1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Can a man made fire be visible a few miles in the night?

  1. Jan 18, 2010 #1
    Can a fire say in a dish plate with a pile camphor be visible from a couple of miles in the night? What would it take to put a fire visible from a few miles? Will a pile of camphor in 1mx1m do the trick? Been puzzled with this question for a while and thought the experts in this forum can help guide me in the right direction.

    The main objective being it (the fire) should be visible to "naked human eyes" at night from a few miles? Can this be achieved? Will a plate of camphor do the trick or will you go for a more conservative 1mx1m fire or even bigger?
    Last edited: Jan 18, 2010
  2. jcsd
  3. Jan 18, 2010 #2
    Just blow something up, that'll be visible
  4. Jan 18, 2010 #3
    It should be a controlled fire rather than blowing something up.
  5. Jan 18, 2010 #4
    Attributes shown by the fire:
    1. It happens quickly and is controlled - does not result in forest fire or visible smokes
    2. The fire blinks (appears/disappears) three times or gives the illusion
    3. Remember the fire is watched from a distance of a few miles

    Key questions:
    1. Can a small/controlled fire be visible from a few miles at night?
    2. If so, what should be the magnitude of the fire?
    3. If some one asks you to create one, how will you pull this off?

    This phenomenon occurs yearly in India. Any idea to explain this would be very helpful.
  6. Jan 18, 2010 #5

    Andy Resnick

    User Avatar
    Science Advisor
    Education Advisor

    Fires can (sometimes) be seen from space, so the answer is 'yes'. In order to calculate how big a fire should be, you need to know the capabilities of the detector (for example, the noise equivalent power, or noise floor), the collection area of the detector, and how much light is given off by a fire- something that will depend on a lot of variables, including what waveband your detector operates at. A single candle has an intensity (W/sr) of about one Candela, so using that, the detector characteristics, and assuming the fire radiates as a point source, you can at least estimate the number of candles needed to be detected a certain distance away.
  7. Jan 18, 2010 #6
    Thanks for the details.

    There are no concrete measurements for this phenomenon. The main objective being it (the fire) should be visible to "naked human eyes" at night from a few miles? Can this be achieved? Will a plate of camphor do the trick or will you go for a more conservative 1mx1m fire?
  8. Jan 18, 2010 #7
    Think about it. You can see the lights on airplanes that are 5 miles high at night. Or, if you are in the plane, you can see car lights and street lights. The human eye is incredibly sensitive to light in the dark, and the atmosphere is very transparent normally (i.e. no rain, snow, fog, pollution etc.). We see stars that have almost no detectable optical power at all. I'll bet a simple campfire is visible at several miles.

    More important than the fire size itself are issues like, air quality/transparency, unobstructed line-of-sight, and amount of background light.
  9. Jan 18, 2010 #8
    Thanks for the excellent details. Here is a pictorial depiction - * is the place of fire, **viewing point and ~ is just a filler to help with white space mgmt. Both are hill location at least a few thousand feet above the ground and separated by at least a couple of miles.

    / \*
    / \ ~~~~~~**/\
    / \~~~~~~~ / \
    / \<- 2 miles-> / \

    Is the size/intensity of the fire important? What is the size of a typical campfire? How can the blinking (appearing/disappearing) thrice in a few seconds be simulated?
  10. Jan 18, 2010 #9
    Been reading a lot on this. From what I have read, it seems the light source should be at least 300-500 cd do to be visible from a few miles. Considering a 100 W incandescent light bulb emits about 120 cd, we need at least 250W - 420W? Is this possible from a fire to match this? If so, using what chemical? Will a campfire have 300-500 cd?

    Share your thoughts.
  11. Jan 19, 2010 #10
    Please help. Thanks
  12. Jan 19, 2010 #11
    It's difficult to create a reference to judge and it's probably easier to figure this out with a few experiments.

    The only idea that comes to mind is to look at the old definition of the unit cd.

    One candlepower was the light produced by a pure spermaceti candle weighing one sixth of a pound and burning at a rate of 120 grains per hour.

    I can't relate to this defintion personally, but the cd is supposed to roughly correspond to the light of a typical candle. It seems to me you can make a fire that outputs the light of 500 candles. I'm not sure if this would be a small campfire, or larger than that, but it does seem feasible.
    Last edited: Jan 19, 2010
  13. Jan 19, 2010 #12
    Thanks for the details. Is there any way to prove (theoretically) that the light source should be at least 300-500cd to be visible from a couple of miles?
  14. Jan 19, 2010 #13
    I think so. The light intensity is known to decay as distance squared (assuming you are not directing a beam with optical components), and the absorption of the atmosphere can be estimated in the visible band, with assumptions of altitude, humidity and polution. Then the sensitivity of the human eye can be looked up, under the assumption of no background light. It will not really be proof, but it can be considered an estimate based on optimium (or at least good) conditions. This needs to be done with some care in calculating and double checking (use of multiple sources) of experimental data used in the calculations.
  15. Jan 19, 2010 #14
    Thanks so much for the time and information. Really appreciate the help on this.
  16. Jan 20, 2010 #15
    AFAIK it's something like "Vision Threshold: A single candle flame from 30 miles on a dark, clear night". Sounds about right.

    Personally with the naked eye I was able to spot camp fires in the hills over ~10 miles. Greatly depends on the air condition and the light pollution.
  17. Jan 20, 2010 #16
    IMO Bear Grills will have the best answer...
  18. Jan 20, 2010 #17
    Based on technical details of beacon lights, 300-500cd light is required to be visible from a couple of miles.

    Link: http://en.wikipedia.org/wiki/Aircraft_warning_lights

    !!!!!The common medium white strobe flashes 40 times in a minute,
    !!!!!at an intensity of 20,000 candelas for daytime/twilight, and
    !!!!!2,000 candelas at nighttime.

    Is it 10 miles from the foot of the hill where as the campfire was at a suitable elevation?
  19. Jan 21, 2010 #18
    Can someone share their thoughts on this?
  20. Feb 20, 2010 #19
    Please share any additional thoughts on this... Thanks
  21. Mar 2, 2010 #20
    Can someone share their thoughts on this? TIA
  22. Mar 2, 2010 #21


    User Avatar

    Staff: Mentor

    I agree.....?

    What do you want to know?
  23. Jan 17, 2011 #22
    Need confirmation on two aspects:
    1. Luminous intensity of a fire should be in the range of 500 candela to be visible from a few miles across in the night
    2. Can a man-made fire on a plate of camphor produce 500 candela of luminous intensity
  24. Jan 21, 2011 #23
    Please share your thoughts on the above observations.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook