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How does glow in the dark work?



A lot of things glow in the dark, from toys to stickers to shoes. Just about anything you can imagine, someone's made it glow in the dark. There are several different categories of things that glow in the dark, but i'll be focusing on what makes most consumer products glow in the dark, since it's more relevant to every day life. While researching for this blog post, the second sentence of the Wikipedia page mentioned quantum mechanics, so this could be even more interesting than I initially thought it would. Anyways, the technical name for "glow in the dark" is phosphorescence and this type of glow in the dark, as mentioned previously, can be found in toys, paint, and stickers, and according to Wikipedia, the study of phosphorescent materials led to radioactivity being discovered in the late 1890's. So, since phosphorescence is a special case of photoluminescence, which is when a photon is absorbed and then quickly released, photons are still absorbed but instead of being released very quickly like on the case of photoluminescence, they rare released slowly over the course of minutes or even hours. This is because when the photons are absorbed they experience strange intersystem crossing, sending them usually into a triplet state, which basically means that an excited electron is not paired with a ground state electron, and has the same spin as a ground state electron. These crossings to a triplet state are not very common since they require and a forbidden spin transition, which is a transition that is possible, however they are electric dipole forbidden and occur at a much lower rate. Still following along? Good, because there's more where that came from. Since the energy from the absorbed light is stuck with an electron that has crossed to a triplet state, the same "forbidden spin" transaction must occor for the electron to return to its original energy state. As such, these significantly less common transactions occur much more slowly, and are therefore able to store light for a long period of time. Once all of the electrons have been restored to their initial energy state, there is no longer any "glow". This turned out to be a shockingly interesting topic to research and write about, especially considering it could behave been explained in about 1 sentence consisting of "you charge it with light then wait for the energy to be released and it doesn't glow anymore." But hey, where's the fun in that?


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