Quote:
Originally Posted by mental4astro
No. Those particular mirrors have an extremely short focal length with a terrible figure, so they are not capable of focusing the sun to the necessary concentration. And this also does not change the amount of energy coming in from the Sun at THAT time of year and time of day. None of this adds up.
I can't get anything to burn at sunset in the middle of summer here in Sydney with a dirty great big magnifying glass, DELIBERATELY trying to cause burning, and a fire started this way in the UK in spring?????
Think about it.
I'll do a Mythbusters experiment myself about this. I'll get one of these magnifying cosmetic mirrors and try to cause a fire this afternoon (or the very first clear sunset). I'll also use a 114mm diameter astro mirror to do the same, knowing that the figure will be much better than that of a cosmetic mirror. I'll try to get a very dry piece of newspaper to catch fire. This eill be from a warmer Sydney than a cooler UK.
Hmm, I know someone in the UK I could also ask to replicate this experiment too.
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I grew up in the UK before moving to Australia - I can assure you that English kids do exactly the same despicable deeds to ant's nests using magnifying glasses / mirrors as Australian kids, despite the weaker Sun at those latitudes.
I've got a 100 mm magnifying glass which will EASILY start a fire on dry grass or paper in just a couple of seconds. (I wouldn't know what effect it has on an ant's nest!
) I've also tried it with a 200 - 250 mm magnifying shaving mirror (focal length about 700 mm?), and it will also do the job easily.
I think the story is very plausible - if the mirror is left sitting on a shelf or window sill at roughly the focal length from the curtains, and angled so that the afternoon sun would strike the curtains, then it could very easily start a fire.