Can you use a magnifying glass and moonlight to light a fire? (2016)
Interesting article. Right in many places. Wrong (possibly) in main conclusion.<p>Entropy argument - correct in the sense that using radiation from black body we cannot use lenses to heat another body to the temperature higher than original. Easy to understand why - the first body has a temperature, radiation has the same temperature, if we apply the radiation to another object it will not heat up more than the radiation&#x27;s temperature.<p>Also the argument about impossibility of concentrating light into a dot is correct (although even if it were possible we still would not be able to get higher temperature - light would not be energetic enough for that). The important part is - we could concentrate light into a dot only if it consist of parallel rays - i.e. only for an object that is infinitely far away.<p>Moon surface temperature argument is incorrect. A body at 100 degrees Celsius does not radiate in visible spectrum, so the light we see is not produced by Moon&#x27;s temperature. It is reflected Sun light. So Moon&#x27;s temperature doesn&#x27;t matter. Moon surface does absorbs some light, changing spectral composition from about 5.7kK (Sun&#x27;s surface temperature) to about 4kK. So we should consider moon to be a part of optics not emitter.<p>Hence the question is now - can we concentrate moon light enough so that intensity at the concentration point is higher than thermal loss into environment (only then we will be able to raise temperature in the concentration area enough for combustion - remember that light is &quot;hot&quot; enough for this)? I don&#x27;t have answer for that - need to do calculations. What can be a deal breaker? Remember that Moon is much closer than Sun, so rays come to us even less parallel, so the area into which we can concentrate light reflected from the Moon is even larger than the Sun&#x27;s, so together with lower intensity of light from Moon we might have trouble achieving the necessary intensity for combustion. However big enough lens probably will work.<p>And yes - I&#x27;m a physicist by training.