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

What happens when light

  1. Feb 25, 2007 #1
    what happens when light ....

    Lets say there is the guy named jim
    Now, jim is doomed to die.
    He is inside of a hollow cube at the bottom of the pacific ocean, and he is unconscious. Now, inside the cube is pretty roomy. If he stood at one corner, and ran diagonally to the other corner, it would take him like 2 minutes, if he was sprinting as fast as possible.
    It's completely dark inside, with enough oxygen for him to live for about 48 hours.
    So basically, his death clock starts at 48 hours and 0 minutes and 0 seconds, and starts counting down exactly when he awakes.
    So, he wakes up and his clock starts counting down. (Jim is completely unaware of any of these facts, he doesnt know he's in a cube at the bottom of the ocean, he doesnt know that he only has 48 hours to lvie)

    He wakes with no recollection of how he got there, and since it's pitch black, he is naturally terrified. He spends exactly one full hour banging around the walls, and screaming, only to find out he's completely alone and the walls are indestructible. However, at the end of the 1 hour, he manages to find a hard object on the ground. He realizes it's a flashlight, and he switches it on. THe entire room illuminates almost immediately, because all the walls, the floor and the cieling are mirrors.

    He realizes now he's in a cube and there is absolutely no way out. Because of this, he starts panicking, and passes out for EXACTLY 10 hours, WITH THE FLASHLIGHT ON. Now, after 8 hours of him falling asleep, the flashlight turns off because the batteries die. When he wakes up 2 hours later, will the room be

    a. illuminated because the light kept bouncing off the mirror walls
    or
    b.completely dark again?

    and if the room is completley dark again, where did the light go? how could it escape a room that is all mirrors?
     
  2. jcsd
  3. Feb 25, 2007 #2

    G01

    User Avatar
    Homework Helper
    Gold Member

    Well, assuming the mirrors are perfect mirrors, the light will stay in the cube. But you have to take into account the light that will be absorbed by his body I guess.........BTW, what does this have to do with quantum physics?Other than maybe the concept of photons?
     
  4. Mar 2, 2007 #3
    The light will be absorbed of course. Mirrors aren't that good at reflecting light are they? I would say that a perfect white colour would reflect light better than a mirror.

    The colours we see is the colours (wavelengths) that is reflected from the material. White light is made up by all colours, and since a mirror also reflect colour and not just white it is a poorer reflector than a perfect white colour, isn't it???
     
  5. Mar 4, 2007 #4
    If you had a constant power source one which keep the level of light coming out of the torch at the same level for as long as you wanted it and a room made out of perfect mirrors reflected 100% of the light delivered by the torch would the level of light in the room slowly increase the longer the light was on and if so how perfect would the mirrors need to be for the level of light trapped in the room to increase the longer the torch was on.
     
  6. Mar 4, 2007 #5

    russ_watters

    User Avatar

    Staff: Mentor

    Since light zips around pretty fast, you'd need awfully good mirrors to notice any increase in brightness. The math here is actually pretty simple. It is a geometric progression: x^y. If you have 99% efficient mirrors, and the light bounces back and forth 100 times, that's .99^100= 36.6% of the light remains. If your room is 300 meters on a side, those 100 trips will take about one millionth of a second.

    And that doesn't even include the light that gets absorbed by you.
     
  7. Mar 4, 2007 #6
    Indeed. Micky's basically describing the resonator cavity in a laser (the light needs to be vastly more intense between the two mirrors; typically only a couple percent of photons are transmitted to form the outgoing beam). But pak's situation will be dark because the back of the torch is absorbing photons an instant after they get emitted: try drawing jim in the box and trace a light ray that reaches his eye, so as to appreciate just how recently the photon must have been emitted.
     
  8. Mar 6, 2007 #7
    I think that the room will be dark at the end. No matter how the light is reflected in the room how much time, there should be light abcorbed by the men right?If it continue, the light will be totally abcorbed by the men .
     
  9. Mar 6, 2007 #8
    or the air in the room??
     
  10. Mar 6, 2007 #9

    DaveC426913

    User Avatar
    Gold Member

    Personally, I think pakmingki will make a better story writer than a scientist. It takes quite an imagination to take such a simple experiment and turn it into a life & death short story! :biggrin:
     
  11. Mar 6, 2007 #10
    Interesting Russ, I have a question for you. Ignoring for a second the light absorbed by Jim's body and the flashlight casing, how do the mirrors absorb 1% of the photons per reflection? In other words, where does the energy of absorbed photons go and how is it conserved? I'm guessing extra thermal jittering of electrons in the mirror, but its just a guess. If this is the case, Jim would wake up to a slightly warmer room too right? :)

    What do you think?
     
  12. Mar 6, 2007 #11

    DaveC426913

    User Avatar
    Gold Member

    Well yes, but you don't have to get caught up in the details of photons and wall atoms to see the bigger picture.

    It's a closed system. As soon as the circuit is completed, releasing the stored energy in the battery, the room's (average) termperature goes up. Whether that energy is distributed into the air, or the objects, or the walls, or whatever, it's just moving the energy around. (Unless the energy is able to escape from the room - meaning it's not a closed system. Then the room actually gets cooler.)
     
    Last edited: Mar 6, 2007
  13. Mar 7, 2007 #12
    The energy of the light wave can also be absorbed by the air Jim's body or the mirror.Finally the amplitude of the light will tends to the general level such that the light will turn dark after two hours. Am my guess right ?
     
  14. Mar 7, 2007 #13
    from grief's psot, i got a thread...what if it so happens that two light beams colliding each other cancel each other out...assuming mirrors are 100% efficient, still the room can become darker if the reflected waves are of opposite and equal phase...so they might add upto zero...it seems it is remotely possible !!
     
  15. Mar 7, 2007 #14

    DaveC426913

    User Avatar
    Gold Member

    This makes no sense.
     
  16. Mar 7, 2007 #15

    DaveC426913

    User Avatar
    Gold Member

    That only happens for an instant - the light rays continue to move past each other. It has no effect other than at that instant.
     
  17. Mar 7, 2007 #16

    russ_watters

    User Avatar

    Staff: Mentor

    It would be tough to do that with light because the wavelength is so short and would only work (as Dave suggests) at a specific point in space.
     
  18. Mar 7, 2007 #17

    HallsofIvy

    User Avatar
    Staff Emeritus
    Science Advisor

    That doesn't follow. A mirror "also" reflects colors because colored light, not white line was shone on it. A mirror could reflect all the light incident on it and if the light was red, then the light reflected is red. A white surface does not show as much color (but will show some) because the light is "diffused". That is, if you were standing at exactly the "angle of reflection" at a mirror, you would see the red light reflected to you. With a white surface, the red light is reflected in every direction while the light you see would also include white (or other colors) from other parts of the room.
     
  19. Mar 7, 2007 #18
    Halls of Ivy, thank you.. I understand that the white surface will diffuse the light that hits it

    But if you shine a red light at a white surface you will see a red dot on it from the light.. so it does reflect quite good I think, but not as concentrated.. Don't you think it reflects as many photons as the mirror, but just diffused?? Or is the white a more bad reflector (judging from the number of "red" photons reflected)
     
  20. Mar 7, 2007 #19
    So what is the difference between a mirror and a white surface then?

    Is it just that the mirror has a more coherent surface shape? For example, white paint is usually a colloidal suspension of micrometer-scale particles of titanium dioxide. If we instead created essentially a large crystal of titanium dioxide (with a perfectly flat face, to the molecular level) would it still appear white, or would it be an unusually good reflecter (compared to common silvered mirrors, since, even if equally flat, any greyish meterial must interact more strongly with EM and so must still be absorbing a significant fraction of the light)?
     
  21. Mar 7, 2007 #20
    Let's find out what the best possible reflector is... :D
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?