This little GIF shows a woman applying sunscreen on regular film in one frame and another that shows Ultraviolet (UV) light. As you can see, the sunscreen quite effectively blocks UV waves.
The best time to wear sunscreen is every day. When your skin tans that is the body responding to damage. This damage increases the chances of wrinkling and skin cancer.
Studies show that the best SPF for day-to-day were is around 15, and that an SPF higher then 50 has no increased effectives, unlike that levels leading up to it. Sunscreen breaks down upon contact with UV waves. That’s why it should be applied at regular intervals – epically if you’re planning on getting wet. Water is a strong refractor of UV waves, which is why you can burn so badly when you swim.
Stay say and enjoy the sun!