To make plausible the idea that light exerts a pressure on matter, some introductory physics texts consider the force exerted by an electromagnetic wave on an electron. The argument as presented is mathematically incorrect and has several serious conceptual difficulties without obvious resolution at the classical, yet alone introductory, level. We discuss these difficulties and propose an alternative argument.

Topics
Faraday's law, Electromagnetism, Physical quantities, Electrostatics, Newtonian mechanics, Electrodynamics, Educational aids, Compton scattering, Scattering theory, Thomson scattering
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Most introductory texts use the integral form of Maxwell’s equations. The derivation can easily be carried out by considering infinitesimal loops in the xz and yz planes as follows: The integral form of Faraday’s law is ∮E⋅ds=−dϕ∕dt for magnetic flux ϕ⁠. For the case of our mirror the right-hand rule gives ∮E⋅ds=Ex(z+dz)dx−Ex(z)dx=(dEx∕dz)dzdx=−(dBy∕dt)dxdz=−(dϕ∕dt) which leads immediately to Eq. (22). Similarly, the integral form of Ampère’s law ∮B⋅ds=4πI leads to Eq. (24).

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Anonymous,
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© 2009 American Association of Physics Teachers.
2009
American Association of Physics Teachers
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