1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Of light and mirrors

  1. May 12, 2009 #1
    Light is an electromagnetic wave that exhibits both wave and matter-like properties yes? I was wondering what happens to light when it reflects off a mirror.

    If a tennis ball is thrown at a wall, the moment of contact between the ball and the wall is very brief, so the deceleration rate is very, very high right? Say the ball was travelling 20ms^-1 then bounces back at around -19ms^-1, making a difference of 39ms^-1. The length of time the ball contacts the wall is around 0.02s (for the sake of example). Then the deceleration would be 1950ms^-2. There is a point in time where the ball is simply stationary.

    Light travels at around 299,792,458ms^-1. The moment of 'contact' is near negligible, most likely in the picoseconds. Even so, there must be a measure of deceleration for light whether it is a particle (matter) or wave (radiation). If this is the case, then what kind of properties would light have at the inevitable point in time of 0 velocity?

    If I have it all wrong and there is a separate set of physical laws governing EM radiation, what is it?
  2. jcsd
  3. May 12, 2009 #2


    User Avatar
    Homework Helper

    In fact there is a separate set of physical laws governing EM radiation:
    [tex]\vec{\nabla}\cdot\vec{D} = \rho_f[/tex]
    [tex]\vec{\nabla}\cdot\vec{B} = 0[/tex]
    [tex]\vec{\nabla}\times\vec{E} = -\frac{\partial\vec{B}}{\partial t}[/tex]
    [tex]\vec{\nabla}\times\vec{H} = \frac{\partial\vec{D}}{\partial t} + \vec{J}_f[/tex]
    The process of solving those equations to figure out what happens at a reflecting surface is kind of long and involved, but you do find that EM waves are reflected without undergoing any change in speed. There's no point at which the light is not traveling at 299792458 m/s (which is an exact value, by the way). That's consistent with relativity, which says that anything that ever travels at the speed of light will always travel at the speed of light.
  4. May 12, 2009 #3

    Andy Resnick

    User Avatar
    Science Advisor
    Education Advisor

    Light can exhibit both wavelike and particlelike behavior; this should not be confused with ponderable matter. Light may act as localized (particle) or nonlocalized (wave) objects.

    Light, when scattering off an interface (including a mirror), does indeed transfer momentum to the scattering object.
  5. May 12, 2009 #4
    Very very erroneous thinking! Light doesn't accelerate, neither decelerate. In the moment it irradiates the mirror, light excites the electrons that make the mirror and, if you wish, stops existing. Electrons stay excited for about a few nanoseconds, and then release their excess energy in the form of light. It happens so that this light will come toward the source, thus reflection (in fact, electrons emit light in all directions, but light in forward direction cancels, while that returning doesn't).
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook