SPF measures protection against burning only, and the numbers are not proportional. SPF 30 blocks about 97 percent of UVB and SPF 50 about 98. The gap that matters is UVA, which SPF does not measure at all.
Sunlight contains every colour. Air scatters the short blue wavelengths far more strongly than the long red ones, so blue arrives at your eyes from every direction at once.
