The Mystery of the Glass Doorknob

An ominous photo has been posted a few times on Reddit, showing fire damage attributed to a glass doorknob illuminated by sunlight.

Any thoughts after seeing the linked photo?  Could you convince a jury of the doorknob’s guilt or innocence?

At first glance, it’s a bit of a mystery, since a ball-shaped lens would focus sunlight very close to its surface, not a couple of feet away.  There appears to be a burn mark on the knob.  Glass doesn’t burn, so it must have been from something flammable that was in direct contact with it. Also, note the burn mark farther down the door.  This suggests that something long enough to reach that point was draped over the metal part of the doorknob – something wide enough to contact the hotspot, but not wide enough to block illumination from the other side, so possibly a scarf or a light shirt.  If the door was more or less closed and there were clothes or papers stacked in the corner, perhaps the flames spread from the draped object to that corner, causing the damage shown.

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5 comments on “The Mystery of the Glass Doorknob
  1. Ronian says:

    Very interesting, Donna. Good insights from you and good points made.

    • webmistress says:


      Any thoughts on the Nutella Jar fire?

      I can see how a fishbowl or jar filled with water could focus light, but not an empty one.

      • Ronian says:

        Oh, I didn’t see that news. Not sure…it’s difficult to tell. Jars have so much astrigmatism, and door knobs so much spherical, and the conditions are hard to re-enact. I suppose one can do a non-sequential analysis of the irradiance from a typical magnifying glass that we know can burn paper, and compare it with the irradiance from a variety of dimensions for glass jars and door knobs, over a variety of conditions. But I like your analysis. Perhaps there may be something there to “optical forensics”. Prof. Donna Waters, “optical forensicist”.

        • webmistress says:

          It would be an awful lot of fun to do an optical version of Mythbusters! I’m very skeptical about the Nutella jar causing that fire, unless perhaps it had alcohol in it to sterilize the hairbands(?). I found a CAD file on GrabCab, but it represents the jar with no obvious variation in thickness. If accurate, the jar wouldn’t have enough optical power to cause a fire. I guess I have a good excuse to go down to World Market and buy a jar, so I can see what it really looks like and oh, nutella…yum!

          • Ronian says:

            Oh! Yes you’re right, I get it now. I didn’t realize it was an empty bowl (did not read the news in full actually!) Yeah I think it would be fun to have an optical version of Mythbusters. You could be on to something 🙂

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