The story of sunscreen photostability is really the story of how sunscreens work. The principal characters are photons and electrons. Their stages are tiny, and the acts in their dramas are extremely short-lived. Take, for example, a UVB photon with a wavelength of 300 nm (3000 Å). If one imagines a photon as a round object and its wavelength as its diameter, one can calculate the time it takes for it to pass a point in space by dividing its diameter, 3 x 103 Å, by its speed—the speed of light, c, or 3 x 1018 Å/sec. Thus the short journey of a 300-nm photon takes just 10-15 sec, a single femtosecond (i.e., one one-thousandth of a trillionth of a second), to complete.