I recently did a presentation on visibility and it was great real life application of the properties of light. This topic really doesn't hit 4th or 5th grade, but it fits right in with light and color for 6th grade.

The presentation was for high schoolers, but I would tune it to water in the atmosphere for 6th grade and only talk about the reflections, refractions, and separation of color by describing the properties of clouds and rainbows. (if classrooms are more interested, i don't mind getting into dust too)

I am in the process of shifting it to an exploration rather than a presentation, but if someone is interested, I'll get it done sooner rather than later.