To understand how sunscreens work, you must first understand what ultraviolet (UV) radiation is. UV radiation has a shorter wavelength than visible light, meaning it penetrates the skin and can damage DNA, causing mutations that lead to skin cancer. Visible light has a longer wavelength, while X-rays are on the other end of the spectrum with even shorter wavelengths than UV. As you can imagine, X-rays can penetrate through the body and stop when they hit bone; UV rays are just short of that and stop when they hit the first layers of skin.
UV is divided into two categories: UV-A and UV-B.
UV-A is to blame for long-term skin damage (sun spots, aging, wrinkles), while UV-B is to blame for sunburn and skin cancer. A good sunscreen is “broad spectrum” meaning it blocks both UV-A and UV-B rays.
Zinc oxide is one ingredient in organic sunscreens which blocks both UV-A and UV-B rays. The way it does this is by forming a physical barrier on the skin. This literally causes UV rays to bounce off, reflect, or scatter off of it. In some ways, it is like a layer of clothing. Therefore, the harmful UV rays are not absorbed into the skin! In contrast, the organic sunscreens containing chemicals such as oxybenzone absorb only UV-B rays and release the energy as heat.
Modern zinc formulations apply far more clearly than older ones. EcoZinc uses ZinClear XP non-nano zinc oxide, which gives a clear finish while keeping zinc oxide as the sole active UV filter — no nanoparticles, and no chemical filters.
References:
http://www.physics.org/article-questions.asp?id=46
https://www.ncbi.nlm.nih.gov/books/NBK537164/#_article-29687_s2_