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

How can you see light that is traveling away from the observer?

  1. Jan 30, 2004 #1
    You are on a salt flat, away from the city lights, no moon and it is cloudy. It is pitch black, you cant see a thing. It is a salt flat so there is nothing for miles. You shine a torch so the beam hits a hundred meters in front of you onto the ground. If the light is travelling away from you at 300 kps and there is nothing to bounce off,back to the observer, then how can you see the light? Do the same with a laser, 500 metres, how can you see the point where the laser hits the ground, when it is travelling away from your eyes, and in a focused beam?
     
  2. jcsd
  3. Jan 30, 2004 #2

    chroot

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    If there truly was nothing to bounce the light back, you would not be able to see the light.

    - Warren
     
  4. Jan 30, 2004 #3
    as experiments have proved that theory wrong would you like to try again?
     
  5. Jan 30, 2004 #4
    ...and if you replaced the 'salt flats' with a really good mirror, then you wouldn't see the light. But when it hits salt, or sand, or something bumpy, then some of the light bounces in every direction. That's why you can see it after the bounce.

    If there's enough water vapor, smoke, dust, or whatever in the air, you'll be able to see the beam even before it bounces off the ground.

    P
     
  6. Jan 30, 2004 #5

    chroot

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Uh.. excuse me? Which experiments?

    - Warren
     
  7. Jan 30, 2004 #6

    Integral

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    When I do this sort of experiment, like every night I am driving, I see light scattered off of the road in front of me. The ground is something whether it be salt flats or asphalt.
     
  8. Jan 30, 2004 #7
    when refering to the "beam" I am not refering to the light contained within the beam travelling from observer to object. As stated, the point where it hits the ground. Mirror does the same, so your guess is wrong. And no tiny bits of light do not bounce off sand, if that were true then if you did the same thing inside a room the amount of light generated by the reflection would be blinding, yet there is no difference between out in the open or direct contact.
     
    Last edited: Jan 30, 2004
  9. Jan 30, 2004 #8
    the one with the torch the laser and the mirror on a dark clouded night, with no moon, what other experiment would you expect? The question wasnt can you see the point of impact, but why can you see it.
     
    Last edited: Jan 30, 2004
  10. Jan 30, 2004 #9

    chroot

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    You can see the point where the beam hits the ground because the ground scatters some of the light back to your eyes.

    - Warren
     
  11. Jan 30, 2004 #10
    Maybe I got the wrong conclusion from all this: but basically you couldn't see light if there weren't any particles for it to hit? (when its not shining directly at you).

    PS: I agree with the above answers.
     
  12. Jan 31, 2004 #11
    Yes you can as stated previously, in an experiment. You see the light that hits the ground be it torch or laser. Being at an angle whereby reflection is in the opposite direction.
     
  13. Jan 31, 2004 #12
    as already stated, in order for that to be true, it would mean that the value of the light energy of the one or two photons that magically come back to you would need to bee the same as the total value of all the other light that hits the surface, since you still see the point of impact with the same degree as if you were shining it directly at a wall.

    What you are basicaly saying is that if a torch was shon at a surface at an angle of 179 degrees, that somehow a small amount of light is always no matter what, returned in your direction, yet despite the miniscul amount it still allows you to see the light at the same level as if all the light was bouncing back. If this were true, then if you were to turn your back and look at your hand, then the "reflected" light you claim exists, would illuminate your hand, but it does not, therefore you are wrong.
     
  14. Jan 31, 2004 #13

    chroot

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Sorry, simon. You're wrong. If you shine your torch or laser into an infinite vacuum (the blackness of space is a pretty good approximation), there will be no particles to scatter the light back to your eyes, and you will not see the beam.

    As has been said, in the real word, air molecules, dust, water vapor, and other particles scatter light. Of course, when your beam hits the ground, more light is scattered, in all directions.

    - Warren
     
  15. Jan 31, 2004 #14

    Umm....thats how human sight works, light bounces off of stuff. Well actually thats the simplified version. More specifically the light excites the molecules of said sand (and some of the photons are absorbed and others bounce off, giving the sight of the red dot) that then release a certain wavelength of light that is the color of the sand.

    However it is an inefficient process (as all antural processes are) meaning that the amount of light reflected is less than the amount recieved. Some of the recieved light is transformed into heat for example, rather than ebing reflected as visible light.
     
  16. Jan 31, 2004 #15

    Janus

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Yes, the light does bounce off the sand. And it also does so off the walls of a room. If you were to place an object and a light in a room with white walls the object will be better lit than if the same light and object were place in open space. This is because the light reflecting off of the wall helps to illuminate the object. (This effect is call radiosity.)

    The light is not blindingly bright because even a white wall does not reflect all of the light, but only a portion. The rest is is absorbed and converted to heat.

    This heat is radiated off to both the inside and outside of the room. (Which is why the room doesn't just keep getting warmer and warmer. It will heat up just to the point where the heat being lost to outside the room equals the heat gained from the light.
     
  17. Jan 31, 2004 #16
    You cant seem to get this through your head, I am not taling about the BEAM of light that can be seen when it hits dust or smoke particles in the air. I am talking about the point where the beam of light hits the ground,that illuminates the surface area, that point and that point only, so vacuum is irrelevant as the same effect would occur on the dark side of the moon if you shon a torch onto the surface of the moon away from you.
     
  18. Jan 31, 2004 #17
    179 degree angle, so your assecrtion wont work.
     
  19. Jan 31, 2004 #18

    chroot

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    The angle is irrelevant. Some of the light is scattered directly back to the light source. Some of it hits your eyes.

    - Warren
     
  20. Jan 31, 2004 #19

    Integral

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    I think you are correct, we do not get your point.

    Are you telling me that I cannot see my head lights hit the road? Just what are you saying?
     
  21. Jan 31, 2004 #20

    Janus

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    The reflected light does illumnate your hand. But since this reflected light is only a small percentage of the light striking the spot of the ground, this illumination is very slight and in most cases will be below the eyes ability to see.(If the Light is very bright to start with, and you are very close to the spot on the ground, it will be visible. Shine a flashlight on a wall in a dark room, and hold your hand out of the beam but close to the wall and you will see it lit up by the light reflected off the wall.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?



Similar Discussions: How can you see light that is traveling away from the observer?
Loading...