Clear glass powder appears opaque, but when some water is poured into the glass powder, why does it appear transparent again?
Answer: Light travels easily through the glass. So the glass appears transparent. But the rough surface of powdered glass reflects scattered light, so the powdered glass appears opaque. But when water is poured into that glass, it looks transparent again. The refractive index of glass relative to water is much lower than that of glass relative to air. So light is easily refracted from water to glass. As a result, the glass powder appears transparent again.