Suppose a fine powder were used as a reflective agent that could be released into the atmosphere. Each particle is about 25 microns in size. Would it reflect the same amount of light if all those particles were joined together side by side like a sheet? More? or less?
That particle size is close enough to the wavelengths of light that you’re definitely going to get diffractive effects, so it definitely wouldn’t be the same as a continuous sheet. Off the top of my head, though, I couldn’t say whether they’d reflect more or less.
As a wild guess - if some particles are below others, then some of the light would be blocked by the particle above, the lower particle partly in shadow; so for “full coverage” with scattered powder you’d need even more than enough to form a continuous flat sheet.
Then consider that the light that hits the lower particles, some will reflect upward and instead of going up into space, will hit a higher particle and be reflected downward. So my WAG would be that a sort-of-solid sheet would be more reflective for the same amount of particles. (That’s not even considering the diffraction issues)
The first scenario is just describing what is usually called “fog”. Fog is not particularly reflective, although it does reflect some light.
In the second scenario, we’d need to know exactly how the powder is “joined together”. If the joining together is so precise that the resulting sheet is perfectly smooth, it might act just like a mirror, so would be quite reflective. If on the other hand the powder is joined together into a rough sheet with a lot of small scale irregularities, it might be very much non-reflective.
Very different situations. The first case – 25 micron particles (presumably spherical) is described by Mie scattering. Mie scattering theory was worked out over a century ago, but the computing power to accurately perform the calculations really came into its own in the past few decades (the result is in the form of an infinite series that converges very slowly, so people used clever dodges to reduce the number needed). In any event, light scattered in this way is not reflected at all like light from a continuous sheet – there is a complex angular dependence, with a lot of the light going through.
As for the sheet, as has been remarked above, it depends on how the sheet is coinstructed. But if we just assume that it’s like a sheet of dielectric material, you’d have a portion of the light reflected, a portion refracted and passing through, and a portion absorbed.
Exacrtly how much of each depends upon the polarization of the incident light, its angle with respect to the normal to the surface, and the complex refractive index of your material. Suffice it to say that there’s no single, simple answer. If your material was highly absorbing, like a metal, then you would certainly get more light reflected from the sheet, and more passing through if in dispersed powder form.