While I was considering an answer to this question, I wondered how much light that enters the atmosphere reaches the ground without colliding with air molecules—if any. I've taken a good bit of physics course (not optics, yet), but I'm not really sure how light interacts with gases or other transparent material. I've always thought that it's periodically absorbed and then reemitted at a similar or identical angle and energy. I know I'll learn about it at some point, and I'll probably read up on it sooner than that, but to make it more fun, I'll ask these questions first:
What fraction of the light that enters the earth's atmosphere reaches the ground "unmolested"—without ever colliding with an air molecule? Is there practically any?
If no light can traverse the atmosphere without interacting with air, then what fraction of it reaches the ground without significant changes to its direction and/or energy? I imagine that this has a frequency dependency (i.e. the sky is blue, UV filtering).
Well, there you have it. Now I'm going to try to focus on my real homework.