S wave -- a shadow zone appears when the velocity suddenly decreases 103-180 degrees from the epicenter. Describe the differences between P and S waves. P waves expand and contract, can move through liquid solid and gas. S wave moves up and down or side to side moves through only solids.
Optimal conditions to see the northern lights in Iceland is when skies are clear and dark (away from the city lights, in cold temperature and ideally from mid-September to mid-April.
2) Ideally, 2 or 3 days in Reykjavik, another 7 - 10 days to circle the country on route 1 (Ring Road), depending on your priority. For a shorter trip, focus on South Shore / Golden Circle. 3) Impossible to see Northern Lights in June - it is daylight all the time. 4) General concensus is yes. 5) I know people who have camped, and thus made a very economical trip out of it. If you rent a car in the first half of June, you will find prices are about half of what they are in end of June or July.