I was at the house of a friend, and noticed that he had put a frosted vinyl film over his windows for privacy in his densely populated neighborhood. This film blurred the image from outside, but it was still possible to make out the outlines of the buildings across the street, and you could see when people and cars were passing by outside. I’m sure that from the sidewalk at night you could make out much of what was going on inside the house, especially for events taking place near the window. So the film transmits a lot of light, but also passes a useful image. On the other end of the spectrum, some of the commercial buildings around here have skylights made of white plastic domes, which don’t pass an image but also probably transmit only a fraction of the incident light.
It got me wondering whether there was some material that would transmit as much visible light as clear glass, but which would maximally scramble the light on the way through? I imagine a pane where all the visible light incident on one side comes out the other side as a kind of uniform blackbody-like radiation. Put another way, every photon incident on one side is emitted on the other side in a random direction from a random location (maybe those aren’t the same definition). Is the tradeoff between transmission and scrambling (scattering? randomization?) a hard reality à la Heisenberg uncertainty, or can we have both? I realize that heavily frosted glass does a pretty good job of both transmitting and scattering light in the real world - I’d just like to know whether a perfectly translucent material exists in theory, and whether there are real-world materials that outperform frosted glass for visible wavelengths.
You could hypothetically do this using fiber optics. Whereas a coherent fiberoptic bundle transmits an image from one end to the other, you could make an incoherent bundle, i.e. one in which each fiber’s location on one end of the bundle does not correspond to its location on the other end of the bundle. This wouldn’t be a thin pane of material, though; you’d need some distance from one face to the other to allow individual fibers some space in which to orient in a transverse direction. Each fiber can be made thick at the very ends and thin through its vast middle portion, further enabling transverse orientation of the mid-portion of each fiber. This wouldn’t give 100% transmission of incident light, but I suspect it would be better than the best conventional translucent material.
One way of making frosted glass is pure clear glass with a rough surface. Refraction sends the light in different directions.
Glass has a greater index of refraction that does air. That means that light travels more slowly through air than through glass. And it also means that light coming in at a flat angle is totally reflected.
If you want to completely avoid total reflection at low angles of incidence, you need a material through which light travels at faster than the speed of light in air. The speed of light in air is close to the maximum possible (the speed of light in a vacuum, aka “the speed of light”), so you won’t find a material that doesn’t reflect at least some light.
As a teenager, I once helped out a friend who did laundry collection and delivery. One housing estate he serviced had frosted glass in all the front doors. At one of the houses, he told me to stay behind him when he rang the bell. After a few moments I could see someone on the inside and it soon resolved into a naked woman - she opened the door a crack with a towel in front of her, completely unaware that we could see clearly through the glass because she was so close.
I think you have that backwards: At low angles, you’re guaranteed to get total reflection going from the high-index material to the low-index material. Even going the other way, reflection does tend to increase at grazing incidence angles, but it’s not total in the way that internal reflection is past the critical angle. And in a material with parallel planar surfaces, you’ll never have to worry about total internal reflection for light incident from outside the material.
Part of the problem with ground glass or other irregular surfaces is that some of the facets will scatter light back toward its source rather than letting it pass. That’s why I’m wondering if translucency must always decrease total light transmission.