The Heavens Declare His Handiwork

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Thomas Lee Abshier, ND
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Energy Conservation in Photon Reflection

By: Thomas Lee Abshier, ND

The photon is absorbed, and reradiated off of reflecting surfaces.  Collision with a relatively massive target will give the target only a very small velocity.  Momentum is conserved in the collision, in that the center of momentum of the photon-mirror system continues on in the same direction before the collision as afterward.  A small velocity is imparted to the mirror, while the photon’s velocity experiences a perpendicular component reversal.  In energetic terms, the conservation of energy dictates that there is some energy lost by the photon, and some energy gained by the mirror.  The amount of energy lost by the photon is small if the collision is with a relatively massive target.  The small velocity increase by the mass corresponds to the amount of energy lost by the photon.  The energy lost by the photon corresponds to a decrease in frequency of the reflected light.  Thus, the kinetic energy of the mirror rises due to its increase in velocity, while the frequency of the reflected light is reduced by an equivalent amount.