Solar mirror danger

[Merlin66]

https://www.banburyguardian.co.uk/read-this/warning-house-fire-sparked-mirror/

[mental4astro]

What the???

A household, FLAT mirror, sparking a fire, in a house, at 8pm in the UK!!!??

What are these people smoking???

No concentration of the sunlight as from a parabolic mirror.

With the Sun shining with its lowest intensity at sunset by going through the greatest amount of atmosphere.

In a location on Earth when a hot SPRING day is maybe 18°, AT 1PM IN THE ARVO.

What were these curtains made of???

I have too much faith in fire inspectors to believe this story as being nothing more than craphouse journalism. It really smacks of a journalist that does not think for themself and is not used to questioning as they lack all logic and are used to believing everything they are spoon fed.

Bullcrap! is my gut feeling about this source of ignition. There is NO mention of a solar mirror. And again, how big would a "solar mirror" need to be to start a fire with all the above conditions???

Alex.

[Merlin66]

What about one of those magnifying mirrors the wife uses for her eyebrows?

[multiweb]

That's right. :thumbsup:

:lol: Alex what are you smoking during your easter break mate? ;)

[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.

[mental4astro]

Well, I just tried on of these magnifying cosmetic mirrors at pretty much noon here in Sydney, during a break in the clouds with full Sun, just to see if with the best conditions possible I could start a fire or burn something.

The sharpest focused image I could get with this mirror was the size of a $1 coin, and the image was very uneven of the Sun due to the poor figure of the mirror.

I put a sheet of old, dry newspaper and no smoke, no fire. I even put my hand in the way of the sharpest beam of sunlight, it got warm, but not hot. My temp regulated hot water system is hotter at 55°.

This is at NOON in Sydney mid April when the Sun will be much stronger than at 7pm in the UK. If I could not burn my hand, little lone cause old newspaper to smolder, how the heck could such fire start this way as described in the UK???

I stand by my claim of CRAP JOURNALISM!

Alex.

[xelasnave]

Hi Alex
I am inclinded to conclude it is a beat up and it would not be the first time so perhaps just relax and know the good that should come out of it is that folk may make a point of putting their dust cap on their scope☺ and get rid of those make up mirrors and obsess less about appearance.
Alex

[julianh72]

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! :innocent: ) 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.

[mental4astro]

But how am I going to so carefully put my lippy on???:question:

[mental4astro]

Julian, read post No 6.

Ever try to roast ants at 7pm with your magnifying glass? Try it at 5pm here in Sydney during summer.

A cosmetic mirror (103mm diameter) doesn't have the quality of figure for a sufficiently focused image to cause combustion, nor burn my hand at noon when the Sun as the least amount of atmosphere to travel through. The late afternoon Sun in the UK when the fire started in that house certainly won't have the energy.

Remember, the article specifically mentions the fire was controlled by 8pm local time. Say the fire started an hour earlier, the amount of energy in the sunlight is what is in question. This is very different from noon.

[gaseous]

In the words of Edmund Blackadder: "I smell something fishy, and I’m not talking about the contents of Baldrick’s apple crumble."

[julianh72]

Do a Google search for "fire caused by mirror" - it's not as rare as you might think! You don't need a well-figured mirror to reach combustion temperatures at the focus of a 200 - 250 mm make-up mirror. (If yours won't do it, it might be exceptionally poorly figured, or just a bit too small. My 200 mm mirror will make a solar image that is about 5 - 8 mm across, and trust me, you won't leave your bare hand there for long!)

The house referenced in the OP was well-alight at 6:00 pm, so could have been smouldering from 5:00 pm or so. Devon is 3 1/2 degrees west of London, and UK is on daylight-saving time, so local sunset is around 8:20 pm right now. At 5:00 pm local time say, the Sun would be at an altitude of about 25 degrees - I think I could light a fire with a make-up mirror in those circumstances.
https://www.bristolpost.co.uk/news/uk-world-news/warning-issued-after-mirror-believed-2784581

https://www.youtube.com/watch?v=v3gHAj_bE1M
Early morning in April in the UK - PVC window frame smouldering / smoking from a make-up mirror.

I maintain that the story is plausible, and unless you want to find out the hard way, don't leave your make-up mirrors where the sun can hit them while unattended.

[Merlin66]

Sticker on solar filter:
"No Ants were killed during the testing of this product"

[raymo]

I, just this minute went outside with my wife's 160mm diam. rotating
mirror, full sun and no wind, so focused on a bedsheet hanging on the line.
About 650-700mm f/l and spot approx 6-7mm diam. After approx 1 second
had to pull my hand away as pain too intense, this would set fire to paper
or material pretty damn quickly.
raymo

[DJT]

I can just see the headlines in the Ilford gazette. SPSP abandoned as astronomers dismantle telescopes in an attempt to prove out that you can indeed burn the legs off an ant at sunset..am staying at home.

[julianh72]

A little bit of basic "order of magnitude" maths:

Solar Radiation Intensity is about 1300 W/m2 (before losses).

A 160 mm mirror has an area of about 0.02 m2.

If the mirror is pointing pretty much directly at the Sun, it receives about 26 watts of incident solar radiation, which it concentrates into a spot less than 10 mm diameter.

Allow for losses (atmospheric absorption / scatter, reflection losses, etc), and we could be looking at something like 10 watts (or more) in a 5 to 10 mm disc - comparable to one of those 10 watt halogen globes for desktop lights - and they get pretty hot!

[mynameiscd]

A while a ago I posted this........

"We have a collection of all different sized stainless bowls and my wife uses one of them to collect vegies from the garden.
Last summer we could see smoke coming from behind the kitchen.
As we lived in the bush, panic stations kicked in and im already running to the fire pump.
My wife came around the corner with the large bowl ( about 40cm) with a cloth in the middle smouldering away.
We then tried a few more bowls but it was only one thst could ignite paper in about 30 seconds.*
This bowl is now banned from our bush block and im still worried with it in the kitchen at our new house.
Pretty scratched now so the mirror effect is not as good but im still worried.*
I never thought about putting a secondary and eyepiece with the bowl.
Hmmm a parabolic for $10......bargain*"

Mind you this was midday summer but it really surprised me.

Also i wouldn't leave on of these around, they actually work!!!
https://www.ebay.com.au/i/303107282818?chn=ps&ul_ref=https%253A%252F%252Frover.eb ay.com%252Frover%252F1%252F705-139619-5960-0%252F2%253Fmpre%253Dhttps%25253A%2 5252F%25252Fwww.ebay.com.au%25252Fi %25252F303107282818%25253Fchn%25253 Dps%2526itemid%253D303107282818%252 6targetid%253D469526444556%2526devi ce%253Dt%2526adtype%253Dpla%2526goo gleloc%253D1000567%2526poi%253D%252 6campaignid%253D1793098769%2526adgr oupid%253D74958852128%2526rlsatarge t%253Dpla-469526444556%2526abcId%253D1139216% 2526merchantid%253D7364522%2526gcli d%253DCjwKCAjw7_rlBRBaEiwAc23rhrC58 OI2kt-gcRNxeudtxBUaSm4hVGhEUB7cDpTN6EtFL9 mDpssDphoCmLgQAvD_BwE%2526srcrot%25 3D705-139619-5960-0%2526rvr_id%253D1943882780118%2526 rvr_ts%253D4950ecee16a0ab6475e64116 ff9faabb

Cheers
Andy

[Ukastronomer]

When I was about 12 my mother left her cigarette lighter on the kitchen window sill and the curtains did catch alight with the reflection from the Sun.

[GrahamL]

Your game raymo.. moving the sacred mirror gets me enough trouble.. trying to ignite the bed linen with it .. is just.. well madness my friend !

[raymo]

Hi Graham, SWMBO was asleep, taking her mandatory afternoon nap.
I held the mirror aimed at the bedsheet just long enough to see the size
of the focal circle and put my finger immediately in front of it.:)
raymo