When it comes down to it, there are two types of sunscreen: chemical and physical. Chemical sunscreen is loaded with various actives (e.g. avobenzone, oxybenzone, helioplex) that chemically bind to skin and absorb (some scatter) UVRs- defusing that energy as heat. Physical sunscreen on the other hand is made up of two actives (specifically: titanium dioxide and zinc oxide) that act like a physical barrier- reflecting and repelling UVRs. However, titanium dioxide and zinc oxide don't just reflect light, they also attenuate UVRs- just like chemical sunscreens. Only about 10% of the UV energy is scattered.
Due to the way chemical sunscreen acts with the skin, it needs to be applied 30 minutes before you're protected. Physical sunscreen, however, acts as a barrier immediately.
