I didn't have much difficulty with this - in the beginning, just knowing what to do initially was a little confusing, but once I got going it was rather simple. The questions weren't particularly taxing, either, except for number 3. I can think of a more complex step to insure that the lines A and B are caused by the earth's atmosphere, but a simple one didn't come to mind, so I don't now if I got that one right or not. XD
It applies to the current unit because of chapter 9, which is all about the sun. It mentions the elements in the sun, and this lab has many similarities to that. It also gives an idea of what elements block what wavelengths, etc.