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**Unformatted text preview: **and the ﬁelds are always in phase with each other. E = E m sin( kx-ωt ) B = B m sin( kx-ωt ) • EM waves in vacuum always travel at the same speed, c , which is called the speed of light. This speed is given by the constants that we have been working with all semester. c = 1 √ μ ± (50) • The ratio of the amplitudes of the electric and magnetic waves is ﬁxed in terms of this speed. E m B m = c (51) 14.2 Energy and Momentum in EM Waves Anyone who has been out in the sun for a while or stood next to a campﬁre at night knows that em waves carry energy. The warmth that we feel in these two cases is the energy from these waves begin deposited in our skin. The quantity which describes the energy carried by an em wave is the Poynting vector. Poynting is the last name of the physicist who ﬁrst discussed its properties, but it turns out to be aptly 56...

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