Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Possible for a torch to light up a wall

  1. Nov 8, 2005 #1
    Is it possible for a torch to light up a wall, but not reflect back to you so that you cannot in fact see the light hitting the wall?
    For instance I can see streetlights from a couple of miles away out of my bedroom window - therefore the light from the streetlights must be reaching me - but anyone looking at me will not be able to see the reflection of this light will they? Does this mean that we never really know how much anything is being lit up by any particular light without being the object it is lighting? Or am I just going down a blind alley here?

    I think what I am really trying to get at is how can we see things such as far off street lamps and yet their light is not seen to be shining on us, or lighting us up.

    I understand that this is probably incredibly basic for the majority of you on this board, but I hope at least someone will spare there time to answer it,

    Thanks,

    Stev.
     
  2. jcsd
  3. Nov 8, 2005 #2

    jtbell

    User Avatar

    Staff: Mentor

    Generally, some of the light from the streetlights will reflect off of you, and in principle it will be possible for others to see you by this reflected light. In practice, if the streetlights are far away, the reflected light will be very faint and it will take sophisticated equipment to detect it. Think of those cameras that TV people use at night, with very sensitive detectors and a lot of signal amplification.
     
  4. Nov 8, 2005 #3

    DaveC426913

    User Avatar
    Gold Member

    Is it dark enough where you are standing that you can be in near total darkness except for the streetlight?

    If so, try the following experiment: Look behind you.

    Is there a faint shadow? That shadow is where the wall is being lit up everywhere except where you are being lit up. (Give your eyes time to adjust after looking at the streetlight. It could be quite faint.)

    Note that, if you cannot achieve total darkness except for that strteetlight, you may still cast a shadow by it, but your shadow will swamped by the ambient light.


    (BTW, did you also know that - under the right conditions - planets can give you a shadow?)
     
    Last edited: Nov 8, 2005
  5. Nov 8, 2005 #4
    Thank you for clearing that up for me, much appreciated.

    But do we ever now just how much any one object is actually lit up? If someone shines a torch on a wall which previously was dark, and I am standing next to that wall, then I will see the torches lighting affect as greater than if I was standing 2 miles away, is this correct?

    If it is then do we ever know how light the torch is actually making the wall as it it looks brighter the nearer we get, but without being the wall, we can never get close enough to know exactly how light the wall really is.

    :confused:
     
  6. Nov 8, 2005 #5
    No I didn't, but after jtbells post I was thinking about how we must therefore reflect the light from the stars as well as lamp posts!

    How could you achieve the conditions needed? I guess you would need to be in the middle of nowhere with a clear sky above and no moonlight?
     
  7. Nov 8, 2005 #6

    russ_watters

    User Avatar

    Staff: Mentor

    There is a word to describe this: albedo. Albedo is the fraction of the light that hits an object that is reflected.

    If you know the brightness of the light, the distance of the light to the object, the albedo of the object, it's distance from you, and what your minimum visible threshold is, you can calculate whether the reflected light is bright enough to see. This is a very useful concept to astronomers.
     
  8. Nov 9, 2005 #7

    DaveC426913

    User Avatar
    Gold Member

    Well, we can tell how big Pluto's two new moons are if we know how reflective their surfaces are (as Russ sez: their albedo). So, that's light coming from a VERY distant streetlight (i.e. Sol, 6 billion miles away), bouncing off the moons and back toward us on Earth (another 5.9 billion miles). So, yeah, with sensitive enough instruments, we can sense how much light is bouncing off something from quite a distance.
    We can stand beside or in front of the wall and measure how much light is reaching us, and thus, the wall.

    BTW, this is stuff you have to know when doing pro photography. You often take a light meter reading of the scene you are about to shoot, so you know what exposure to use. But when taking pictures of objects that have an odd or unreliable reflectivity (such as a black wall or a white wall), you won't get a good reading (eg.: your black wall will read dark, causing you to boost the exposure so that the wall is exposed in the middle (grey) range. But that's wrong, because it *should* look dark).

    So what you do, is turn yourself away from your subject (the wall), and point your light meter at the source (the streetlight), and take a reading there. That way, regardless of your subject, you can set your exposure so that the scene is exposed properly.
     
    Last edited: Nov 9, 2005
  9. Nov 9, 2005 #8

    DaveC426913

    User Avatar
    Gold Member

    We do. It's called starlight. On a moonless night, if you are in the wilderness, and it is a clear night, and your eyes are adjusted**, you should still be able to see well enough to stumble about. What you are seeing is the very faint light of stars reflecting off every surface.
    Yup. Mars is awfully bright this month. Again, in otherwise pitch dark conditions, once your eyes are adjusted** you might just be able to make out a very faint shadow from Mars.

    (**Could be as much as 10-30 minutes without any exposure to lights.)

    BTW, the shadows are still there when the Moon is up, they're just swamped by brighter light messing with your night vision.
     
  10. Nov 9, 2005 #9

    jtbell

    User Avatar

    Staff: Mentor

    I think I've read about people seeing shadows cast by the light from Venus, which can get much brighter than Mars.
    This reminds me... a few days ago I was watching the local evening news on TV, and the weatherman was talking about how you could easily locate and see Mercury that evening. It was very close to the crescent moon, with Venus not far away. I don't think I've ever knowingly seen Mercury myself, so I dashed outside to take a look. The problem was, I was watching a recording on my video recorder a couple of hours after the actual broadcast, and by then the moon and Mercury had already disappeared below the horizon. :cry:
    I could still see Venus, though, and it was very bright, even in the twilight sky.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?



Similar Discussions: Possible for a torch to light up a wall
  1. Possible to Add Light? (Replies: 12)

  2. Torch light in space (Replies: 11)

Loading...