Wikipedia:Reference desk/Archives/Miscellaneous/2015 May 25

= May 25 =

Long periscope?
Would it be possible to build a periscope that is say, 100 miles long so you could see things that far away as if you were in the same room. Or is that more of a telescope of sorts? — Preceding unsigned comment added by 82.28.140.226 (talk) 20:59, 25 May 2015 (UTC)
 * Yes, perhaps using something like fiber optic cables which you called a periscope. But not in the traditional sense of the word, since the light would attenuate to a uniform blackness over such a distance. μηδείς (talk) 21:39, 25 May 2015 (UTC)


 * As Medeis says, it would be like looking at something 100 miles away... Just through a tube. Dismas |(talk) 23:08, 25 May 2015 (UTC)


 * Certainly you can do that - but these days a camera, a long wire and a TV is easier! The glass that they use for optical fibre is amazingly clear (0.2dB/km according to our article)- so you'd get a reasonably bright image - but to maintain a focussed image over that distance would require that the light bounces repeatedly off the sides of the fibre.  This means that you need a lot of fibres to maintain a complete image - and hence the result is going to be very pixelated.  Obviously you're not reasonably able to pull a few million optical fibres over 100 miles - and without doing that, you just get better images with a digital camera.  I suppose a 100 mile long telescope (or periscope...it makes no practical difference) would be possible.  But the attenuation of the light due to the air inside the tube would require you to pump the air out - and the precision with which the pipe would have to be straight and the lenses perpendicular and free of even the slightest vibration would be daunting.  Electronics just do it better. SteveBaker (talk) 23:13, 25 May 2015 (UTC)
 * IF you were looking into somebody else's room with a 100 mile long telescope, the vision would be distorted, due to the the rotation of the earth, and the telescope itself may be broken by window frames, etc. You would also need planning permission from the city council. If you really want to spy on someone 100 miles away, install TeamViewer on their PC, and remotely switch on their webcam. KägeTorä - (影虎)  ( もしもし！ ) 01:45, 26 May 2015 (UTC)
 * How would the rotation of the Earth distort anything? AndyTheGrump (talk) 16:49, 27 May 2015 (UTC)
 * Perhaps he meant "curvature"? Given it is assumed by definition that periscopes have mirros/things to deal with angles, I wasn't too worried about pointing that out. μηδείς (talk) 21:07, 27 May 2015 (UTC)
 * Yeah - that's what I'd assume too - and in that case, you could (in the case of a thought-experiment like this) dig a straight tunnel to carry the tube that started out at the surface, gradually increased in depth and then shallowed out to appear at the surface 100 miles away. That's not a problem in principle...and might actually make it easier to pump most of the air out of the tube to preserve optical clarity.  But there are plenty of other issues here. SteveBaker (talk) 17:43, 28 May 2015 (UTC)
 * Yes, and to make it clear, my original answer implied the use of multiple cables of optic fibers, each in essence a pixel, not just one fibre. Kind of like a very long compound eye. μηδείς (talk) 01:27, 29 May 2015 (UTC)