I was at CVS the other day, and a lady near the front door handed me a sample of some sort of fancy "age-defying" cream. I'm not especially interested in it, as it's expensive by CVS standards and contains a lot of unnecessary glop in the base that my face won't appreciate, but I found the ingredients list rather interesting. It's essentially using sunscreen compounds for what Mog and I refer to as "the other use".

[Mog also speaks Japanese, at least to the extent that she didn't die while in Tokyo for a semester. The phrase on English-language instructions that's customarily rendered as "only to be used as directed" has a Japanese-language equivalent that literally translates to "not to be used for the other use". Moggie has taken this as a guiding principle of life, and uses things for the other use whenever possible.]

There are two kinds of sunscreens in the world: physical and chemical. Physical sunscreens work by reflecting UV light. Zinc oxide and titanium dioxide work like this, as does whatever clothing you're wearing. You can only use them in applications where you don't mind seeing the stuff -- titania is used a lot in SPF 15 foundation, since the idea of foundation is to cover your skin anyway, but zinc is mostly used by lifeguards who mind white noses less than they mind sunburns.

Chemical sunscreens work by absorbing UV photons into their chemical structure. Shelf-stable chemical structures don't usually have a lot of room for extra energy like that, so one of two things happens, after the new photon has finished shoving its way in. Either the substance undergoes some kind of chemical transformation, with the new influx of energy going towards breaking and reforming molecular bonds, or the substance kicks the energy back out in the form of other photons. Compounds that break down under UV are single-use only, and have to be continually reapplied as the reaction will proceed for as long as you're exposing them to ultraviolet light, but compounds that re-radiate the energy will continue to function until they decompose for some other environmental reason, or until you sweat them off, which makes them quite popular.

[Glow paint isn't always fussy about the sort of photons it takes in, either. Back in the bad old days of radiation science, they used to use radium salts as the source of photons to make a phosphorous pigment glow. You might be thinking this was a terrible idea, and you would be right. Radium is a gamma emitter, and as it turns out, gamma-level photons react with a lot of things, including humans, for whom they are rather unhealthy. Phosphorous was also not a walk in the park, as it liked to elbow calcium out of bones and cause all manner of structural problems there. More modern glow pigments use metallic salts that are much less toxic -- I mean, don't go eating the stuff, but it won't kill you with proximity.]

In many cases, the photons kicked back out are in the same part of the spectrum as the photons that went in. The best explanation I can get as to why metal is shiny is the same reason metal is conductive, which is that it looooves to meet and get to know new particles. In the case of electrons, they're welcomed right in and passed through the internal structure like a game of Hot Potato. In the case of photons, they're greeted warmly and then immediately given the boot. Casting visible light on metal makes it kick back a visible shine; casting invisible light on it, such as infrafred, makes it kick back energy in the IR part of the spectrum instead, i.e., heat.

In the case of sunscreens, the absorbed photons are in the UV part of the spectrum, but the emitted photons are not. That would defeat the purpose of sunscreen -- the whole point is to keep UV away from the skin, so something that took in UV energy and then re-emitted UV energy would be exactly what you don't want. The compounds they use in this "age-defying" stuff absorb UV from sunlight and then kick it back out as a faint visible glow. This is the exact same thing your "brightening" laundry detergent does, except your laundry soap has compounds meant to permanently stick to (dye) your clothes, and emit in a slightly bluer wavelength to counteract the natural yellowing that many fabrics undergo as a result of age and use.

It also has much the same effect as the blur filter so popular on the old Star Trek series, which is that when everything looks like it's glowing slightly, it's a lot harder to pick out pores and wrinkles. The top layer of your skin is slightly translucent to begin with, and has some diffusion effect on the light reflected off your face, but how much varies with your pigmentation, your age, the moisture content of the air and your epidermis, the surface texture of your skin, the angle of incidence, whatever else you've painted on your face, the phase of the moon, etc.