How does sunblock block out UVa and UVb rays?

Answer

1. Light scattering. This mechanism causes the ultraviolet radiation to be reflected in many directions from microscopic faces of crystals of some pigment. The pigment is typically titanium dioxide (TiO₂), the same pigment used to make white paints. The principle is to put an opaque film on the skin.
2. The second mechanism is absorption by some substance in the wavelength range of UVa and UVb. A typical substance is para amino benzoic acid (PABA). The absorbed radiation is then re-emitted as heat by the vibrational de-excitation of the excited state of the PABA. There are many substances that absorb ultraviolet radiation in the UVa and UVb range; however, the substance must have numerous other properties that limit the possible choices. Some of these other properties are: low odor, low toxicity (people may coat their lips), low eye and nasal irritation, and so on...