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Reflecting photons off a surface

  1. Jul 31, 2009 #1
    I would like to know the answers to these questions:

    How does a photon reflect off a surface and if so is energy lost (i do realise that photons travel at the speed of light but i was just wondering)?

    When a photon reflects off a surface is this point equal to the angle of incidence?

    When a photon reflects off a surface of a material, what does it actually reflect off?

    Please explain it to me simply. Thankyou
  2. jcsd
  3. Jul 31, 2009 #2
    You could say that the photon "bounces" against the electron clouds of atoms on the surface. But to analyse this in more detail you would have to consider the wave properties of the photon. You can't consider the photon localized or hitting the surface at a very precise location. Instead you should consider it as kind of smeared in the form of a wave packet. In order to consider the interaction as being a reflection, the photon should not be absorbed by the atom. If it is absorbed and then re-emmited then that is a different phenomenon.
  4. Jul 31, 2009 #3
    I think in general you'll understand reflection better if you start from the wave rather than particle (photon) perspective. Try Wikipedia at http://en.wikipedia.org/wiki/Reflection_(physics)

    Your questions are "simple" but the answers have many aspects. I'm no expert but I know the answers are dependent on the reflective material.

    Simple/practical answer: When a brick reflects light,for example, you know it's reflecting some light because you can see it; if it absorbed all light you could not see it. You also know some energy is transferred from the light to the brick because it warms up. And you know in general dark colors absorb more energy than light colors...dark surfaces melt snow faster in the winter, for example. And likely you also know smoothness of a surface affects how much light is reflected.

    Also, in everyday existence, visible light, say on the earth from the sun, is unlikely to be energetic enough to affect nuclei...so effects you are interested in have to do with electrons absorbing energy from light, subsequently oscillating more vigorously (which generate sheat) and subsequently radiating heat in the invisible infrared range.

    Not so simple:

    There are many types of "reflection". In the photoelectric effect, for example, some lightwave energy clearly becomes an electric current so there is clearly an energy transfer. Some materials may reflect most light and others absorb most light...perfect absorbers are "black bodies", the opposite of a perfect reflection medium (and not easy to find).

    In general when light gains or loses energy it's frequency changes which mean so does the color..so look for that in discussions. Light can also become partially or totally polarized by reflection and that would seem to affect reflected energy. Refraction occurs when light beams penetrate a medium, even a reflective one, and as a result bound electrons may be set in oscillation which causes reflected radiation. In that case clealy some light energy is transferred to the medium...I'd guess glass is such a medium. So I'm saying reflection, refraction and absorption characteristics of the medium all have an effect.
    Last edited: Jul 31, 2009
  5. Aug 1, 2009 #4
    i would also like to know why certain photons are absorbed by materials and why light (photons) sometimes reflects off surfaces. What does this depend on?
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