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What is the momentum of a photon propagating in a medium?

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If you have a question about this talk, please contact Dr Jonathan Keeling.

We are used to identifying the momentum of a photon as ħk or h/λ. This fundamental idea underlies much of quantum physics and, indeed, was used by de Broglie in his derivation of de Broglie waves. But what happens to this momentum when the photon enters a (transparent) medium? A very simple (even unanswerable?) derivation based on Newton’s first law of motion leads to Abraham’s result that the momentum is reduced by the refractive index. Unfortunately, an equally simple derivation points unambiguously to Minkowski’s result that that the momentum is increased by the refractive index. Even in 2009, after more than a century of debate, experimental papers are being published in high-profile journals supporting one or other of these momenta. So what is going on? In a sense both momenta are correct …

This talk is part of the Irregular seminars in TCM series.

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