When focusing sunlight on a piece of paper, e.g. with magnifying glass, the paper will be charred and might eventually even burn (assuming low cloudiness). To what extent is the heat a result of the focus of the visible light, rather than to other parts of the electromagnetic spectrum (i.e. ultraviolet or infrared) that are invisible to the naked eye?
It's mostly just the visible light, with some infrared.
Typically, glass doesn't transmit so well outside the visible spectrum, as shown in the graph in this answer. Near infrared gets through ok, but ultraviolet transmittance drops off fairly quickly. And then you have to take dispersion into account: different wavelengths are in focus at different distances. So when the visible wavelengths are mostly in focus the infrared components won't be fully focused.
But it would be interesting (IMHO) to do the experiment, and see if a lens with relatively high UV transmittance, like a quartz lens, heats & burns stuff significantly faster than a similar lens of "normal" glass.