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where does light go?

  • 21-08-2008 11:55am
    #1
    protective
    Closed Accounts Posts: 25 ✭


    i am probably been lazy by asking and not researching and to some it might seem a very basic question..but..

    is there a point where light stops travelling??? and also where does it go...

    i am basing my question on the light we receive(or see) from stars..say for example it takes that light 6years to reach us,where does it go then??

    i have some more questions on this but kinda be helpful to have this answered first..

    thanks..


Comments

  • #2
    Spear
    Moderators, Computer Games Moderators, Technology & Internet Moderators, Help & Feedback Category Moderators Posts: 25,756 CMod ✭✭✭✭Spear


    protective wrote: »
    i am probably been lazy by asking and not researching and to some it might seem a very basic question..but..

    is there a point where light stops travelling??? and also where does it go...

    i am basing my question on the light we receive(or see) from stars..say for example it takes that light 6years to reach us,where does it go then??

    i have some more questions on this but kinda be helpful to have this answered first..

    thanks..

    Light stops when absorbed into the electrons. Or potentially undergo pair creation into electon positron pairs. Othewise it keeps going.


  • #3
    protective
    Closed Accounts Posts: 25 ✭protective


    Spear wrote: »
    Light stops when absorbed into the electrons. Or potentially undergo pair creation into electon positron pairs. Othewise it keeps going.

    am i right in assuming that the speed of light slows as approaches us. say when it enters our atmosphere for example?


  • #4
    Podge_irl
    Moderators, Sports Moderators Posts: 28,119 Mod ✭✭✭✭Podge_irl


    Light doesn't "stop" so to speak, but it does get absorbed. Photons, the constituents of light, can be viewed rather simply as packets of energy. When we see starlight it is the photons that have travelled from that star entering our eye and being absorbed (the exact biological mechanism of this is not something I know off hand). Similarly sunlight is absorbed by materials - this causes the material to heat up for one - and certain frequencies of light also cause electrons to become excited. When this happens the electrons change energy level (think of it as them moving further away from the nucleus), they then drop back down to their original level, but release energy in this process which is released as a photon. This process is what gives objects their colour.


  • #5
    Podge_irl
    Moderators, Sports Moderators Posts: 28,119 Mod ✭✭✭✭Podge_irl


    protective wrote: »
    am i right in assuming that the speed of light slows as approaches us. say when it enters our atmosphere for example?

    Light slows whenever it enters any medium - be that air in the atmosphere or glass etc. It travels at a constant velocity in a vacuum though.


  • #6
    ga2re2t
    Closed Accounts Posts: 134 ✭✭ga2re2t


    Podge_irl wrote: »
    Light slows whenever it enters any medium - be that air in the atmosphere or glass etc. It travels at a constant velocity in a vacuum though.

    The speed of light in air is practically the same as for that in a vacuum. So to answer protective's question, light does not slow down in any appreciable way as it approaches us. It "hits" our eyes at roughly 1 thousand million km/h. We don't feel this impact because light does not have any mass.

    When light hits, say, a leaf:
    - some of the light photons have 'low' energy (long wavelengths) and cause the atoms + molecules of the leaf to vibrate more. This is what we call heating.
    - some of the light photons have enough energy to cause the electrons in the atoms and molecules to get 'hotter' - what Podge referred to as changing energy levels. But they can't keep this extra energy for long and as Podge pointed out they release it as light again. In the case of a perfectly green leaf, only the green light undergoes this process which is why the leaf is green.
    - some of the light photons have a lot of energy (e.g. UV light) and may cause the electrons in a molecule to get so 'hot' that they break the molecule. This may damage the leaf a bit. The leaf will reflect most of this light to protect itself and repairs itself faster than the rate of damage. Our skin isn't good at this job which is why we get sunburnt.


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  • #7
    Podge_irl
    Moderators, Sports Moderators Posts: 28,119 Mod ✭✭✭✭Podge_irl


    ga2re2t wrote: »
    The speed of light in air is practically the same as for that in a vacuum. So to answer protective's question, light does not slow down in any appreciable way as it approaches us. It "hits" our eyes at roughly 1 thousand million km/h. We don't feel this impact because light does not have any mass.

    It has no rest mass, but then its not at rest when it enters our eyes. It has both momentum and energy when it enters our eyes. It's momentum is just quite low.


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