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Basic kinematics

  1. Nov 10, 2005 #1
    I know that scientists have two meanings on masses. One is the resistance against motion and other the quantity of existence. Since photons travel with the speed of light, they are considered to be massless. And it also supports the fact that they never resist motion. Infact they always move with the speed of light. But in photoelectric effect it is said that due to high frequency light the electrons get emitted due to the photoelectric effect. But how can photons which are massless have kinetic energy.
    Now when a photon strikes a mirror it reflects back. The action of reflection here is instantaneous or takes no time here since the photons tend to continue to move with the speed of light. Similar is the case of refraction. I think I am correct. I need an expert's verification of my statements. (A small doubt - what is the difference between deviation and deflection?)
  2. jcsd
  3. Nov 10, 2005 #2
    K.E = .5 m v^2
    this is the equation u are lookin upto while having photon and photoelectric effect in ur mind, as u should know this is a equation from Newtonian Mechanics and Newtonian mechanics doesn't hold on at relevastic speeds.
    I don't know what are you talking abt continue motion of photon. and theres change in speed of photons while passing through a material it happenes in refration also in mirror, mirror is a glass slab with one side covered with reflecting material, u know. But i still can't get the ppoint u r talkin about.
  4. Nov 11, 2005 #3


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    A photon has energy (E=hf) where h is Planck's constant and f is the photon frequency. The electron in the metal can absorb this energy and transfer it into kinetic energy of the electron, and if sufficient free the electron from the metal. While a photon has energy it is never said to have kinetic energy because a photon is a different entity to a particle with mass. A photon does however have momentum as derived by Einstein which is (p=E/c).
  5. Nov 12, 2005 #4
    thank you. you made a major part where i have misunderstanding very clear.

    yes, i recall of reading about force neededto stop a photon as such. it has mommentum and not kinetic energy. how does such a thind happen? am i too small to understand such things?

    jimmy, i am sorry about refraction anyway, think about reflection. what i want to know is whether the act of reflection takes any time. just like in basic kinematics if you hit a ball straight back down the ground, the ball is at rest for an instant when it touches the bat. but such property should not be shown by photons, isn' t it?
  6. Nov 12, 2005 #5


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    Don't push these analogies too far, but:

    In a sense, reflection from a mirror metal surface takes a half-cycle of the wave -
    the reflected wave is half a wave out-of-synch from the incoming wave.
    The Electric field switches direction when reflecting at the metal surface,
    and the magnetic field penetrates into the metal about 1/4 wavelength.

    In a sense, the Energy of a photon is half Kinetic Energy (KE = 1/2 p v)
    and is half Potential Energy, residing in the Electric Field of the wave.

    Now, the "more correct" understanding of reflection is that the original
    photon dives into the material and is eventually absorbed,
    while the photon that comes out, as if it was the reflected photon,
    is actually a *different* photon than the one that hit the metal.
    Photons are *bosons*, and boson number is NOT conserved.
    Last edited: Nov 12, 2005
  7. Nov 12, 2005 #6
    well it depends on your point of view. In case of refraction lets say, you know the lenght of glass slab you are using, you know the speed of light while it passes through that galss slab, and obviously you can calculate the time taken by, lets say a photon, in traveling across the slab.
    And same would happen in case of reflaction. but we are just too slow to recognise it.
  8. Nov 13, 2005 #7


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    Most reflections take place off a metallic surface. When a photon hits that surface an electron will absorb the photon for a brief time, typically 10^-8 seconds (correct me if I'm wrong i'm working from memory here). In that snse it takes a finite time to reflect the image. For refraction through an object a similar absorption process occurs thus making it appear that the light has reduced speed when in fact it hasn't, it is just being absorbed and emitted many times which takes a finite time too.
  9. Nov 14, 2005 #8
    I assume that the boson number is the spin number. Anyway photons show particle nature. If a ball is spun and thrown on a wall, while coming back the direction of spin would have changed. Why not such a thing could have happened and we feel that the photon is another photon but infact the boson number has changed due to the collision with the mirror surface. The boson number changes is the only observation that led the scientist to think in such a way or is there in other proof I don’t know? Anyway a metal surface is opaque. So where does that photon which dives go if you theorem is any way correct. I also think that according to this theory the energy in the other photon that comes back is being transferred from this photon.
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