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Where does light go when i turn off the.light

  1. Jun 22, 2012 #1
    I am having a debate with my brother. He reasons that when he turns off the light it is no longer there. You cant see headlights when you turn them off or a lighters light bouncing around the room when you turn off the lighter. So where does it go? He reasons that since you cant see the light anymore its just gone and it no longer exists. So when a star explodes its light doesn't keep going toward us on earth for billions of years. So scientists are wrong and light just disappears when a star explodes. What is his flaw in LOGIC? I cant seem to think of how to refute him. Its a good point but he's clearly wrong.
     
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  3. Jun 22, 2012 #2

    haruspex

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    After you turn the light off in the room, photons continue to bounce around off the walls and objects in the room. But a bouncing photon would cross a 4m wide room 100 million times a second. At each bounce, a large portion of the light will be absorbed instead of bouncing. Even if we replace the walls with mirrors that reflect 99.9% of the light, after one thousandth of a second the fraction left bouncing around would be 1 in 1043. You've no chance of seeing that.
    Light from a star billions of light years away is almost unimpeded.
     
  4. Jun 23, 2012 #3

    DaveC426913

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    Can't really add to what haruspex said. Can only reiterate.

    1] When light hits the walls, it gets absorbed, heating up the wall a tiny bit.
    2] It takes light little more than a billionth of a second to cross the room and hit the wall.
    3] By contrast, the nearest star is more than 40 million billion meters away, meaning it takes 4 years to cross space.
     
  5. Jun 23, 2012 #4

    haruspex

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    Funny, I thought the nearest star was 147 billion metres away (8 minutes) :wink:
     
  6. Jun 23, 2012 #5

    DaveC426913

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    Oops. Well played. Well played. :rofl:
     
  7. Jun 23, 2012 #6

    mfb

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    While it is not possible to see it by eye, it is possible to measure the time difference between "light switches off" and "light got absorbed, everything is dark". However, you cannot do this with a regular room light, as it does not get switched off quick enough (if you look close enough, it looks like a good dimmer).
     
  8. Jun 23, 2012 #7

    Drakkith

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    To clarify, the filament in the light bulb is heated to high temperatures and emits light because of it. When you turn it off it must first cool down, which takes a finite amount of time and is not instant. So the light dims, gradually going off.
     
  9. Jun 23, 2012 #8

    Nabeshin

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    As an exercise for the reader, calculate how long it would take for the light to dim to below the visibility threshold for a tungsten filament bulb in a normal room.
     
  10. Jun 23, 2012 #9

    HallsofIvy

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    The time would not be determined by how long it takes a photon to be absorbed, which is what others are talking about, but how long it takes the filament to cool below the heat necessary to produce visible light.
     
  11. Jun 23, 2012 #10
    I think OP got his answer. So, let me hijack this thread for a similar question, light , EM wave, the same thing.

    It struck me as strange while I was driving very fast and listening to the radio. How does the radio keep up with every beat of music and never lose a single beat, doesn't mess up a tone, doesn't make the music noisy, even when the receiver is constantly moving in space? It doesn't matter if the receiver is moving towards or away from the transmitter or moving side ways. The radio always catches every sequential moments (msec, usec) of tune.

    I know many of you will explain as speed of EM versus speed of car. Is there more to it? If light stays on for a moment once for an observer, shouldn't a particular RF signals also be received once and go away?
     
  12. Jun 23, 2012 #11
    There is a risk of detuning from the same frequency (Doppler effect), so that you could loose signal - but that's only an issue with high frequency bands such as mobile phone TV (perhaps due to narrower bandwidths?). Talking of Doppler, it's similar to sound. If a car with loud radio passes by, you might hear a slight variation in pitch, but your ear will normally (a wind blast could mess it up) catch every sequential moment.
     
  13. Jun 24, 2012 #12

    Drakkith

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    The radio signal received by your radio is an EM wave that is absorbed by the antenna as photons. These photons do disappear once the antenna picks them up. However the signal is being sent in an omnidirectional pattern, so as your car moves you are simply picking up the signal that was being sent to that location to begin with. The transmitter is constantly sending out the signal, analogous to a light bulb that is left on.
     
  14. Jun 24, 2012 #13
    May I ask how they make it omnidirectional, I was under the assumption that when the electron jumped down energy levels 1 photon was emitted?
     
  15. Jun 24, 2012 #14

    ZapperZ

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    This is a common myth, and I don't understand why. The myth here being that ALL light (or EM radiation) is created ONLY via an atomic transition. This is FALSE.

    The radio waves that is extremely common is created out of transmission lines. Anyone taking classical E&M would have seen such a thing, where in the simplest case, it is just a line of conductor with an oscillating current! In other words, strip down to its bare simplicity, it is nothing more than bunches of electrons oscillating back and forth! An accelerating charge can produce EM radiation! No atomic transition of any kind here! In fact, this general principle applies to the light generated out of the common household incandescent light bulbs!

    So when one can create such wave out of transmission lines, one can have various configurations and geometry of such lines so that one can control the field pattern. That is how one can have directional transmission.

    Zz.
     
  16. Jun 24, 2012 #15
    Great explanation Zz, thankyou
     
  17. Jun 24, 2012 #16
    I think I have figured it out. Well, its only my explanation.

    Probably the fastest musical sounds that follows one another is a
    strum on a guitar strings. (I may be wrong here, I'm not a musician)

    Even if we consider the fastest humanly possible hand to go from string-1 to string-2
    is around .01 sec. The sound (RF signal) of string-1 will travel 1860 miles before string-2 is touched.
    This 1860 miles is far outside almost all FM stations' radius of broadcast. At this point I'm not sure about AM stations which works by reflection from ionosphere.
    This means, when sound of string-1 is heard by all cars in FM stations listening area,
    sound of string-2 is still in the studio getting ready for the air.

    But I still have a distortion problem. If RF signals travels in spherical waves,
    a moving car break the wavefront and catch only part of the sound. As we can not
    hear the distortion in sound, we humans may have a hearing resolution, like visual resolution.
     
  18. Jun 24, 2012 #17

    davenn

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    well thats some seriously weird ideas rock2012 proposes
    cant say I agree with any of it

    just to take 1 point ....
    not in any physics text books I have seen. that statement would infer that billions of tungsten filament atoms are being destroyed every time I switch on an incandescent light
    to account for the billions of photons being released. I doubt it wouldnt take much time before there was no filament left ?? Most filaments I have seen fail have been from the repeated surge current stresses.

    Dave
     
  19. Jun 24, 2012 #18

    DaveC426913

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    Reported and disposed of as a crank.
     
  20. Jun 24, 2012 #19

    Drakkith

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    True, but the station is broadcasting a signal in between the two separate tones. There is no break in the signal, it just doesn't have any music encoded in it until the next note begins.

    No the wavefront contain EM radiation, not a sound wave. The modulation and encoding determine how the data is "read" by your radio, decoded, and sent over the speakers. Only catching part of the wavefront does not present a problem as all of the data necessary to playback the original signal is present.
     
  21. Jun 24, 2012 #20

    davenn

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    thanks Dave

    I looked in the reporting tab but it didnt really fit into the spam and abuse etc choices

    was hoping one of our fearless leaders would act accordingly

    Dave
     
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