I’m reading a book called ‘The 500 Best Urban Legends Ever’ by Yorick Brown and Mike Flynn. Some I’m discounting as I read, some I’ve already read debunkings of, and some I’m putting into a ‘eh, maybe something like that happened sometime, somewhere.’ category.
But here’s a new one, and I’m pretty sure I don’t buy it.
Seems a guy bought a satellite dish, installed it himself, and enjoyed a weekend of TV. Next door neighbor’s house burns down. Fire chief investigating catches a ‘flash of sunlight’ off of newly installed dish and sez ‘ah ha’. It seems the dish was concentrating the rays of the sun enough to catch the neighbors curtains on fire.
Uhhhhhh. If the focal point of the parabolic dish is on the curtains, how were they getting reception and enjoying the newfound joys of satellite TV? And from what I recall of trying to make a solar oven, those focal distances are pretty short. He’d have to have the thing damn near pointed into his neighbor’s bedroom from about three feet. Plus, those dishes aren’t all that shiney, they tend to be painted, from what I’ve seen.
What say ye Dopers?
I searched Cecils archives, and the boards, and this doesn’t seem to have been addressed before.
Yeah, I don’t get it. Why would you read a book that’s purposefully called “500 Best Urban Legends” and then question whether they are real or not?
Doesn’t the title imply that they’ve all been debunked already?
OK, OK, call me gullible. I don’t consider an Urban Legend automatic BS. A UR, to me is just a story that’s been passed around so much the serial numbers have been filed off and the original details changed, exaggerations added, etc. The Legend of Billy the Kid had roots in a real person who lived in real towns.
Some URs are plausible, I was looking for help as to why this one wasn’t.
The book opens with the story about a guy losing a finger being careless with a table saw. A coworker asks him how he did it. He demonstrates, and loses another finger. I can see someone being stoic (or in shock) enough for that to actually happen. The UL about the man who was killed when a saguaro cactus he was shooting at fell on him? Really happened. David Grundman in 1982. But by now, it’s an Urban Legend.
That is a joke my father told me back when I was a kid in the 1950’s. Guy comes home from his first day at the sawmill missing both index fingers.
Wife: What happened?
Idiot: The boss was showing me the saw I was gonna use, and he told me it was impossible to cut your finger off with it. I showed him he was wrong.
Wife: What about the other one?
Idiot: He couldn’t believe it, so I had to show him again.
And the race does not always to go the swift and the strong, but that is the best way to bet.
The focal length of the parabola is the spot that light would converge on. The curtains would have to be at the focal point or close to it to ignite from the concentration of the light energy.
I was thinking about what you could do with an old C-band satellite dish and some tinfoil the other day:[ol][li]Cover the dish with the tinfoil.[]Mount a diverging lens just before the focus (so that the converging rays reflected from the dish are made parallel again). []Mount a planar reflecting mirror on a pivot just beyond the diverging lens.[/ol] Voila! Instant aimable solar-powered death* ray! [/li][sub]*or at least “inconvenience”[/sub]
The focal point of a satellite dish can be identified by the location of the feed horn or downconverter. That’s the element that sits out in front of the dish and receives the focused energey that the disc collects.
You can play around with parabolic reflection on this page. No matter where you move the slider, any incoming rays should be directed towards the feed horn.
I’m thinking that for the focal point to be the next door neighbor’s house, the mirror would have to be almost flat. Or at least a LOT flatter than a satellite dish is.
Not in the least. Take an ordinary magnifying makeup mirror and a penlight. Shine the light on the mirror and play the reflection off the mirror onto a wall. The focal point moves all over the place. The ideal placement is having the light aimed straight into the mirror. But aimed at an angle works too, just not as well. The focal point becomes less point-like.
For camera optics, a bad focal point really messes things up. For sat signals and such, it only results in a weaker signal.
As to the story in the OP: I call shenanigans. The optic reflectivity of a sat dish is quite low. The focal length is going to be short no matter what the incoming angle of the sunlight is. And over a couple extra feet, the focal “point” won’t exist.
The app pointed to in Patty O’Furniture’s link only applies to straight-incoming light. So it is not germaine to this discussion.
Even if the angle of incidence changes, the only affect is that the focal point moves on the x and y axis (relative to the “face” of the mirror). Regard.
Use this applet and see how disordered the incoming light rays would have to be in order to have the focal point move on the z axis, out away from the dish (click the source button to create a focal point, then you can move it around).
OK, I think this particular UL we can put outside the realm of possibilities. I remember trying to build a solar reflector in an alternative energy class, decades ago, and we couldn’t even cook an egg with the thing. Melted a hole in my sneaker sole with a chalkboard size fresnel lens, though. Leave one of those unattended near your house, as the sun sets…
I thank nearly everyone in this thread for their input.
