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Are photons necessary for an object to show colour?

  1. Jun 25, 2012 #1

    I know without light, an object can't give out radiations thus, show no colour.

    So if we keep any object in a completly dark room, it would not exhibit colour.
    Suppose we send in a large amount of sound energy into the same room. The object in the room would absorb some amount of this sound energy. So the electrons get excited, come back to the ground level and emit radiations.
    So does this mean an object can show colour without photons stricking on them?

    Or am I just typing blunder?

    Thank you.
  2. jcsd
  3. Jun 25, 2012 #2


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    Welcome to PF, Swetasuria.
    Photons are the carriers of EM, including visible light, and thus are the carriers of colour. No light and no colour can exist without them. An atom does not have to be stimulated by EM in order to radiate. There are other triggers.
  4. Jun 25, 2012 #3


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    This question is really a philosophical one.
    What we perceive as 'colour' is just the particular mix of visible wavelengths that enters our eyes and stimulates the receptors. I think we would normally say that a purple object is still purple when the light is turned off; its purple colour will only look that way when its illuminated by a white light source but we know how it will look when the light's turned on again.

    In the same way, would we not say that a 10cm line is still 10cm long even when the ruler is taken away?
  5. Jun 25, 2012 #4
    1) Aren't radiations from electrons necessary for an object to show colour? Or are radiations of the visible range necessary to show colour?

    2) Is EM electromagnetism or electromagnetic wave?
  6. Jun 25, 2012 #5
    For example I could have an electron orbit a planet, and as it went around it would radiate.
    So I could use gravity to cause light emission. Or I could accelerate a proton with the nuclear force and cause it to radiate light.
  7. Jun 25, 2012 #6
    But how would the electron radiate because according to Bohr's theory, an elctron cannot lose or gain energy while in a fixed orbit. Unless we can't apply Bohr's theory in this senario.
  8. Jun 25, 2012 #7
    I an afraid your question is a little ambiguous, so the answers you got so far may not answer your intended question.
    The ambiguous part is "on them".
    I think you may want to say "on it" where "it" means the object.
    If this is the case, then yes, reflecting light from it is only one possible way to make an object visible.

    The object may emit "its own" light even if there is no incoming light on it. You can excite this emission thermally, electrically and even with ultrasound (which is another way of heating up so it's not really a different method).
    Now, the color emitted by a hot object may not be the same color you see when light reflects on the cold object.
  9. Jun 25, 2012 #8


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    It helps if you keep in mind that most energy eventually converts to heat, which is EM. So, you can rub your "object in the dark" and the kinetic energy you input will convert to heat. This heat will be visible with an infrared camera, or if you generate enough heat, the object will begin to glow in the visible spectrum, starting with red.

    Bombarding the object with soundwaves is no different; kinetic energy in the form of soundwaves is converted to heat energy. You could also hit the object wth a hammer, creating a shower of sparks.

    Or, if the object happens to be a star, it creates its own EM during the act of steller fusion (this I suppose is te most familiar example of an object that emits visibloe light without a external source reflecting off of it). The suprising thing is, every physical object undergoes some form of radiative decay. Not many are as obvous as fusion, but all release some form of energy; usually heat.
  10. Jun 25, 2012 #9


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    The electron is not in the ground-state (for reasonable planets), therefore it will radiate photons and go into a lower orbit. As the involved quantum numbers are extremely large, it is a good approximation to treat this in a classical way.
  11. Jun 25, 2012 #10


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    Put a 'fast' electron in a strong magnetic field and it will go round in a curve ('orbit') radiating EM energy and losing its energy. It's referred to as synchrotron radiation. The energy level situation is different from an electron around an atom so the result is different. No one's theory is violated.
  12. Jun 25, 2012 #11


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    A very good example of that is a typical electric stove. An element is normally black, but turns red when in operation. That colour is a function of temperature.

    I thought that we agreed that your sex life would no longer be mentioned in public.
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