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What makes light glow ?

  1. Aug 25, 2004 #1
    What makes light "glow"?


    First post from a newbie... so forgive the naivety...

    Light is described as an electromagnetic wave/particle... but what actually gives light its "brightness", allowing it to illuminate the surroundings through which it travels?

    Also, another question if i may... I know that the speed of light, 186,000 miles per second, refers to light speed in a vacuum, however is the speed of light on earth (through air that is) much different? How much slower is it?

    Thanks in advance for all your help.
  2. jcsd
  3. Aug 25, 2004 #2
    light does not illuminate something by traveling by it or in it. Light has to reflect off something, and eneter your eye in order to be senced by your retna, making the object seem brighter by the light.

    To answer your second question. The speed of light in air is a little less, not much. But the reason there is a difference is because as light propagates throught a medium such as air, it has to go throught atoms (oxygen atoms, nitrogen etc..) What happens is a photon of light will hit an electron in the atom, cause it to shift to a higher energy level, and then go back down to the original and emit the photon again so it goes to the next atom. It is like a chain reaction. The proces of absorbtion and reemitttion is what makes light seem slower while traveling in air, because of this little delay in propagation.
  4. Aug 25, 2004 #3
    Oh yes, silly me... so the retina/brain "converts" the electromagnetic wave to give vision...

    I read recently that photons are largely responsible for why matter doesn't collapse in on itself!? For example, the electrons (negatively charged) and protons (positively charged) inside each of the atoms that make up our bodies should ordinarily attract each other and thereby cause us to collapse in a heap(?), however, because photons interact between electrons and protons this interaction suffices to keep the two apart. Does that sound about right? (p.s. I may also post this in the quantum physics forum)

    Related to the last point above... are there still photons in the air when it is pitch black?
    Last edited: Aug 25, 2004
  5. Aug 25, 2004 #4
    to answer you atomic question. Yes this is right, according to quantom theory, there are things called virtual photons which are responsible for the elctromagnetic force. These virtual photons ar what keeps elevtrons and protons appart in an atom.

    yes, try it. Turn off the light in a dark room and shine a flashlight past the door, you will see that you see nothing in the room. You might see dust particles, but this is because they bounce the photons to your eyes, but you do not see the back of the room.
  6. Aug 25, 2004 #5
    Hmm, then, why is the colour of light white? Is it because of its speed? Because, if a matter goes fast, it heats, so it turns into an incandescent matter, and as i know when a matter turns into an incandescent matter, its colour turns into white. Is it becaouse of that, or....?
  7. Aug 25, 2004 #6
    its not actually white, its a combination of 5 main coulours in the elctromagnetic spectrum. The waveleangth of visible light varies from 450nm to 700nm, and this variation produces different colours (violet, blue, green, yellow, red). And about your second claim, when matter heats up, it releases different colours of light. This is called blackbody radiation. It can be summed up using Weins law.
    [tex] {\lambda}T = 2.9 * 10^{-3} m*K [/tex]
  8. Aug 25, 2004 #7


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    Light is one of the hottest things there is, since heat is really kinetic energy. But light can't go faster or slower, so I doubt that makes it red-green-blue...

    Speaking liberally, light has no "color", it only has a frequency. Our eyes interpret this and we assigned the different frequencies the name of "color".
  9. Aug 25, 2004 #8
    Hmm, now i could understand clearly! Thanks.....
  10. Aug 25, 2004 #9
    In regards to light traveling through a medium:
    When a photon collides with an electron, the electron momentarily goes up an energy level and then back down at which point another photon is emitted. I’m curious as to why the photon that gets emitted continues on in the same direction as the initial colliding photon. Can anyone elaborate?
  11. Aug 25, 2004 #10


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    Conservation of momentum.

    Yep, that about sums up my knowledge on that one.
  12. Aug 25, 2004 #11
    I'm aware that the atom produces a photon of the same frequency as the one that hit it in the same direction, but what happens to the initial photon? If I remember correctly, lasers use this property to function properly. If the photon that strikes the atom doesn't continue, then how would lasers function?
  13. Aug 25, 2004 #12


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    Photons have weight?
  14. Aug 25, 2004 #13


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    Photons have no rest mass, but they have momentum. I believe it is calculated with their frequency, the higher the frequency the higher the momentum.
  15. Aug 25, 2004 #14
    How could you say that light glows, had you ever saw a path of light. It make things possible to see when it transfer information about the thing to our eyes and our brain visulize it. it something like the thing you want to see has coded some information and send it as EM wave and our eyes and brain decoded in the same manner in which it was encoded. Take an example of a computer as you save any ingormation in it like a written text then it doesn't save itself as the text it appears.
  16. Aug 26, 2004 #15


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    Uhhh... Another thing... In software programs on the computer like photoshop... there's options of magenta, yellow and cyan, and red, blue and green. When you mix equal amounts of 100% red, blue, and yellow you get white. When you do the same with magenta, cyan, and yellow, you get black. On the TV there's red, green, and blue. Why the differences? Couldn't you create violet, and green out of the red, blue and yellow combo?
  17. Aug 26, 2004 #16
    Thank you... but this specific point has been asked and answered already above. See my second post.
  18. Aug 26, 2004 #17
    Im not an artist, so I cant answer you question. But quantum mechanics is not art. The reson there is violet and blue at the bottom of the spectrum is because of its high frequency and very low waveleagth. You cant mix monochromatic light in order to get a high frequency colour.
  19. Aug 26, 2004 #18


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    So Nenad, you're looking at a color monitor. Look closely: all colors are composed of red green and blue. So how does your monitor display violet? The reason you can get any perceived color out of just 3 color guns is because your eye itself has 3 different sensors each with its own spectral sensitivity. These all overlap heavily. Thus completely different colors actually can look exactly the same. E.g. a pure violet looks exactly the same as a combination of RGB. Here is a really excellent explanation (needs powerpoint viewer): http://people.cs.uct.ac.za/~jgain/courses/advgfx/colour.ppt
    Last edited: Aug 27, 2004
  20. Aug 27, 2004 #19


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    If the RGB combination = violet, than are there also cyan, magenta, and yellow combinations for violet?, about RGB, on a cathode-ray tube based monitor, how does yellow appear?
  21. Aug 27, 2004 #20
    meh, if you think about it, that is only a way for us to see it. If we were actually to think about the quantum particles, it would be impossoble.
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