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Is a lens attracted to the light?

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    Is a lens attracted to the light?

    Is a lens attracted to the light, like a moth?

    Hint: When light is refracted by a lens the light's momentum will change (or will it) and momentum must be conserved by the system as a whole.

    #2
    No it will be repelled, at an angle.
    The photons of light have momentum in a direction directly away from the source of light. Then the lens changes it to an angle. Since the size of the momentum must remain the same, the vector part of it in the direction away from the original source of the light must reduce (by the cosine of the angle defracted by). Therefore to conserve momentum the lens must acquire momentum away from the light source to make up this difference.

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      #3
      Originally posted by TimberWolf View Post
      the light's momentum will change (or will it) and momentum must be conserved by the system as a whole.
      Light has momentum? Fantastic...
      "Never argue with stupid people, they will drag you down to their level and beat you with experience". Mark Twain

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        #4
        Originally posted by TimberWolf View Post
        Is a lens attracted to the light, like a moth?
        Speaking as a member of the non BSc bunch, my days at university were spent by having my lenses distracted by norks and beer.

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          #5
          Originally posted by scooterscot View Post
          Light has momentum? Fantastic...
          How did this happen? Who's to blame? Well certainly there are those more responsible than others, and they will be held accountable, but again truth be told, if you're looking for the guilty, you need only look into a mirror.

          Follow me on Twitter - LinkedIn Profile - The HAB blog - New Blog: Mad Cameron
          Xeno points: +5 - Asperger rating: 36 - Paranoid Schizophrenic rating: 44%

          "We hang the petty thieves and appoint the great ones to high office" - Aesop

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            #6
            Originally posted by KentPhilip View Post
            No it will be repelled, at an angle.
            The photons of light have momentum in a direction directly away from the source of light. Then the lens changes it to an angle. Since the size of the momentum must remain the same, the vector part of it in the direction away from the original source of the light must reduce (by the cosine of the angle defracted by). Therefore to conserve momentum the lens must acquire momentum away from the light source to make up this difference.

            Okay. That’s the case when parallel rays are converged by a convex lens. But what about a convex lens that takes in diverging light rays and outputs parallel light rays? E.g. if the lens were close to a small light source, or the lens is really big compared to the source?
            Last edited by TimberWolf; 18 June 2009, 06:51. Reason: Simplified setup

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              #7
              Originally posted by TimberWolf View Post
              Okay. That’s the case when parallel rays are converged by a convex lens. But what about a convex lens that takes in diverging light rays and outputs parallel light rays? E.g. if the lens were close to a small light source, or the lens is really big compared to the source?
              Parallelism is an illusion. All lines converge at infinity.
              "take me to your leader"

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                #8
                Originally posted by Grinder View Post
                Parallelism is an illusion. All lines converge at infinity.
                If that's true, would rays that are diverging by an infinitely small amount not cross at infinity? If not, would all divergent rays converge at infinity?

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                  #9
                  Originally posted by Grinder View Post
                  Parallelism is an illusion. All lines converge at infinity.
                  Only with the one-point compactification.

                  Otherwise infinity is an illusion.

                  Anyway, parallelism is an option (postulate, or axiom).

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                    #10
                    Originally posted by TimberWolf View Post
                    Okay. That’s the case when parallel rays are converged by a convex lens. But what about a convex lens that takes in diverging light rays and outputs parallel light rays? E.g. if the lens were close to a small light source, or the lens is really big compared to the source?
                    Hmm yes, I suppose you are right.

                    Make sure you don't set fire to the lens when it flies into the light

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