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?

Related to “If visible light has more energy than microwaves, why isn't visible light dangerous?”


1 Answer 1


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.

  • $\begingroup$ Typical lens glass will certainly transmit nearly all of the IR energy in the solar spectrum. The NIR portion (~700-3000nm) is slightly dominant, making up just over 50% of the total energy in sunlight. The remaining half is mostly visible light and about 5% UV (by energy). You don't need to do the experiment - this data is very well characterized and understood. The UV portion is minimal. You would do better to apply a broadband antireflection coating to the lens - this would reduce the reflection losses (~4% per surface) and would make up much more than you are losing in the UV. $\endgroup$
    – J...
    Jan 10, 2019 at 16:31
  • $\begingroup$ @J... It seems I didn't read that graph correctly. :( I'll adjust my answer. $\endgroup$
    – PM 2Ring
    Jan 10, 2019 at 16:43
  • $\begingroup$ Re, "...won't be fully focused." That matters if you want to get sharp photographs with your infrared camera, but it's not quite as noticeable when you're playing with a burning glass. $\endgroup$ Jan 10, 2019 at 17:38
  • $\begingroup$ @Solomon Perhaps not, although when I've played with a burning glass (or Fresnel sheet) a focused image of the Sun tends to make the difference between scorching the wood or paper and actually setting it aflame. $\endgroup$
    – PM 2Ring
    Jan 10, 2019 at 17:46

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