquid freon to make paper transparent so they can see through envelopes without needing to open them and without leaving any trace. Is this the same principle?
Cite for this phenomenon? Videos and pictures preferred.
No, vacuum has for most practical purposes the same index of refraction as air (vacuum has an index of exactly 1, while air has an index of about 1.0003). Any two substances with differing index of refraction will cause some scattering, while substances with closer indices will cause less scattering. For comparison, water has an index of about 1.33, while glass is somewhere in the vicinity of 1.5ish (it varies with the kind of glass).
Addendum: Cellulose, apparently, has an index of refraction of 1.54. So there’s still some difference of index of refraction with water, but not nearly as much as there is with air. But benzene (1.50) would make for much better wet T-shirt contests than water (other than the minor problem of the participants being poisoned).
So why is Polyethylene (1.54) transparent while Cellulose (1.54) isn’t?
Light that goes from air into Polyethylene should scatter at the same rate as when it goes from air into Cellulose, no?
Polyethylene is only transparent if it has a smooth surface. Light rays will always be bent some when they cross a surface between two different indices of refraction (unless they hit the surface exactly straight on), and the angle they bend by depends on the angle they hit the surface at. If you’ve got a smooth surface, then all of the light from some source will be hitting the surface at more or less the same angle, and so it’ll all bend together and stay unscattered. But if you’ve got an irregular surface, like a T-shirt or frosted glass, then the light rays will hit the surface at all sorts of different angles, and therefore bend in all sorts of different directions, and thus scatter.
Incidentally, am I the only one who thinks of Eliot’s Hollow Men every time I see the title? “…Like wind through dried grass, or Scotch tape on frosted glass…”
Anyone who has cleaned that type of glass has probably noticed the same effect when the liquid cleaner is smeared on but hasn’t yet evaporated.
I’d also like to note that there are generally two kinds of frosted/non-glare glass. The kind used in the OP video is the type that has a etched surface, often only on one side. The other kind has multitudes of minuscule bubble throughout the interior of the glass. The trick shown would not work on this second type, but it’s an old-fashioned method for non-glare and unusual to find.
Then why did you say the close index of refraction between water and cellulose makes a wet T-shirt transparent. Doesn’t the water just make the surface of the cellulose smoother and therefore more transparent?
No, it also makes the light refract less while going through the cellulose/water “blend”. And if water didn’t have a closer index of refraction to cellulose than air has, neither this nor the smoother surface would matter.
Also, if you shred the polyethylene into fibers the size of cellulose fibers, it will be opaque, and turning cellulose into a thin film, as in cellophane, makes it transparent.
I see.
So if I were to somehow get liquid polyethylene and throw it on a cotton t-shirt, that shirt would then become transparent?
Yes, or as Chronos mentioned, benzene.
Have I mentioned that I love this place?