The answer I usually get (and I'm paraphrasing here) is that they disappear and are instead absorbed as heat energy.
But I find it hard to believe that the photon simply "disappears." Common sense tells me it must turn into something or other, not just simply poof out of existence; then again, common sense has betrayed me before.
Forgive me if this is obvious; high school physics student here who's just learned about light and is greatly confused by all this.
Answer
Well, the answer you usually get is half right. They do disappear (more on this in a second). I'd hesitate to say they turn into "heat energy," both because we don't use the term "heat" that way in a technical sense and because most of the time we like to talk about atoms absorbing photons. In this case the energy of the photon becomes potential energy of the electron that made the transition, and there's no need to talk about heat.
Now, can the photon disappear? The short answer is yes. When you talk about things "not simply poofing out of existence" what you're really describing is like a conservation law. For instance, we say that energy is neither created nor destroyed. Your intuition that things aren't just "poofed" out of existence is probably due to your everyday experience that objects generally can be broken into parts, but not usually destroyed. This isn't true in the particle physics sense, usually. The energy carried by that photon has to be accounted for, as does its momentum and angular momentum. But "photon number" is not a conserved quantity the way that energy or (for instance) electric charge are. A photon really is just a way of looking at disturbances/excitations in the electric field, and so its "destruction" just represents that energy that was present in the field has been moved into some other mode.
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